typecheck/TcSplice.hs

    {-
    (c) The University of Glasgow 2006
    (c) The GRASP/AQUA Project, Glasgow University, 1992-1998

    TcSplice: Template Haskell splices
    -}

    {-# LANGUAGE CPP #-}
    {-# LANGUAGE FlexibleInstances #-}
    {-# LANGUAGE MagicHash #-}
    {-# LANGUAGE ScopedTypeVariables #-}
    {-# LANGUAGE InstanceSigs #-}
    {-# LANGUAGE GADTs #-}
    {-# LANGUAGE RecordWildCards #-}
    {-# LANGUAGE MultiWayIf #-}
    {-# LANGUAGE TypeFamilies #-}
    {-# OPTIONS_GHC -fno-warn-orphans #-}

    module TcSplice(
         tcSpliceExpr, tcTypedBracket, tcUntypedBracket,
    --     runQuasiQuoteExpr, runQuasiQuotePat,
    --     runQuasiQuoteDecl, runQuasiQuoteType,
         runAnnotation,

         runMetaE, runMetaP, runMetaT, runMetaD, runQuasi,
         tcTopSpliceExpr, lookupThName_maybe,
         defaultRunMeta, runMeta', runRemoteModFinalizers,
         finishTH
          ) where

    #include "HsVersions.h"

    import GhcPrelude

    import HsSyn
    import Annotations
    import Finder
    import Name
    import TcRnMonad
    import TcType

    import Outputable
    import TcExpr
    import SrcLoc
    import THNames
    import TcUnify
    import TcEnv
    import FileCleanup ( newTempName, TempFileLifetime(..) )

    import Control.Monad

    import GHCi.Message
    import GHCi.RemoteTypes
    import GHCi
    import HscMain
            -- These imports are the reason that TcSplice
            -- is very high up the module hierarchy
    import FV
    import RnSplice( traceSplice, SpliceInfo(..) )
    import RdrName
    import HscTypes
    import Convert
    import RnExpr
    import RnEnv
    import RnUtils ( HsDocContext(..) )
    import RnFixity ( lookupFixityRn_help )
    import RnTypes
    import TcHsSyn
    import TcSimplify
    import Type
    import NameSet
    import TcMType
    import TcHsType
    import TcIface
    import TyCoRep
    import FamInst
    import FamInstEnv
    import InstEnv
    import Inst
    import NameEnv
    import PrelNames
    import TysWiredIn
    import OccName
    import Hooks
    import Var
    import Module
    import LoadIface
    import Class
    import TyCon
    import CoAxiom
    import PatSyn
    import ConLike
    import DataCon
    import TcEvidence( TcEvBinds(..) )
    import Id
    import IdInfo
    import DsExpr
    import DsMonad
    import GHC.Serialized
    import ErrUtils
    import Util
    import Unique
    import VarSet
    import Data.List        ( find )
    import Data.Maybe
    import FastString
    import BasicTypes hiding( SuccessFlag(..) )
    import Maybes( MaybeErr(..) )
    import DynFlags
    import Panic
    import Lexeme
    import qualified EnumSet
    import Plugins

    import qualified Language.Haskell.TH as TH
    -- THSyntax gives access to internal functions and data types
    import qualified Language.Haskell.TH.Syntax as TH

    -- Because GHC.Desugar might not be in the base library of the bootstrapping compiler
    import GHC.Desugar      ( AnnotationWrapper(..) )

    import Control.Exception
    import Data.Binary
    import Data.Binary.Get
    import qualified Data.ByteString as B
    import qualified Data.ByteString.Lazy as LB
    import Data.Dynamic  ( fromDynamic, toDyn )
    import qualified Data.Map as Map
    import Data.Typeable ( typeOf, Typeable, TypeRep, typeRep )
    import Data.Data (Data)
    import Data.Proxy    ( Proxy (..) )
    import GHC.Exts         ( unsafeCoerce# )

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Main interface + stubs for the non-GHCI case
    *                                                                      *
    ************************************************************************
    -}

    tcTypedBracket   :: HsExpr GhcRn -> HsBracket GhcRn -> ExpRhoType -> TcM (HsExpr GhcTcId)
    tcUntypedBracket :: HsExpr GhcRn -> HsBracket GhcRn -> [PendingRnSplice] -> ExpRhoType
                     -> TcM (HsExpr GhcTcId)
    tcSpliceExpr     :: HsSplice GhcRn  -> ExpRhoType -> TcM (HsExpr GhcTcId)
            -- None of these functions add constraints to the LIE

    -- runQuasiQuoteExpr :: HsQuasiQuote RdrName -> RnM (LHsExpr RdrName)
    -- runQuasiQuotePat  :: HsQuasiQuote RdrName -> RnM (LPat RdrName)
    -- runQuasiQuoteType :: HsQuasiQuote RdrName -> RnM (LHsType RdrName)
    -- runQuasiQuoteDecl :: HsQuasiQuote RdrName -> RnM [LHsDecl RdrName]

    runAnnotation     :: CoreAnnTarget -> LHsExpr GhcRn -> TcM Annotation
    {-
    ************************************************************************
    *                                                                      *
    \subsection{Quoting an expression}
    *                                                                      *
    ************************************************************************
    -}

    -- See Note [How brackets and nested splices are handled]
    -- tcTypedBracket :: HsBracket Name -> TcRhoType -> TcM (HsExpr TcId)
    tcTypedBracket rn_expr brack@(TExpBr _ expr) res_ty
      = addErrCtxt (quotationCtxtDoc brack) $
        do { cur_stage <- getStage
           ; ps_ref <- newMutVar []
           ; lie_var <- getConstraintVar   -- Any constraints arising from nested splices
                                           -- should get thrown into the constraint set
                                           -- from outside the bracket

           -- Typecheck expr to make sure it is valid,
           -- Throw away the typechecked expression but return its type.
           -- We'll typecheck it again when we splice it in somewhere
           ; (_tc_expr, expr_ty) <- setStage (Brack cur_stage (TcPending ps_ref lie_var)) $
                                    tcInferRhoNC expr
                                    -- NC for no context; tcBracket does that

           ; meta_ty <- tcTExpTy expr_ty
           ; ps' <- readMutVar ps_ref
           ; texpco <- tcLookupId unsafeTExpCoerceName
           ; tcWrapResultO (Shouldn'tHappenOrigin "TExpBr")
                           rn_expr
                           (unLoc (mkHsApp (nlHsTyApp texpco [expr_ty])
                                          (noLoc (HsTcBracketOut noExt brack ps'))))
                           meta_ty res_ty }
    tcTypedBracket _ other_brack _
      = pprPanic "tcTypedBracket" (ppr other_brack)

    -- tcUntypedBracket :: HsBracket Name -> [PendingRnSplice] -> ExpRhoType -> TcM (HsExpr TcId)
    tcUntypedBracket rn_expr brack ps res_ty
      = do { traceTc "tc_bracket untyped" (ppr brack $$ ppr ps)
           ; ps' <- mapM tcPendingSplice ps
           ; meta_ty <- tcBrackTy brack
           ; traceTc "tc_bracket done untyped" (ppr meta_ty)
           ; tcWrapResultO (Shouldn'tHappenOrigin "untyped bracket")
                           rn_expr (HsTcBracketOut noExt brack ps') meta_ty res_ty }

    ---------------
    tcBrackTy :: HsBracket GhcRn -> TcM TcType
    tcBrackTy (VarBr {})  = tcMetaTy nameTyConName
                                               -- Result type is Var (not Q-monadic)
    tcBrackTy (ExpBr {})  = tcMetaTy expQTyConName  -- Result type is ExpQ (= Q Exp)
    tcBrackTy (TypBr {})  = tcMetaTy typeQTyConName -- Result type is Type (= Q Typ)
    tcBrackTy (DecBrG {}) = tcMetaTy decsQTyConName -- Result type is Q [Dec]
    tcBrackTy (PatBr {})  = tcMetaTy patQTyConName  -- Result type is PatQ (= Q Pat)
    tcBrackTy (DecBrL {})   = panic "tcBrackTy: Unexpected DecBrL"
    tcBrackTy (TExpBr {})   = panic "tcUntypedBracket: Unexpected TExpBr"
    tcBrackTy (XBracket {}) = panic "tcUntypedBracket: Unexpected XBracket"

    ---------------
    tcPendingSplice :: PendingRnSplice -> TcM PendingTcSplice
    tcPendingSplice (PendingRnSplice flavour splice_name expr)
      = do { res_ty <- tcMetaTy meta_ty_name
           ; expr' <- tcMonoExpr expr (mkCheckExpType res_ty)
           ; return (PendingTcSplice splice_name expr') }
      where
         meta_ty_name = case flavour of
                           UntypedExpSplice  -> expQTyConName
                           UntypedPatSplice  -> patQTyConName
                           UntypedTypeSplice -> typeQTyConName
                           UntypedDeclSplice -> decsQTyConName

    ---------------
    -- Takes a tau and returns the type Q (TExp tau)
    tcTExpTy :: TcType -> TcM TcType
    tcTExpTy exp_ty
      = do { unless (isTauTy exp_ty) $ addErr (err_msg exp_ty)
           ; q    <- tcLookupTyCon qTyConName
           ; texp <- tcLookupTyCon tExpTyConName
           ; return (mkTyConApp q [mkTyConApp texp [exp_ty]]) }
      where
        err_msg ty
          = vcat [ text "Illegal polytype:" <+> ppr ty
                 , text "The type of a Typed Template Haskell expression must" <+>
                   text "not have any quantification." ]

    quotationCtxtDoc :: HsBracket GhcRn -> SDoc
    quotationCtxtDoc br_body
      = hang (text "In the Template Haskell quotation")
             2 (ppr br_body)

      -- The whole of the rest of the file is the else-branch (ie stage2 only)

    {-
    Note [How top-level splices are handled]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Top-level splices (those not inside a [| .. |] quotation bracket) are handled
    very straightforwardly:

      1. tcTopSpliceExpr: typecheck the body e of the splice $(e)

      2. runMetaT: desugar, compile, run it, and convert result back to
         HsSyn RdrName (of the appropriate flavour, eg HsType RdrName,
         HsExpr RdrName etc)

      3. treat the result as if that's what you saw in the first place
         e.g for HsType, rename and kind-check
             for HsExpr, rename and type-check

         (The last step is different for decls, because they can *only* be
          top-level: we return the result of step 2.)

    Note [How brackets and nested splices are handled]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Nested splices (those inside a [| .. |] quotation bracket),
    are treated quite differently.

    Remember, there are two forms of bracket
             typed   [|| e ||]
       and untyped   [|  e  |]

    The life cycle of a typed bracket:
       * Starts as HsBracket

       * When renaming:
            * Set the ThStage to (Brack s RnPendingTyped)
            * Rename the body
            * Result is still a HsBracket

       * When typechecking:
            * Set the ThStage to (Brack s (TcPending ps_var lie_var))
            * Typecheck the body, and throw away the elaborated result
            * Nested splices (which must be typed) are typechecked, and
              the results accumulated in ps_var; their constraints
              accumulate in lie_var
            * Result is a HsTcBracketOut rn_brack pending_splices
              where rn_brack is the incoming renamed bracket

    The life cycle of a un-typed bracket:
       * Starts as HsBracket

       * When renaming:
            * Set the ThStage to (Brack s (RnPendingUntyped ps_var))
            * Rename the body
            * Nested splices (which must be untyped) are renamed, and the
              results accumulated in ps_var
            * Result is still (HsRnBracketOut rn_body pending_splices)

       * When typechecking a HsRnBracketOut
            * Typecheck the pending_splices individually
            * Ignore the body of the bracket; just check that the context
              expects a bracket of that type (e.g. a [p| pat |] bracket should
              be in a context needing a (Q Pat)
            * Result is a HsTcBracketOut rn_brack pending_splices
              where rn_brack is the incoming renamed bracket

    In both cases, desugaring happens like this:
      * HsTcBracketOut is desugared by DsMeta.dsBracket.  It

          a) Extends the ds_meta environment with the PendingSplices
             attached to the bracket

          b) Converts the quoted (HsExpr Name) to a CoreExpr that, when
             run, will produce a suitable TH expression/type/decl.  This
             is why we leave the *renamed* expression attached to the bracket:
             the quoted expression should not be decorated with all the goop
             added by the type checker

      * Each splice carries a unique Name, called a "splice point", thus
        ${n}(e).  The name is initialised to an (Unqual "splice") when the
        splice is created; the renamer gives it a unique.

      * When DsMeta (used to desugar the body of the bracket) comes across
        a splice, it looks up the splice's Name, n, in the ds_meta envt,
        to find an (HsExpr Id) that should be substituted for the splice;
        it just desugars it to get a CoreExpr (DsMeta.repSplice).

    Example:
        Source:       f = [| Just $(g 3) |]
          The [| |] part is a HsBracket

        Typechecked:  f = [| Just ${s7}(g 3) |]{s7 = g Int 3}
          The [| |] part is a HsBracketOut, containing *renamed*
            (not typechecked) expression
          The "s7" is the "splice point"; the (g Int 3) part
            is a typechecked expression

        Desugared:    f = do { s7 <- g Int 3
                             ; return (ConE "Data.Maybe.Just" s7) }

    Note [Template Haskell state diagram]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Here are the ThStages, s, their corresponding level numbers
    (the result of (thLevel s)), and their state transitions.
    The top level of the program is stage Comp:

         Start here
             |
             V
          -----------     $      ------------   $
          |  Comp   | ---------> |  Splice  | -----|
          |   1     |            |    0     | <----|
          -----------            ------------
            ^     |                ^      |
          $ |     | [||]         $ |      | [||]
            |     v                |      v
       --------------          ----------------
       | Brack Comp |          | Brack Splice |
       |     2      |          |      1       |
       --------------          ----------------

    * Normal top-level declarations start in state Comp
           (which has level 1).
      Annotations start in state Splice, since they are
           treated very like a splice (only without a '$')

    * Code compiled in state Splice (and only such code)
      will be *run at compile time*, with the result replacing
      the splice

    * The original paper used level -1 instead of 0, etc.

    * The original paper did not allow a splice within a
      splice, but there is no reason not to. This is the
      $ transition in the top right.

    Note [Template Haskell levels]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    * Imported things are impLevel (= 0)

    * However things at level 0 are not *necessarily* imported.
          eg  $( \b -> ... )   here b is bound at level 0

    * In GHCi, variables bound by a previous command are treated
      as impLevel, because we have bytecode for them.

    * Variables are bound at the "current level"

    * The current level starts off at outerLevel (= 1)

    * The level is decremented by splicing $(..)
                   incremented by brackets [| |]
                   incremented by name-quoting 'f

    When a variable is used, we compare
            bind:  binding level, and
            use:   current level at usage site

      Generally
            bind > use      Always error (bound later than used)
                            [| \x -> $(f x) |]

            bind = use      Always OK (bound same stage as used)
                            [| \x -> $(f [| x |]) |]

            bind < use      Inside brackets, it depends
                            Inside splice, OK
                            Inside neither, OK

      For (bind < use) inside brackets, there are three cases:
        - Imported things   OK      f = [| map |]
        - Top-level things  OK      g = [| f |]
        - Non-top-level     Only if there is a liftable instance
                                    h = \(x:Int) -> [| x |]

      To track top-level-ness we use the ThBindEnv in TcLclEnv

      For example:
               f = ...
               g1 = $(map ...)         is OK
               g2 = $(f ...)           is not OK; because we havn't compiled f yet

    -}

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Splicing an expression}
    *                                                                      *
    ************************************************************************
    -}

    tcSpliceExpr splice@(HsTypedSplice _ _ name expr) res_ty
      = addErrCtxt (spliceCtxtDoc splice) $
        setSrcSpan (getLoc expr)    $ do
        { stage <- getStage
        ; case stage of
              Splice {}            -> tcTopSplice expr res_ty
              Brack pop_stage pend -> tcNestedSplice pop_stage pend name expr res_ty
              RunSplice _          ->
                -- See Note [RunSplice ThLevel] in "TcRnTypes".
                pprPanic ("tcSpliceExpr: attempted to typecheck a splice when " ++
                          "running another splice") (ppr splice)
              Comp                 -> tcTopSplice expr res_ty
        }
    tcSpliceExpr splice _
      = pprPanic "tcSpliceExpr" (ppr splice)

    {- Note [Collecting modFinalizers in typed splices]
       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    'qAddModFinalizer' of the @Quasi TcM@ instance adds finalizers in the local
    environment (see Note [Delaying modFinalizers in untyped splices] in
    "RnSplice"). Thus after executing the splice, we move the finalizers to the
    finalizer list in the global environment and set them to use the current local
    environment (with 'addModFinalizersWithLclEnv').

    -}

    tcNestedSplice :: ThStage -> PendingStuff -> Name
                    -> LHsExpr GhcRn -> ExpRhoType -> TcM (HsExpr GhcTc)
        -- See Note [How brackets and nested splices are handled]
        -- A splice inside brackets
    tcNestedSplice pop_stage (TcPending ps_var lie_var) splice_name expr res_ty
      = do { res_ty <- expTypeToType res_ty
           ; meta_exp_ty <- tcTExpTy res_ty
           ; expr' <- setStage pop_stage $
                      setConstraintVar lie_var $
                      tcMonoExpr expr (mkCheckExpType meta_exp_ty)
           ; untypeq <- tcLookupId unTypeQName
           ; let expr'' = mkHsApp (nlHsTyApp untypeq [res_ty]) expr'
           ; ps <- readMutVar ps_var
           ; writeMutVar ps_var (PendingTcSplice splice_name expr'' : ps)

           -- The returned expression is ignored; it's in the pending splices
           ; return (panic "tcSpliceExpr") }

    tcNestedSplice _ _ splice_name _ _
      = pprPanic "tcNestedSplice: rename stage found" (ppr splice_name)

    tcTopSplice :: LHsExpr GhcRn -> ExpRhoType -> TcM (HsExpr GhcTc)
    tcTopSplice expr res_ty
      = do { -- Typecheck the expression,
             -- making sure it has type Q (T res_ty)
             res_ty <- expTypeToType res_ty
           ; meta_exp_ty <- tcTExpTy res_ty
           ; zonked_q_expr <- tcTopSpliceExpr Typed $
                              tcMonoExpr expr (mkCheckExpType meta_exp_ty)

             -- See Note [Collecting modFinalizers in typed splices].
           ; modfinalizers_ref <- newTcRef []
             -- Run the expression
           ; expr2 <- setStage (RunSplice modfinalizers_ref) $
                        runMetaE zonked_q_expr
           ; mod_finalizers <- readTcRef modfinalizers_ref
           ; addModFinalizersWithLclEnv $ ThModFinalizers mod_finalizers
           ; traceSplice (SpliceInfo { spliceDescription = "expression"
                                     , spliceIsDecl      = False
                                     , spliceSource      = Just expr
                                     , spliceGenerated   = ppr expr2 })

             -- Rename and typecheck the spliced-in expression,
             -- making sure it has type res_ty
             -- These steps should never fail; this is a *typed* splice
           ; addErrCtxt (spliceResultDoc expr) $ do
           { (exp3, _fvs) <- rnLExpr expr2
           ; exp4 <- tcMonoExpr exp3 (mkCheckExpType res_ty)
           ; return (unLoc exp4) } }

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Error messages}
    *                                                                      *
    ************************************************************************
    -}

    spliceCtxtDoc :: HsSplice GhcRn -> SDoc
    spliceCtxtDoc splice
      = hang (text "In the Template Haskell splice")
             2 (pprSplice splice)

    spliceResultDoc :: LHsExpr GhcRn -> SDoc
    spliceResultDoc expr
      = sep [ text "In the result of the splice:"
            , nest 2 (char '$' <> ppr expr)
            , text "To see what the splice expanded to, use -ddump-splices"]

    -------------------
    tcTopSpliceExpr :: SpliceType -> TcM (LHsExpr GhcTc) -> TcM (LHsExpr GhcTc)
    -- Note [How top-level splices are handled]
    -- Type check an expression that is the body of a top-level splice
    --   (the caller will compile and run it)
    -- Note that set the level to Splice, regardless of the original level,
    -- before typechecking the expression.  For example:
    --      f x = $( ...$(g 3) ... )
    -- The recursive call to tcPolyExpr will simply expand the
    -- inner escape before dealing with the outer one

    tcTopSpliceExpr isTypedSplice tc_action
      = checkNoErrs $  -- checkNoErrs: must not try to run the thing
                       -- if the type checker fails!
        unsetGOptM Opt_DeferTypeErrors $
                       -- Don't defer type errors.  Not only are we
                       -- going to run this code, but we do an unsafe
                       -- coerce, so we get a seg-fault if, say we
                       -- splice a type into a place where an expression
                       -- is expected (Trac #7276)
        setStage (Splice isTypedSplice) $
        do {    -- Typecheck the expression
             (expr', wanted) <- captureConstraints tc_action
           ; const_binds     <- simplifyTop wanted

              -- Zonk it and tie the knot of dictionary bindings
           ; zonkTopLExpr (mkHsDictLet (EvBinds const_binds) expr') }

    {-
    ************************************************************************
    *                                                                      *
            Annotations
    *                                                                      *
    ************************************************************************
    -}

    runAnnotation target expr = do
        -- Find the classes we want instances for in order to call toAnnotationWrapper
        loc <- getSrcSpanM
        data_class <- tcLookupClass dataClassName
        to_annotation_wrapper_id <- tcLookupId toAnnotationWrapperName

        -- Check the instances we require live in another module (we want to execute it..)
        -- and check identifiers live in other modules using TH stage checks. tcSimplifyStagedExpr
        -- also resolves the LIE constraints to detect e.g. instance ambiguity
        zonked_wrapped_expr' <- tcTopSpliceExpr Untyped $
               do { (expr', expr_ty) <- tcInferRhoNC expr
                    -- We manually wrap the typechecked expression in a call to toAnnotationWrapper
                    -- By instantiating the call >here< it gets registered in the
                    -- LIE consulted by tcTopSpliceExpr
                    -- and hence ensures the appropriate dictionary is bound by const_binds
                  ; wrapper <- instCall AnnOrigin [expr_ty] [mkClassPred data_class [expr_ty]]
                  ; let specialised_to_annotation_wrapper_expr
                          = L loc (mkHsWrap wrapper
                                     (HsVar noExt (L loc to_annotation_wrapper_id)))
                  ; return (L loc (HsApp noExt
                                    specialised_to_annotation_wrapper_expr expr')) }

        -- Run the appropriately wrapped expression to get the value of
        -- the annotation and its dictionaries. The return value is of
        -- type AnnotationWrapper by construction, so this conversion is
        -- safe
        serialized <- runMetaAW zonked_wrapped_expr'
        return Annotation {
                   ann_target = target,
                   ann_value = serialized
               }

    convertAnnotationWrapper :: ForeignHValue -> TcM (Either MsgDoc Serialized)
    convertAnnotationWrapper fhv = do
      dflags <- getDynFlags
      if gopt Opt_ExternalInterpreter dflags
        then do
          Right <$> runTH THAnnWrapper fhv
        else do
          annotation_wrapper <- liftIO $ wormhole dflags fhv
          return $ Right $
            case unsafeCoerce# annotation_wrapper of
               AnnotationWrapper value | let serialized = toSerialized serializeWithData value ->
                   -- Got the value and dictionaries: build the serialized value and
                   -- call it a day. We ensure that we seq the entire serialized value
                   -- in order that any errors in the user-written code for the
                   -- annotation are exposed at this point.  This is also why we are
                   -- doing all this stuff inside the context of runMeta: it has the
                   -- facilities to deal with user error in a meta-level expression
                   seqSerialized serialized `seq` serialized

    -- | Force the contents of the Serialized value so weknow it doesn't contain any bottoms
    seqSerialized :: Serialized -> ()
    seqSerialized (Serialized the_type bytes) = the_type `seq` bytes `seqList` ()

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Running an expression}
    *                                                                      *
    ************************************************************************
    -}

    runQuasi :: TH.Q a -> TcM a
    runQuasi act = TH.runQ act

    runRemoteModFinalizers :: ThModFinalizers -> TcM ()
    runRemoteModFinalizers (ThModFinalizers finRefs) = do
      dflags <- getDynFlags
      let withForeignRefs [] f = f []
          withForeignRefs (x : xs) f = withForeignRef x $ \r ->
            withForeignRefs xs $ \rs -> f (r : rs)
      if gopt Opt_ExternalInterpreter dflags then do
        hsc_env <- env_top <$> getEnv
        withIServ hsc_env $ \i -> do
          tcg <- getGblEnv
          th_state <- readTcRef (tcg_th_remote_state tcg)
          case th_state of
            Nothing -> return () -- TH was not started, nothing to do
            Just fhv -> do
              liftIO $ withForeignRef fhv $ \st ->
                withForeignRefs finRefs $ \qrefs ->
                  writeIServ i (putMessage (RunModFinalizers st qrefs))
              () <- runRemoteTH i []
              readQResult i
      else do
        qs <- liftIO (withForeignRefs finRefs $ mapM localRef)
        runQuasi $ sequence_ qs

    runQResult
      :: (a -> String)
      -> (SrcSpan -> a -> b)
      -> (ForeignHValue -> TcM a)
      -> SrcSpan
      -> ForeignHValue {- TH.Q a -}
      -> TcM b
    runQResult show_th f runQ expr_span hval
      = do { th_result <- runQ hval
           ; traceTc "Got TH result:" (text (show_th th_result))
           ; return (f expr_span th_result) }

    -----------------
    runMeta :: (MetaHook TcM -> LHsExpr GhcTc -> TcM hs_syn)
            -> LHsExpr GhcTc
            -> TcM hs_syn
    runMeta unwrap e
      = do { h <- getHooked runMetaHook defaultRunMeta
           ; unwrap h e }

    defaultRunMeta :: MetaHook TcM
    defaultRunMeta (MetaE r)
      = fmap r . runMeta' True ppr (runQResult TH.pprint convertToHsExpr runTHExp)
    defaultRunMeta (MetaP r)
      = fmap r . runMeta' True ppr (runQResult TH.pprint convertToPat runTHPat)
    defaultRunMeta (MetaT r)
      = fmap r . runMeta' True ppr (runQResult TH.pprint convertToHsType runTHType)
    defaultRunMeta (MetaD r)
      = fmap r . runMeta' True ppr (runQResult TH.pprint convertToHsDecls runTHDec)
    defaultRunMeta (MetaAW r)
      = fmap r . runMeta' False (const empty) (const convertAnnotationWrapper)
        -- We turn off showing the code in meta-level exceptions because doing so exposes
        -- the toAnnotationWrapper function that we slap around the user's code

    ----------------
    runMetaAW :: LHsExpr GhcTc         -- Of type AnnotationWrapper
              -> TcM Serialized
    runMetaAW = runMeta metaRequestAW

    runMetaE :: LHsExpr GhcTc          -- Of type (Q Exp)
             -> TcM (LHsExpr GhcPs)
    runMetaE = runMeta metaRequestE

    runMetaP :: LHsExpr GhcTc          -- Of type (Q Pat)
             -> TcM (LPat GhcPs)
    runMetaP = runMeta metaRequestP

    runMetaT :: LHsExpr GhcTc          -- Of type (Q Type)
             -> TcM (LHsType GhcPs)
    runMetaT = runMeta metaRequestT

    runMetaD :: LHsExpr GhcTc          -- Of type Q [Dec]
             -> TcM [LHsDecl GhcPs]
    runMetaD = runMeta metaRequestD

    ---------------
    runMeta' :: Bool                 -- Whether code should be printed in the exception message
             -> (hs_syn -> SDoc)                                    -- how to print the code
             -> (SrcSpan -> ForeignHValue -> TcM (Either MsgDoc hs_syn))        -- How to run x
             -> LHsExpr GhcTc        -- Of type x; typically x = Q TH.Exp, or
                                     --    something like that
             -> TcM hs_syn           -- Of type t
    runMeta' show_code ppr_hs run_and_convert expr
      = do  { traceTc "About to run" (ppr expr)
            ; recordThSpliceUse -- seems to be the best place to do this,
                                -- we catch all kinds of splices and annotations.

            -- Check that we've had no errors of any sort so far.
            -- For example, if we found an error in an earlier defn f, but
            -- recovered giving it type f :: forall a.a, it'd be very dodgy
            -- to carry ont.  Mind you, the staging restrictions mean we won't
            -- actually run f, but it still seems wrong. And, more concretely,
            -- see Trac #5358 for an example that fell over when trying to
            -- reify a function with a "?" kind in it.  (These don't occur
            -- in type-correct programs.
            ; failIfErrsM

            -- run plugins
            ; hsc_env <- getTopEnv
            ; expr' <- withPlugins (hsc_dflags hsc_env) spliceRunAction expr

            -- Desugar
            ; ds_expr <- initDsTc (dsLExpr expr')
            -- Compile and link it; might fail if linking fails
            ; src_span <- getSrcSpanM
            ; traceTc "About to run (desugared)" (ppr ds_expr)
            ; either_hval <- tryM $ liftIO $
                             HscMain.hscCompileCoreExpr hsc_env src_span ds_expr
            ; case either_hval of {
                Left exn   -> fail_with_exn "compile and link" exn ;
                Right hval -> do

            {       -- Coerce it to Q t, and run it

                    -- Running might fail if it throws an exception of any kind (hence tryAllM)
                    -- including, say, a pattern-match exception in the code we are running
                    --
                    -- We also do the TH -> HS syntax conversion inside the same
                    -- exception-cacthing thing so that if there are any lurking
                    -- exceptions in the data structure returned by hval, we'll
                    -- encounter them inside the try
                    --
                    -- See Note [Exceptions in TH]
              let expr_span = getLoc expr
            ; either_tval <- tryAllM $
                             setSrcSpan expr_span $ -- Set the span so that qLocation can
                                                    -- see where this splice is
                 do { mb_result <- run_and_convert expr_span hval
                    ; case mb_result of
                        Left err     -> failWithTc err
                        Right result -> do { traceTc "Got HsSyn result:" (ppr_hs result)
                                           ; return $! result } }

            ; case either_tval of
                Right v -> return v
                Left se -> case fromException se of
                             Just IOEnvFailure -> failM -- Error already in Tc monad
                             _ -> fail_with_exn "run" se -- Exception
            }}}
      where
        -- see Note [Concealed TH exceptions]
        fail_with_exn :: Exception e => String -> e -> TcM a
        fail_with_exn phase exn = do
            exn_msg <- liftIO $ Panic.safeShowException exn
            let msg = vcat [text "Exception when trying to" <+> text phase <+> text "compile-time code:",
                            nest 2 (text exn_msg),
                            if show_code then text "Code:" <+> ppr expr else empty]
            failWithTc msg

    {-
    Note [Exceptions in TH]
    ~~~~~~~~~~~~~~~~~~~~~~~
    Suppose we have something like this
            $( f 4 )
    where
            f :: Int -> Q [Dec]
            f n | n>3       = fail "Too many declarations"
                | otherwise = ...

    The 'fail' is a user-generated failure, and should be displayed as a
    perfectly ordinary compiler error message, not a panic or anything
    like that.  Here's how it's processed:

      * 'fail' is the monad fail.  The monad instance for Q in TH.Syntax
        effectively transforms (fail s) to
            qReport True s >> fail
        where 'qReport' comes from the Quasi class and fail from its monad
        superclass.

      * The TcM monad is an instance of Quasi (see TcSplice), and it implements
        (qReport True s) by using addErr to add an error message to the bag of errors.
        The 'fail' in TcM raises an IOEnvFailure exception

     * 'qReport' forces the message to ensure any exception hidden in unevaluated
       thunk doesn't get into the bag of errors. Otherwise the following splice
       will triger panic (Trac #8987):
            $(fail undefined)
       See also Note [Concealed TH exceptions]

      * So, when running a splice, we catch all exceptions; then for
            - an IOEnvFailure exception, we assume the error is already
                    in the error-bag (above)
            - other errors, we add an error to the bag
        and then fail

    Note [Concealed TH exceptions]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    When displaying the error message contained in an exception originated from TH
    code, we need to make sure that the error message itself does not contain an
    exception.  For example, when executing the following splice:

        $( error ("foo " ++ error "bar") )

    the message for the outer exception is a thunk which will throw the inner
    exception when evaluated.

    For this reason, we display the message of a TH exception using the
    'safeShowException' function, which recursively catches any exception thrown
    when showing an error message.

    To call runQ in the Tc monad, we need to make TcM an instance of Quasi:
    -}

    instance TH.Quasi TcM where
      qNewName s = do { u <- newUnique
                      ; let i = getKey u
                      ; return (TH.mkNameU s i) }

      -- 'msg' is forced to ensure exceptions don't escape,
      -- see Note [Exceptions in TH]
      qReport True msg  = seqList msg $ addErr  (text msg)
      qReport False msg = seqList msg $ addWarn NoReason (text msg)

      qLocation = do { m <- getModule
                     ; l <- getSrcSpanM
                     ; r <- case l of
                            UnhelpfulSpan _ -> pprPanic "qLocation: Unhelpful location"
                                                        (ppr l)
                            RealSrcSpan s -> return s
                     ; return (TH.Loc { TH.loc_filename = unpackFS (srcSpanFile r)
                                      , TH.loc_module   = moduleNameString (moduleName m)
                                      , TH.loc_package  = unitIdString (moduleUnitId m)
                                      , TH.loc_start = (srcSpanStartLine r, srcSpanStartCol r)
                                      , TH.loc_end = (srcSpanEndLine   r, srcSpanEndCol   r) }) }

      qLookupName       = lookupName
      qReify            = reify
      qReifyFixity nm   = lookupThName nm >>= reifyFixity
      qReifyInstances   = reifyInstances
      qReifyRoles       = reifyRoles
      qReifyAnnotations = reifyAnnotations
      qReifyModule      = reifyModule
      qReifyConStrictness nm = do { nm' <- lookupThName nm
                                  ; dc  <- tcLookupDataCon nm'
                                  ; let bangs = dataConImplBangs dc
                                  ; return (map reifyDecidedStrictness bangs) }

            -- For qRecover, discard error messages if
            -- the recovery action is chosen.  Otherwise
            -- we'll only fail higher up.
      qRecover recover main = tryTcDiscardingErrs recover main

      qAddDependentFile fp = do
        ref <- fmap tcg_dependent_files getGblEnv
        dep_files <- readTcRef ref
        writeTcRef ref (fp:dep_files)

      qAddTempFile suffix = do
        dflags <- getDynFlags
        liftIO $ newTempName dflags TFL_GhcSession suffix

      qAddTopDecls thds = do
          l <- getSrcSpanM
          let either_hval = convertToHsDecls l thds
          ds <- case either_hval of
                  Left exn -> pprPanic "qAddTopDecls: can't convert top-level declarations" exn
                  Right ds -> return ds
          mapM_ (checkTopDecl . unLoc) ds
          th_topdecls_var <- fmap tcg_th_topdecls getGblEnv
          updTcRef th_topdecls_var (\topds -> ds ++ topds)
        where
          checkTopDecl :: HsDecl GhcPs -> TcM ()
          checkTopDecl (ValD _ binds)
            = mapM_ bindName (collectHsBindBinders binds)
          checkTopDecl (SigD _ _)
            = return ()
          checkTopDecl (AnnD _ _)
            = return ()
          checkTopDecl (ForD _ (ForeignImport { fd_name = L _ name }))
            = bindName name
          checkTopDecl _
            = addErr $ text "Only function, value, annotation, and foreign import declarations may be added with addTopDecl"

          bindName :: RdrName -> TcM ()
          bindName (Exact n)
            = do { th_topnames_var <- fmap tcg_th_topnames getGblEnv
                 ; updTcRef th_topnames_var (\ns -> extendNameSet ns n)
                 }

          bindName name =
              addErr $
              hang (text "The binder" <+> quotes (ppr name) <+> ptext (sLit "is not a NameU."))
                 2 (text "Probable cause: you used mkName instead of newName to generate a binding.")

      qAddForeignFilePath lang fp = do
        var <- fmap tcg_th_foreign_files getGblEnv
        updTcRef var ((lang, fp) :)

      qAddModFinalizer fin = do
          r <- liftIO $ mkRemoteRef fin
          fref <- liftIO $ mkForeignRef r (freeRemoteRef r)
          addModFinalizerRef fref

      qAddCorePlugin plugin = do
          hsc_env <- env_top <$> getEnv
          r <- liftIO $ findHomeModule hsc_env (mkModuleName plugin)
          let err = hang
                (text "addCorePlugin: invalid plugin module "
                   <+> text (show plugin)
                )
                2
                (text "Plugins in the current package can't be specified.")
          case r of
            Found {} -> addErr err
            FoundMultiple {} -> addErr err
            _ -> return ()
          th_coreplugins_var <- tcg_th_coreplugins <$> getGblEnv
          updTcRef th_coreplugins_var (plugin:)

      qGetQ :: forall a. Typeable a => TcM (Maybe a)
      qGetQ = do
          th_state_var <- fmap tcg_th_state getGblEnv
          th_state <- readTcRef th_state_var
          -- See #10596 for why we use a scoped type variable here.
          return (Map.lookup (typeRep (Proxy :: Proxy a)) th_state >>= fromDynamic)

      qPutQ x = do
          th_state_var <- fmap tcg_th_state getGblEnv
          updTcRef th_state_var (\m -> Map.insert (typeOf x) (toDyn x) m)

      qIsExtEnabled = xoptM

      qExtsEnabled =
        EnumSet.toList . extensionFlags . hsc_dflags <$> getTopEnv

    -- | Adds a mod finalizer reference to the local environment.
    addModFinalizerRef :: ForeignRef (TH.Q ()) -> TcM ()
    addModFinalizerRef finRef = do
        th_stage <- getStage
        case th_stage of
          RunSplice th_modfinalizers_var -> updTcRef th_modfinalizers_var (finRef :)
          -- This case happens only if a splice is executed and the caller does
          -- not set the 'ThStage' to 'RunSplice' to collect finalizers.
          -- See Note [Delaying modFinalizers in untyped splices] in RnSplice.
          _ ->
            pprPanic "addModFinalizer was called when no finalizers were collected"
                     (ppr th_stage)

    -- | Releases the external interpreter state.
    finishTH :: TcM ()
    finishTH = do
      dflags <- getDynFlags
      when (gopt Opt_ExternalInterpreter dflags) $ do
        tcg <- getGblEnv
        writeTcRef (tcg_th_remote_state tcg) Nothing

    runTHExp :: ForeignHValue -> TcM TH.Exp
    runTHExp = runTH THExp

    runTHPat :: ForeignHValue -> TcM TH.Pat
    runTHPat = runTH THPat

    runTHType :: ForeignHValue -> TcM TH.Type
    runTHType = runTH THType

    runTHDec :: ForeignHValue -> TcM [TH.Dec]
    runTHDec = runTH THDec

    runTH :: Binary a => THResultType -> ForeignHValue -> TcM a
    runTH ty fhv = do
      hsc_env <- env_top <$> getEnv
      dflags <- getDynFlags
      if not (gopt Opt_ExternalInterpreter dflags)
        then do
           -- Run it in the local TcM
          hv <- liftIO $ wormhole dflags fhv
          r <- runQuasi (unsafeCoerce# hv :: TH.Q a)
          return r
        else
          -- Run it on the server.  For an overview of how TH works with
          -- Remote GHCi, see Note [Remote Template Haskell] in
          -- libraries/ghci/GHCi/TH.hs.
          withIServ hsc_env $ \i -> do
            rstate <- getTHState i
            loc <- TH.qLocation
            liftIO $
              withForeignRef rstate $ \state_hv ->
              withForeignRef fhv $ \q_hv ->
                writeIServ i (putMessage (RunTH state_hv q_hv ty (Just loc)))
            runRemoteTH i []
            bs <- readQResult i
            return $! runGet get (LB.fromStrict bs)

    -- | communicate with a remotely-running TH computation until it finishes.
    -- See Note [Remote Template Haskell] in libraries/ghci/GHCi/TH.hs.
    runRemoteTH
      :: IServ
      -> [Messages]   --  saved from nested calls to qRecover
      -> TcM ()
    runRemoteTH iserv recovers = do
      THMsg msg <- liftIO $ readIServ iserv getTHMessage
      case msg of
        RunTHDone -> return ()
        StartRecover -> do -- Note [TH recover with -fexternal-interpreter]
          v <- getErrsVar
          msgs <- readTcRef v
          writeTcRef v emptyMessages
          runRemoteTH iserv (msgs : recovers)
        EndRecover caught_error -> do
          v <- getErrsVar
          let (prev_msgs, rest) = case recovers of
                 [] -> panic "EndRecover"
                 a : b -> (a,b)
          if caught_error
            then writeTcRef v prev_msgs
            else updTcRef v (unionMessages prev_msgs)
          runRemoteTH iserv rest
        _other -> do
          r <- handleTHMessage msg
          liftIO $ writeIServ iserv (put r)
          runRemoteTH iserv recovers

    -- | Read a value of type QResult from the iserv
    readQResult :: Binary a => IServ -> TcM a
    readQResult i = do
      qr <- liftIO $ readIServ i get
      case qr of
        QDone a -> return a
        QException str -> liftIO $ throwIO (ErrorCall str)
        QFail str -> fail str

    {- Note [TH recover with -fexternal-interpreter]

    Recover is slightly tricky to implement.

    The meaning of "recover a b" is
     - Do a
       - If it finished successfully, then keep the messages it generated
       - If it failed, discard any messages it generated, and do b

    The messages are managed by GHC in the TcM monad, whereas the
    exception-handling is done in the ghc-iserv process, so we have to
    coordinate between the two.

    On the server:
      - emit a StartRecover message
      - run "a" inside a catch
        - if it finishes, emit EndRecover False
        - if it fails, emit EndRecover True, then run "b"

    Back in GHC, when we receive:

      StartRecover
        save the current messages and start with an empty set.
      EndRecover caught_error
        Restore the previous messages,
        and merge in the new messages if caught_error is false.
    -}

    -- | Retrieve (or create, if it hasn't been created already), the
    -- remote TH state.  The TH state is a remote reference to an IORef
    -- QState living on the server, and we have to pass this to each RunTH
    -- call we make.
    --
    -- The TH state is stored in tcg_th_remote_state in the TcGblEnv.
    --
    getTHState :: IServ -> TcM (ForeignRef (IORef QState))
    getTHState i = do
      tcg <- getGblEnv
      th_state <- readTcRef (tcg_th_remote_state tcg)
      case th_state of
        Just rhv -> return rhv
        Nothing -> do
          hsc_env <- env_top <$> getEnv
          fhv <- liftIO $ mkFinalizedHValue hsc_env =<< iservCall i StartTH
          writeTcRef (tcg_th_remote_state tcg) (Just fhv)
          return fhv

    wrapTHResult :: TcM a -> TcM (THResult a)
    wrapTHResult tcm = do
      e <- tryM tcm   -- only catch 'fail', treat everything else as catastrophic
      case e of
        Left e -> return (THException (show e))
        Right a -> return (THComplete a)

    handleTHMessage :: THMessage a -> TcM a
    handleTHMessage msg = case msg of
      NewName a -> wrapTHResult $ TH.qNewName a
      Report b str -> wrapTHResult $ TH.qReport b str
      LookupName b str -> wrapTHResult $ TH.qLookupName b str
      Reify n -> wrapTHResult $ TH.qReify n
      ReifyFixity n -> wrapTHResult $ TH.qReifyFixity n
      ReifyInstances n ts -> wrapTHResult $ TH.qReifyInstances n ts
      ReifyRoles n -> wrapTHResult $ TH.qReifyRoles n
      ReifyAnnotations lookup tyrep ->
        wrapTHResult $ (map B.pack <$> getAnnotationsByTypeRep lookup tyrep)
      ReifyModule m -> wrapTHResult $ TH.qReifyModule m
      ReifyConStrictness nm -> wrapTHResult $ TH.qReifyConStrictness nm
      AddDependentFile f -> wrapTHResult $ TH.qAddDependentFile f
      AddTempFile s -> wrapTHResult $ TH.qAddTempFile s
      AddModFinalizer r -> do
        hsc_env <- env_top <$> getEnv
        wrapTHResult $ liftIO (mkFinalizedHValue hsc_env r) >>= addModFinalizerRef
      AddCorePlugin str -> wrapTHResult $ TH.qAddCorePlugin str
      AddTopDecls decs -> wrapTHResult $ TH.qAddTopDecls decs
      AddForeignFilePath lang str -> wrapTHResult $ TH.qAddForeignFilePath lang str
      IsExtEnabled ext -> wrapTHResult $ TH.qIsExtEnabled ext
      ExtsEnabled -> wrapTHResult $ TH.qExtsEnabled
      _ -> panic ("handleTHMessage: unexpected message " ++ show msg)

    getAnnotationsByTypeRep :: TH.AnnLookup -> TypeRep -> TcM [[Word8]]
    getAnnotationsByTypeRep th_name tyrep
      = do { name <- lookupThAnnLookup th_name
           ; topEnv <- getTopEnv
           ; epsHptAnns <- liftIO $ prepareAnnotations topEnv Nothing
           ; tcg <- getGblEnv
           ; let selectedEpsHptAnns = findAnnsByTypeRep epsHptAnns name tyrep
           ; let selectedTcgAnns = findAnnsByTypeRep (tcg_ann_env tcg) name tyrep
           ; return (selectedEpsHptAnns ++ selectedTcgAnns) }

    {-
    ************************************************************************
    *                                                                      *
                Instance Testing
    *                                                                      *
    ************************************************************************
    -}

    reifyInstances :: TH.Name -> [TH.Type] -> TcM [TH.Dec]
    reifyInstances th_nm th_tys
       = addErrCtxt (text "In the argument of reifyInstances:"
                     <+> ppr_th th_nm <+> sep (map ppr_th th_tys)) $
         do { loc <- getSrcSpanM
            ; rdr_ty <- cvt loc (mkThAppTs (TH.ConT th_nm) th_tys)
              -- #9262 says to bring vars into scope, like in HsForAllTy case
              -- of rnHsTyKi
            ; free_vars <- extractHsTyRdrTyVars rdr_ty
            ; let tv_rdrs = freeKiTyVarsAllVars free_vars
              -- Rename  to HsType Name
            ; ((tv_names, rn_ty), _fvs)
                <- checkNoErrs $ -- If there are out-of-scope Names here, then we
                                 -- must error before proceeding to typecheck the
                                 -- renamed type, as that will result in GHC
                                 -- internal errors (#13837).
                   bindLRdrNames tv_rdrs $ \ tv_names ->
                   do { (rn_ty, fvs) <- rnLHsType doc rdr_ty
                      ; return ((tv_names, rn_ty), fvs) }
            ; (_tvs, ty)
                <- solveEqualities $
                   tcImplicitTKBndrs ReifySkol tv_names $
                   fst <$> tcLHsType rn_ty
            ; ty <- zonkTcTypeToType emptyZonkEnv ty
                    -- Substitute out the meta type variables
                    -- In particular, the type might have kind
                    -- variables inside it (Trac #7477)

            ; traceTc "reifyInstances" (ppr ty $$ ppr (typeKind ty))
            ; case splitTyConApp_maybe ty of   -- This expands any type synonyms
                Just (tc, tys)                 -- See Trac #7910
                   | Just cls <- tyConClass_maybe tc
                   -> do { inst_envs <- tcGetInstEnvs
                         ; let (matches, unifies, _) = lookupInstEnv False inst_envs cls tys
                         ; traceTc "reifyInstances1" (ppr matches)
                         ; reifyClassInstances cls (map fst matches ++ unifies) }
                   | isOpenFamilyTyCon tc
                   -> do { inst_envs <- tcGetFamInstEnvs
                         ; let matches = lookupFamInstEnv inst_envs tc tys
                         ; traceTc "reifyInstances2" (ppr matches)
                         ; reifyFamilyInstances tc (map fim_instance matches) }
                _  -> bale_out (hang (text "reifyInstances:" <+> quotes (ppr ty))
                                   2 (text "is not a class constraint or type family application")) }
      where
        doc = ClassInstanceCtx
        bale_out msg = failWithTc msg

        cvt :: SrcSpan -> TH.Type -> TcM (LHsType GhcPs)
        cvt loc th_ty = case convertToHsType loc th_ty of
                          Left msg -> failWithTc msg
                          Right ty -> return ty

    {-
    ************************************************************************
    *                                                                      *
                            Reification
    *                                                                      *
    ************************************************************************
    -}

    lookupName :: Bool      -- True  <=> type namespace
                            -- False <=> value namespace
               -> String -> TcM (Maybe TH.Name)
    lookupName is_type_name s
      = do { lcl_env <- getLocalRdrEnv
           ; case lookupLocalRdrEnv lcl_env rdr_name of
               Just n  -> return (Just (reifyName n))
               Nothing -> do { mb_nm <- lookupGlobalOccRn_maybe rdr_name
                             ; return (fmap reifyName mb_nm) } }
      where
        th_name = TH.mkName s       -- Parses M.x into a base of 'x' and a module of 'M'

        occ_fs :: FastString
        occ_fs = mkFastString (TH.nameBase th_name)

        occ :: OccName
        occ | is_type_name
            = if isLexVarSym occ_fs || isLexCon occ_fs
                                 then mkTcOccFS    occ_fs
                                 else mkTyVarOccFS occ_fs
            | otherwise
            = if isLexCon occ_fs then mkDataOccFS occ_fs
                                 else mkVarOccFS  occ_fs

        rdr_name = case TH.nameModule th_name of
                     Nothing  -> mkRdrUnqual occ
                     Just mod -> mkRdrQual (mkModuleName mod) occ

    getThing :: TH.Name -> TcM TcTyThing
    getThing th_name
      = do  { name <- lookupThName th_name
            ; traceIf (text "reify" <+> text (show th_name) <+> brackets (ppr_ns th_name) <+> ppr name)
            ; tcLookupTh name }
            -- ToDo: this tcLookup could fail, which would give a
            --       rather unhelpful error message
      where
        ppr_ns (TH.Name _ (TH.NameG TH.DataName  _pkg _mod)) = text "data"
        ppr_ns (TH.Name _ (TH.NameG TH.TcClsName _pkg _mod)) = text "tc"
        ppr_ns (TH.Name _ (TH.NameG TH.VarName   _pkg _mod)) = text "var"
        ppr_ns _ = panic "reify/ppr_ns"

    reify :: TH.Name -> TcM TH.Info
    reify th_name
      = do  { traceTc "reify 1" (text (TH.showName th_name))
            ; thing <- getThing th_name
            ; traceTc "reify 2" (ppr thing)
            ; reifyThing thing }

    lookupThName :: TH.Name -> TcM Name
    lookupThName th_name = do
        mb_name <- lookupThName_maybe th_name
        case mb_name of
            Nothing   -> failWithTc (notInScope th_name)
            Just name -> return name

    lookupThName_maybe :: TH.Name -> TcM (Maybe Name)
    lookupThName_maybe th_name
      =  do { names <- mapMaybeM lookup (thRdrNameGuesses th_name)
              -- Pick the first that works
              -- E.g. reify (mkName "A") will pick the class A in preference to the data constructor A
            ; return (listToMaybe names) }
      where
        lookup rdr_name
            = do {  -- Repeat much of lookupOccRn, because we want
                    -- to report errors in a TH-relevant way
                 ; rdr_env <- getLocalRdrEnv
                 ; case lookupLocalRdrEnv rdr_env rdr_name of
                     Just name -> return (Just name)
                     Nothing   -> lookupGlobalOccRn_maybe rdr_name }

    tcLookupTh :: Name -> TcM TcTyThing
    -- This is a specialised version of TcEnv.tcLookup; specialised mainly in that
    -- it gives a reify-related error message on failure, whereas in the normal
    -- tcLookup, failure is a bug.
    tcLookupTh name
      = do  { (gbl_env, lcl_env) <- getEnvs
            ; case lookupNameEnv (tcl_env lcl_env) name of {
                    Just thing -> return thing;
                    Nothing    ->

              case lookupNameEnv (tcg_type_env gbl_env) name of {
                    Just thing -> return (AGlobal thing);
                    Nothing    ->

              -- EZY: I don't think this choice matters, no TH in signatures!
              if nameIsLocalOrFrom (tcg_semantic_mod gbl_env) name
              then  -- It's defined in this module
                    failWithTc (notInEnv name)

              else
         do { mb_thing <- tcLookupImported_maybe name
            ; case mb_thing of
                Succeeded thing -> return (AGlobal thing)
                Failed msg      -> failWithTc msg
        }}}}

    notInScope :: TH.Name -> SDoc
    notInScope th_name = quotes (text (TH.pprint th_name)) <+>
                         text "is not in scope at a reify"
            -- Ugh! Rather an indirect way to display the name

    notInEnv :: Name -> SDoc
    notInEnv name = quotes (ppr name) <+>
                         text "is not in the type environment at a reify"

    ------------------------------
    reifyRoles :: TH.Name -> TcM [TH.Role]
    reifyRoles th_name
      = do { thing <- getThing th_name
           ; case thing of
               AGlobal (ATyCon tc) -> return (map reify_role (tyConRoles tc))
               _ -> failWithTc (text "No roles associated with" <+> (ppr thing))
           }
      where
        reify_role Nominal          = TH.NominalR
        reify_role Representational = TH.RepresentationalR
        reify_role Phantom          = TH.PhantomR

    ------------------------------
    reifyThing :: TcTyThing -> TcM TH.Info
    -- The only reason this is monadic is for error reporting,
    -- which in turn is mainly for the case when TH can't express
    -- some random GHC extension

    reifyThing (AGlobal (AnId id))
      = do  { ty <- reifyType (idType id)
            ; let v = reifyName id
            ; case idDetails id of
                ClassOpId cls -> return (TH.ClassOpI v ty (reifyName cls))
                RecSelId{sel_tycon=RecSelData tc}
                              -> return (TH.VarI (reifySelector id tc) ty Nothing)
                _             -> return (TH.VarI     v ty Nothing)
        }

    reifyThing (AGlobal (ATyCon tc))   = reifyTyCon tc
    reifyThing (AGlobal (AConLike (RealDataCon dc)))
      = do  { let name = dataConName dc
            ; ty <- reifyType (idType (dataConWrapId dc))
            ; return (TH.DataConI (reifyName name) ty
                                  (reifyName (dataConOrigTyCon dc)))
            }

    reifyThing (AGlobal (AConLike (PatSynCon ps)))
      = do { let name = reifyName ps
           ; ty <- reifyPatSynType (patSynSig ps)
           ; return (TH.PatSynI name ty) }

    reifyThing (ATcId {tct_id = id})
      = do  { ty1 <- zonkTcType (idType id) -- Make use of all the info we have, even
                                            -- though it may be incomplete
            ; ty2 <- reifyType ty1
            ; return (TH.VarI (reifyName id) ty2 Nothing) }

    reifyThing (ATyVar tv tv1)
      = do { ty1 <- zonkTcTyVar tv1
           ; ty2 <- reifyType ty1
           ; return (TH.TyVarI (reifyName tv) ty2) }

    reifyThing thing = pprPanic "reifyThing" (pprTcTyThingCategory thing)

    -------------------------------------------
    reifyAxBranch :: TyCon -> CoAxBranch -> TcM TH.TySynEqn
    reifyAxBranch fam_tc (CoAxBranch { cab_lhs = lhs, cab_rhs = rhs })
                -- remove kind patterns (#8884)
      = do { let lhs_types_only = filterOutInvisibleTypes fam_tc lhs
           ; lhs' <- reifyTypes lhs_types_only
           ; annot_th_lhs <- zipWith3M annotThType (mkIsPolyTvs fam_tvs)
                                       lhs_types_only lhs'
           ; rhs'  <- reifyType rhs
           ; return (TH.TySynEqn annot_th_lhs rhs') }
      where
        fam_tvs = tyConVisibleTyVars fam_tc

    reifyTyCon :: TyCon -> TcM TH.Info
    reifyTyCon tc
      | Just cls <- tyConClass_maybe tc
      = reifyClass cls

      | isFunTyCon tc
      = return (TH.PrimTyConI (reifyName tc) 2                False)

      | isPrimTyCon tc
      = return (TH.PrimTyConI (reifyName tc) (tyConArity tc) (isUnliftedTyCon tc))

      | isTypeFamilyTyCon tc
      = do { let tvs      = tyConTyVars tc
                 res_kind = tyConResKind tc
                 resVar   = famTcResVar tc

           ; kind' <- reifyKind res_kind
           ; let (resultSig, injectivity) =
                     case resVar of
                       Nothing   -> (TH.KindSig kind', Nothing)
                       Just name ->
                         let thName   = reifyName name
                             injAnnot = tyConInjectivityInfo tc
                             sig = TH.TyVarSig (TH.KindedTV thName kind')
                             inj = case injAnnot of
                                     NotInjective -> Nothing
                                     Injective ms ->
                                         Just (TH.InjectivityAnn thName injRHS)
                                       where
                                         injRHS = map (reifyName . tyVarName)
                                                      (filterByList ms tvs)
                         in (sig, inj)
           ; tvs' <- reifyTyVars (tyConVisibleTyVars tc)
           ; let tfHead =
                   TH.TypeFamilyHead (reifyName tc) tvs' resultSig injectivity
           ; if isOpenTypeFamilyTyCon tc
             then do { fam_envs <- tcGetFamInstEnvs
                     ; instances <- reifyFamilyInstances tc
                                      (familyInstances fam_envs tc)
                     ; return (TH.FamilyI (TH.OpenTypeFamilyD tfHead) instances) }
             else do { eqns <-
                         case isClosedSynFamilyTyConWithAxiom_maybe tc of
                           Just ax -> mapM (reifyAxBranch tc) $
                                      fromBranches $ coAxiomBranches ax
                           Nothing -> return []
                     ; return (TH.FamilyI (TH.ClosedTypeFamilyD tfHead eqns)
                          []) } }

      | isDataFamilyTyCon tc
      = do { let res_kind = tyConResKind tc

           ; kind' <- fmap Just (reifyKind res_kind)

           ; tvs' <- reifyTyVars (tyConVisibleTyVars tc)
           ; fam_envs <- tcGetFamInstEnvs
           ; instances <- reifyFamilyInstances tc (familyInstances fam_envs tc)
           ; return (TH.FamilyI
                           (TH.DataFamilyD (reifyName tc) tvs' kind') instances) }

      | Just (_, rhs) <- synTyConDefn_maybe tc  -- Vanilla type synonym
      = do { rhs' <- reifyType rhs
           ; tvs' <- reifyTyVars (tyConVisibleTyVars tc)
           ; return (TH.TyConI
                       (TH.TySynD (reifyName tc) tvs' rhs'))
           }

      | otherwise
      = do  { cxt <- reifyCxt (tyConStupidTheta tc)
            ; let tvs      = tyConTyVars tc
                  dataCons = tyConDataCons tc
                  isGadt   = isGadtSyntaxTyCon tc
            ; cons <- mapM (reifyDataCon isGadt (mkTyVarTys tvs)) dataCons
            ; r_tvs <- reifyTyVars (tyConVisibleTyVars tc)
            ; let name = reifyName tc
                  deriv = []        -- Don't know about deriving
                  decl | isNewTyCon tc =
                           TH.NewtypeD cxt name r_tvs Nothing (head cons) deriv
                       | otherwise     =
                           TH.DataD    cxt name r_tvs Nothing       cons  deriv
            ; return (TH.TyConI decl) }

    reifyDataCon :: Bool -> [Type] -> DataCon -> TcM TH.Con
    reifyDataCon isGadtDataCon tys dc
      = do { let -- used for H98 data constructors
                 (ex_tvs, theta, arg_tys)
                     = dataConInstSig dc tys
                 -- used for GADTs data constructors
                 g_user_tvs' = dataConUserTyVars dc
                 (g_univ_tvs, _, g_eq_spec, g_theta', g_arg_tys', g_res_ty')
                     = dataConFullSig dc
                 (srcUnpks, srcStricts)
                     = mapAndUnzip reifySourceBang (dataConSrcBangs dc)
                 dcdBangs  = zipWith TH.Bang srcUnpks srcStricts
                 fields    = dataConFieldLabels dc
                 name      = reifyName dc
                 -- Universal tvs present in eq_spec need to be filtered out, as
                 -- they will not appear anywhere in the type.
                 eq_spec_tvs = mkVarSet (map eqSpecTyVar g_eq_spec)

           ; (univ_subst, _)
                  -- See Note [Freshen reified GADT constructors' universal tyvars]
               <- freshenTyVarBndrs $
                  filterOut (`elemVarSet` eq_spec_tvs) g_univ_tvs
           ; let (tvb_subst, g_user_tvs) = substTyVarBndrs univ_subst g_user_tvs'
                 g_theta   = substTys tvb_subst g_theta'
                 g_arg_tys = substTys tvb_subst g_arg_tys'
                 g_res_ty  = substTy  tvb_subst g_res_ty'

           ; r_arg_tys <- reifyTypes (if isGadtDataCon then g_arg_tys else arg_tys)

           ; main_con <-
               if | not (null fields) && not isGadtDataCon ->
                      return $ TH.RecC name (zip3 (map reifyFieldLabel fields)
                                             dcdBangs r_arg_tys)
                  | not (null fields) -> do
                      { res_ty <- reifyType g_res_ty
                      ; return $ TH.RecGadtC [name]
                                         (zip3 (map (reifyName . flSelector) fields)
                                          dcdBangs r_arg_tys) res_ty }
                    -- We need to check not isGadtDataCon here because GADT
                    -- constructors can be declared infix.
                    -- See Note [Infix GADT constructors] in TcTyClsDecls.
                  | dataConIsInfix dc && not isGadtDataCon ->
                      ASSERT( arg_tys `lengthIs` 2 ) do
                      { let [r_a1, r_a2] = r_arg_tys
                            [s1,   s2]   = dcdBangs
                      ; return $ TH.InfixC (s1,r_a1) name (s2,r_a2) }
                  | isGadtDataCon -> do
                      { res_ty <- reifyType g_res_ty
                      ; return $ TH.GadtC [name] (dcdBangs `zip` r_arg_tys) res_ty }
                  | otherwise ->
                      return $ TH.NormalC name (dcdBangs `zip` r_arg_tys)

           ; let (ex_tvs', theta') | isGadtDataCon = (g_user_tvs, g_theta)
                                   | otherwise     = (ex_tvs, theta)
                 ret_con | null ex_tvs' && null theta' = return main_con
                         | otherwise                   = do
                             { cxt <- reifyCxt theta'
                             ; ex_tvs'' <- reifyTyVars ex_tvs'
                             ; return (TH.ForallC ex_tvs'' cxt main_con) }
           ; ASSERT( arg_tys `equalLength` dcdBangs )
             ret_con }

    {-
    Note [Freshen reified GADT constructors' universal tyvars]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Suppose one were to reify this GADT:

      data a :~: b where
        Refl :: forall a b. (a ~ b) => a :~: b

    We ought to be careful here about the uniques we give to the occurrences of `a`
    and `b` in this definition. That is because in the original DataCon, all uses
    of `a` and `b` have the same unique, since `a` and `b` are both universally
    quantified type variables--that is, they are used in both the (:~:) tycon as
    well as in the constructor type signature. But when we turn the DataCon
    definition into the reified one, the `a` and `b` in the constructor type
    signature becomes differently scoped than the `a` and `b` in `data a :~: b`.

    While it wouldn't technically be *wrong* per se to re-use the same uniques for
    `a` and `b` across these two different scopes, it's somewhat annoying for end
    users of Template Haskell, since they wouldn't be able to rely on the
    assumption that all TH names have globally distinct uniques (#13885). For this
    reason, we freshen the universally quantified tyvars that go into the reified
    GADT constructor type signature to give them distinct uniques from their
    counterparts in the tycon.
    -}

    ------------------------------
    reifyClass :: Class -> TcM TH.Info
    reifyClass cls
      = do  { cxt <- reifyCxt theta
            ; inst_envs <- tcGetInstEnvs
            ; insts <- reifyClassInstances cls (InstEnv.classInstances inst_envs cls)
            ; assocTys <- concatMapM reifyAT ats
            ; ops <- concatMapM reify_op op_stuff
            ; tvs' <- reifyTyVars (tyConVisibleTyVars (classTyCon cls))
            ; let dec = TH.ClassD cxt (reifyName cls) tvs' fds' (assocTys ++ ops)
            ; return (TH.ClassI dec insts) }
      where
        (_, fds, theta, _, ats, op_stuff) = classExtraBigSig cls
        fds' = map reifyFunDep fds
        reify_op (op, def_meth)
          = do { ty <- reifyType (idType op)
               ; let nm' = reifyName op
               ; case def_meth of
                    Just (_, GenericDM gdm_ty) ->
                      do { gdm_ty' <- reifyType gdm_ty
                         ; return [TH.SigD nm' ty, TH.DefaultSigD nm' gdm_ty'] }
                    _ -> return [TH.SigD nm' ty] }

        reifyAT :: ClassATItem -> TcM [TH.Dec]
        reifyAT (ATI tycon def) = do
          tycon' <- reifyTyCon tycon
          case tycon' of
            TH.FamilyI dec _ -> do
              let (tyName, tyArgs) = tfNames dec
              (dec :) <$> maybe (return [])
                                (fmap (:[]) . reifyDefImpl tyName tyArgs . fst)
                                def
            _ -> pprPanic "reifyAT" (text (show tycon'))

        reifyDefImpl :: TH.Name -> [TH.Name] -> Type -> TcM TH.Dec
        reifyDefImpl n args ty =
          TH.TySynInstD n . TH.TySynEqn (map TH.VarT args) <$> reifyType ty

        tfNames :: TH.Dec -> (TH.Name, [TH.Name])
        tfNames (TH.OpenTypeFamilyD (TH.TypeFamilyHead n args _ _))
          = (n, map bndrName args)
        tfNames d = pprPanic "tfNames" (text (show d))

        bndrName :: TH.TyVarBndr -> TH.Name
        bndrName (TH.PlainTV n)    = n
        bndrName (TH.KindedTV n _) = n

    ------------------------------
    -- | Annotate (with TH.SigT) a type if the first parameter is True
    -- and if the type contains a free variable.
    -- This is used to annotate type patterns for poly-kinded tyvars in
    -- reifying class and type instances. See #8953 and th/T8953.
    annotThType :: Bool   -- True <=> annotate
                -> TyCoRep.Type -> TH.Type -> TcM TH.Type
      -- tiny optimization: if the type is annotated, don't annotate again.
    annotThType _    _  th_ty@(TH.SigT {}) = return th_ty
    annotThType True ty th_ty
      | not $ isEmptyVarSet $ filterVarSet isTyVar $ tyCoVarsOfType ty
      = do { let ki = typeKind ty
           ; th_ki <- reifyKind ki
           ; return (TH.SigT th_ty th_ki) }
    annotThType _    _ th_ty = return th_ty

    -- | For every type variable in the input,
    -- report whether or not the tv is poly-kinded. This is used to eventually
    -- feed into 'annotThType'.
    mkIsPolyTvs :: [TyVar] -> [Bool]
    mkIsPolyTvs = map is_poly_tv
      where
        is_poly_tv tv = not $
                        isEmptyVarSet $
                        filterVarSet isTyVar $
                        tyCoVarsOfType $
                        tyVarKind tv

    ------------------------------
    reifyClassInstances :: Class -> [ClsInst] -> TcM [TH.Dec]
    reifyClassInstances cls insts
      = mapM (reifyClassInstance (mkIsPolyTvs tvs)) insts
      where
        tvs = tyConVisibleTyVars (classTyCon cls)

    reifyClassInstance :: [Bool]  -- True <=> the corresponding tv is poly-kinded
                                  -- includes only *visible* tvs
                       -> ClsInst -> TcM TH.Dec
    reifyClassInstance is_poly_tvs i
      = do { cxt <- reifyCxt theta
           ; let vis_types = filterOutInvisibleTypes cls_tc types
           ; thtypes <- reifyTypes vis_types
           ; annot_thtypes <- zipWith3M annotThType is_poly_tvs vis_types thtypes
           ; let head_ty = mkThAppTs (TH.ConT (reifyName cls)) annot_thtypes
           ; return $ (TH.InstanceD over cxt head_ty []) }
      where
         (_tvs, theta, cls, types) = tcSplitDFunTy (idType dfun)
         cls_tc   = classTyCon cls
         dfun     = instanceDFunId i
         over     = case overlapMode (is_flag i) of
                      NoOverlap _     -> Nothing
                      Overlappable _  -> Just TH.Overlappable
                      Overlapping _   -> Just TH.Overlapping
                      Overlaps _      -> Just TH.Overlaps
                      Incoherent _    -> Just TH.Incoherent

    ------------------------------
    reifyFamilyInstances :: TyCon -> [FamInst] -> TcM [TH.Dec]
    reifyFamilyInstances fam_tc fam_insts
      = mapM (reifyFamilyInstance (mkIsPolyTvs fam_tvs)) fam_insts
      where
        fam_tvs = tyConVisibleTyVars fam_tc

    reifyFamilyInstance :: [Bool] -- True <=> the corresponding tv is poly-kinded
                                  -- includes only *visible* tvs
                        -> FamInst -> TcM TH.Dec
    reifyFamilyInstance is_poly_tvs inst@(FamInst { fi_flavor = flavor
                                                  , fi_fam = fam
                                                  , fi_tvs = fam_tvs
                                                  , fi_tys = lhs
                                                  , fi_rhs = rhs })
      = case flavor of
          SynFamilyInst ->
                   -- remove kind patterns (#8884)
            do { let lhs_types_only = filterOutInvisibleTypes fam_tc lhs
               ; th_lhs <- reifyTypes lhs_types_only
               ; annot_th_lhs <- zipWith3M annotThType is_poly_tvs lhs_types_only
                                                       th_lhs
               ; th_rhs <- reifyType rhs
               ; return (TH.TySynInstD (reifyName fam)
                                       (TH.TySynEqn annot_th_lhs th_rhs)) }

          DataFamilyInst rep_tc ->
            do { let rep_tvs = tyConTyVars rep_tc
                     fam' = reifyName fam

                       -- eta-expand lhs types, because sometimes data/newtype
                       -- instances are eta-reduced; See Trac #9692
                       -- See Note [Eta reduction for data family axioms]
                       -- in TcInstDcls
                     (_rep_tc, rep_tc_args) = splitTyConApp rhs
                     etad_tyvars            = dropList rep_tc_args rep_tvs
                     etad_tys               = mkTyVarTys etad_tyvars
                     eta_expanded_tvs = mkTyVarTys fam_tvs `chkAppend` etad_tys
                     eta_expanded_lhs = lhs `chkAppend` etad_tys
                     dataCons         = tyConDataCons rep_tc
                     isGadt           = isGadtSyntaxTyCon rep_tc
               ; cons <- mapM (reifyDataCon isGadt eta_expanded_tvs) dataCons
               ; let types_only = filterOutInvisibleTypes fam_tc eta_expanded_lhs
               ; th_tys <- reifyTypes types_only
               ; annot_th_tys <- zipWith3M annotThType is_poly_tvs types_only th_tys
               ; return $
                   if isNewTyCon rep_tc
                   then TH.NewtypeInstD [] fam' annot_th_tys Nothing (head cons) []
                   else TH.DataInstD    [] fam' annot_th_tys Nothing       cons  []
               }
      where
        fam_tc = famInstTyCon inst

    ------------------------------
    reifyType :: TyCoRep.Type -> TcM TH.Type
    -- Monadic only because of failure
    reifyType ty                | tcIsLiftedTypeKind ty = return TH.StarT
      -- Make sure to use tcIsLiftedTypeKind here, since we don't want to confuse it
      -- with Constraint (#14869).
    reifyType ty@(ForAllTy {})  = reify_for_all ty
    reifyType (LitTy t)         = do { r <- reifyTyLit t; return (TH.LitT r) }
    reifyType (TyVarTy tv)      = return (TH.VarT (reifyName tv))
    reifyType (TyConApp tc tys) = reify_tc_app tc tys   -- Do not expand type synonyms here
    reifyType (AppTy t1 t2)     = do { [r1,r2] <- reifyTypes [t1,t2] ; return (r1 `TH.AppT` r2) }
    reifyType ty@(FunTy t1 t2)
      | isPredTy t1 = reify_for_all ty  -- Types like ((?x::Int) => Char -> Char)
      | otherwise   = do { [r1,r2] <- reifyTypes [t1,t2] ; return (TH.ArrowT `TH.AppT` r1 `TH.AppT` r2) }
    reifyType (CastTy t _)      = reifyType t -- Casts are ignored in TH
    reifyType ty@(CoercionTy {})= noTH (sLit "coercions in types") (ppr ty)

    reify_for_all :: TyCoRep.Type -> TcM TH.Type
    reify_for_all ty
      = do { cxt' <- reifyCxt cxt;
           ; tau' <- reifyType tau
           ; tvs' <- reifyTyVars tvs
           ; return (TH.ForallT tvs' cxt' tau') }
      where
        (tvs, cxt, tau) = tcSplitSigmaTy ty

    reifyTyLit :: TyCoRep.TyLit -> TcM TH.TyLit
    reifyTyLit (NumTyLit n) = return (TH.NumTyLit n)
    reifyTyLit (StrTyLit s) = return (TH.StrTyLit (unpackFS s))

    reifyTypes :: [Type] -> TcM [TH.Type]
    reifyTypes = mapM reifyType

    reifyPatSynType
      :: ([TyVar], ThetaType, [TyVar], ThetaType, [Type], Type) -> TcM TH.Type
    -- reifies a pattern synonym's type and returns its *complete* type
    -- signature; see NOTE [Pattern synonym signatures and Template
    -- Haskell]
    reifyPatSynType (univTyVars, req, exTyVars, prov, argTys, resTy)
      = do { univTyVars' <- reifyTyVars univTyVars
           ; req'        <- reifyCxt req
           ; exTyVars'   <- reifyTyVars exTyVars
           ; prov'       <- reifyCxt prov
           ; tau'        <- reifyType (mkFunTys argTys resTy)
           ; return $ TH.ForallT univTyVars' req'
                    $ TH.ForallT exTyVars' prov' tau' }

    reifyKind :: Kind -> TcM TH.Kind
    reifyKind = reifyType

    reifyCxt :: [PredType] -> TcM [TH.Pred]
    reifyCxt   = mapM reifyPred

    reifyFunDep :: ([TyVar], [TyVar]) -> TH.FunDep
    reifyFunDep (xs, ys) = TH.FunDep (map reifyName xs) (map reifyName ys)

    reifyTyVars :: [TyVar] -> TcM [TH.TyVarBndr]
    reifyTyVars tvs = mapM reify_tv tvs
      where
        -- even if the kind is *, we need to include a kind annotation,
        -- in case a poly-kind would be inferred without the annotation.
        -- See #8953 or test th/T8953
        reify_tv tv = TH.KindedTV name <$> reifyKind kind
          where
            kind = tyVarKind tv
            name = reifyName tv

    {-
    Note [Kind annotations on TyConApps]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    A poly-kinded tycon sometimes needs a kind annotation to be unambiguous.
    For example:

       type family F a :: k
       type instance F Int  = (Proxy :: * -> *)
       type instance F Bool = (Proxy :: (* -> *) -> *)

    It's hard to figure out where these annotations should appear, so we do this:
    Suppose we have a tycon application (T ty1 ... tyn). Assuming that T is not
    oversatured (more on this later), we can assume T's declaration is of the form
    T (tvb1 :: s1) ... (tvbn :: sn) :: p. If any kind variable that
    is free in p is not free in an injective position in tvb1 ... tvbn,
    then we put on a kind annotation, since we would not otherwise be able to infer
    the kind of the whole tycon application.

    The injective positions in a tyvar binder are the injective positions in the
    kind of its tyvar, provided the tyvar binder is either:

    * Anonymous. For example, in the promoted data constructor '(:):

        '(:) :: forall a. a -> [a] -> [a]

      The second and third tyvar binders (of kinds `a` and `[a]`) are both
      anonymous, so if we had '(:) 'True '[], then the inferred kinds of 'True and
      '[] would contribute to the inferred kind of '(:) 'True '[].
    * Has required visibility. For example, in the type family:

        type family Wurble k (a :: k) :: k
        Wurble :: forall k -> k -> k

      The first tyvar binder (of kind `forall k`) has required visibility, so if
      we had Wurble (Maybe a) Nothing, then the inferred kind of Maybe a would
      contribute to the inferred kind of Wurble (Maybe a) Nothing.

    An injective position in a type is one that does not occur as an argument to
    a non-injective type constructor (e.g., non-injective type families). See
    injectiveVarsOfType.

    How can be sure that this is correct? That is, how can we be sure that in the
    event that we leave off a kind annotation, that one could infer the kind of the
    tycon application from its arguments? It's essentially a proof by induction: if
    we can infer the kinds of every subtree of a type, then the whole tycon
    application will have an inferrable kind--unless, of course, the remainder of
    the tycon application's kind has uninstantiated kind variables.

    An earlier implementation of this algorithm only checked if p contained any
    free variables. But this was unsatisfactory, since a datatype like this:

      data Foo = Foo (Proxy '[False, True])

    Would be reified like this:

      data Foo = Foo (Proxy ('(:) False ('(:) True ('[] :: [Bool])
                                         :: [Bool]) :: [Bool]))

    Which has a rather excessive amount of kind annotations. With the current
    algorithm, we instead reify Foo to this:

      data Foo = Foo (Proxy ('(:) False ('(:) True ('[] :: [Bool]))))

    Since in the case of '[], the kind p is [a], and there are no arguments in the
    kind of '[]. On the other hand, in the case of '(:) True '[], the kind p is
    (forall a. [a]), but a occurs free in the first and second arguments of the
    full kind of '(:), which is (forall a. a -> [a] -> [a]). (See Trac #14060.)

    What happens if T is oversaturated? That is, if T's kind has fewer than n
    arguments, in the case that the concrete application instantiates a result
    kind variable with an arrow kind? If we run out of arguments, we do not attach
    a kind annotation. This should be a rare case, indeed. Here is an example:

       data T1 :: k1 -> k2 -> *
       data T2 :: k1 -> k2 -> *

       type family G (a :: k) :: k
       type instance G T1 = T2

       type instance F Char = (G T1 Bool :: (* -> *) -> *)   -- F from above

    Here G's kind is (forall k. k -> k), and the desugared RHS of that last
    instance of F is (G (* -> (* -> *) -> *) (T1 * (* -> *)) Bool). According to
    the algorithm above, there are 3 arguments to G so we should peel off 3
    arguments in G's kind. But G's kind has only two arguments. This is the
    rare special case, and we choose not to annotate the application of G with
    a kind signature. After all, we needn't do this, since that instance would
    be reified as:

       type instance F Char = G (T1 :: * -> (* -> *) -> *) Bool

    So the kind of G isn't ambiguous anymore due to the explicit kind annotation
    on its argument. See #8953 and test th/T8953.
    -}

    reify_tc_app :: TyCon -> [Type.Type] -> TcM TH.Type
    reify_tc_app tc tys
      = do { tys' <- reifyTypes (filterOutInvisibleTypes tc tys)
           ; maybe_sig_t (mkThAppTs r_tc tys') }
      where
        arity       = tyConArity tc
        tc_binders  = tyConBinders tc
        tc_res_kind = tyConResKind tc

        r_tc | isUnboxedSumTyCon tc           = TH.UnboxedSumT (arity `div` 2)
             | isUnboxedTupleTyCon tc         = TH.UnboxedTupleT (arity `div` 2)
             | isPromotedTupleTyCon tc        = TH.PromotedTupleT (arity `div` 2)
                 -- See Note [Unboxed tuple RuntimeRep vars] in TyCon
             | isTupleTyCon tc                = if isPromotedDataCon tc
                                                then TH.PromotedTupleT arity
                                                else TH.TupleT arity
             | tc `hasKey` constraintKindTyConKey
                                              = TH.ConstraintT
             | tc `hasKey` funTyConKey        = TH.ArrowT
             | tc `hasKey` listTyConKey       = TH.ListT
             | tc `hasKey` nilDataConKey      = TH.PromotedNilT
             | tc `hasKey` consDataConKey     = TH.PromotedConsT
             | tc `hasKey` heqTyConKey        = TH.EqualityT
             | tc `hasKey` eqPrimTyConKey     = TH.EqualityT
             | tc `hasKey` eqReprPrimTyConKey = TH.ConT (reifyName coercibleTyCon)
             | isPromotedDataCon tc           = TH.PromotedT (reifyName tc)
             | otherwise                      = TH.ConT (reifyName tc)

        -- See Note [Kind annotations on TyConApps]
        maybe_sig_t th_type
          | needs_kind_sig
          = do { let full_kind = typeKind (mkTyConApp tc tys)
               ; th_full_kind <- reifyKind full_kind
               ; return (TH.SigT th_type th_full_kind) }
          | otherwise
          = return th_type

        needs_kind_sig
          | GT <- compareLength tys tc_binders
          = False
          | otherwise
          = let (dropped_binders, remaining_binders)
                  = splitAtList  tys tc_binders
                result_kind  = mkTyConKind remaining_binders tc_res_kind
                result_vars  = tyCoVarsOfType result_kind
                dropped_vars = fvVarSet $
                               mapUnionFV injectiveVarsOfBinder dropped_binders

            in not (subVarSet result_vars dropped_vars)

    reifyPred :: TyCoRep.PredType -> TcM TH.Pred
    reifyPred ty
      -- We could reify the invisible parameter as a class but it seems
      -- nicer to support them properly...
      | isIPPred ty = noTH (sLit "implicit parameters") (ppr ty)
      | otherwise   = reifyType ty

    ------------------------------
    reifyName :: NamedThing n => n -> TH.Name
    reifyName thing
      | isExternalName name = mk_varg pkg_str mod_str occ_str
      | otherwise           = TH.mkNameU occ_str (getKey (getUnique name))
            -- Many of the things we reify have local bindings, and
            -- NameL's aren't supposed to appear in binding positions, so
            -- we use NameU.  When/if we start to reify nested things, that
            -- have free variables, we may need to generate NameL's for them.
      where
        name    = getName thing
        mod     = ASSERT( isExternalName name ) nameModule name
        pkg_str = unitIdString (moduleUnitId mod)
        mod_str = moduleNameString (moduleName mod)
        occ_str = occNameString occ
        occ     = nameOccName name
        mk_varg | OccName.isDataOcc occ = TH.mkNameG_d
                | OccName.isVarOcc  occ = TH.mkNameG_v
                | OccName.isTcOcc   occ = TH.mkNameG_tc
                | otherwise             = pprPanic "reifyName" (ppr name)

    -- See Note [Reifying field labels]
    reifyFieldLabel :: FieldLabel -> TH.Name
    reifyFieldLabel fl
      | flIsOverloaded fl
                  = TH.Name (TH.mkOccName occ_str) (TH.NameQ (TH.mkModName mod_str))
      | otherwise = TH.mkNameG_v pkg_str mod_str occ_str
      where
        name    = flSelector fl
        mod     = ASSERT( isExternalName name ) nameModule name
        pkg_str = unitIdString (moduleUnitId mod)
        mod_str = moduleNameString (moduleName mod)
        occ_str = unpackFS (flLabel fl)

    reifySelector :: Id -> TyCon -> TH.Name
    reifySelector id tc
      = case find ((idName id ==) . flSelector) (tyConFieldLabels tc) of
          Just fl -> reifyFieldLabel fl
          Nothing -> pprPanic "reifySelector: missing field" (ppr id $$ ppr tc)

    ------------------------------
    reifyFixity :: Name -> TcM (Maybe TH.Fixity)
    reifyFixity name
      = do { (found, fix) <- lookupFixityRn_help name
           ; return (if found then Just (conv_fix fix) else Nothing) }
        where
          conv_fix (BasicTypes.Fixity _ i d) = TH.Fixity i (conv_dir d)
          conv_dir BasicTypes.InfixR = TH.InfixR
          conv_dir BasicTypes.InfixL = TH.InfixL
          conv_dir BasicTypes.InfixN = TH.InfixN

    reifyUnpackedness :: DataCon.SrcUnpackedness -> TH.SourceUnpackedness
    reifyUnpackedness NoSrcUnpack = TH.NoSourceUnpackedness
    reifyUnpackedness SrcNoUnpack = TH.SourceNoUnpack
    reifyUnpackedness SrcUnpack   = TH.SourceUnpack

    reifyStrictness :: DataCon.SrcStrictness -> TH.SourceStrictness
    reifyStrictness NoSrcStrict = TH.NoSourceStrictness
    reifyStrictness SrcStrict   = TH.SourceStrict
    reifyStrictness SrcLazy     = TH.SourceLazy

    reifySourceBang :: DataCon.HsSrcBang
                    -> (TH.SourceUnpackedness, TH.SourceStrictness)
    reifySourceBang (HsSrcBang _ u s) = (reifyUnpackedness u, reifyStrictness s)

    reifyDecidedStrictness :: DataCon.HsImplBang -> TH.DecidedStrictness
    reifyDecidedStrictness HsLazy     = TH.DecidedLazy
    reifyDecidedStrictness HsStrict   = TH.DecidedStrict
    reifyDecidedStrictness HsUnpack{} = TH.DecidedUnpack

    ------------------------------
    lookupThAnnLookup :: TH.AnnLookup -> TcM CoreAnnTarget
    lookupThAnnLookup (TH.AnnLookupName th_nm) = fmap NamedTarget (lookupThName th_nm)
    lookupThAnnLookup (TH.AnnLookupModule (TH.Module pn mn))
      = return $ ModuleTarget $
        mkModule (stringToUnitId $ TH.pkgString pn) (mkModuleName $ TH.modString mn)

    reifyAnnotations :: Data a => TH.AnnLookup -> TcM [a]
    reifyAnnotations th_name
      = do { name <- lookupThAnnLookup th_name
           ; topEnv <- getTopEnv
           ; epsHptAnns <- liftIO $ prepareAnnotations topEnv Nothing
           ; tcg <- getGblEnv
           ; let selectedEpsHptAnns = findAnns deserializeWithData epsHptAnns name
           ; let selectedTcgAnns = findAnns deserializeWithData (tcg_ann_env tcg) name
           ; return (selectedEpsHptAnns ++ selectedTcgAnns) }

    ------------------------------
    modToTHMod :: Module -> TH.Module
    modToTHMod m = TH.Module (TH.PkgName $ unitIdString  $ moduleUnitId m)
                             (TH.ModName $ moduleNameString $ moduleName m)

    reifyModule :: TH.Module -> TcM TH.ModuleInfo
    reifyModule (TH.Module (TH.PkgName pkgString) (TH.ModName mString)) = do
      this_mod <- getModule
      let reifMod = mkModule (stringToUnitId pkgString) (mkModuleName mString)
      if (reifMod == this_mod) then reifyThisModule else reifyFromIface reifMod
        where
          reifyThisModule = do
            usages <- fmap (map modToTHMod . moduleEnvKeys . imp_mods) getImports
            return $ TH.ModuleInfo usages

          reifyFromIface reifMod = do
            iface <- loadInterfaceForModule (text "reifying module from TH for" <+> ppr reifMod) reifMod
            let usages = [modToTHMod m | usage <- mi_usages iface,
                                         Just m <- [usageToModule (moduleUnitId reifMod) usage] ]
            return $ TH.ModuleInfo usages

          usageToModule :: UnitId -> Usage -> Maybe Module
          usageToModule _ (UsageFile {}) = Nothing
          usageToModule this_pkg (UsageHomeModule { usg_mod_name = mn }) = Just $ mkModule this_pkg mn
          usageToModule _ (UsagePackageModule { usg_mod = m }) = Just m
          usageToModule _ (UsageMergedRequirement { usg_mod = m }) = Just m

    ------------------------------
    mkThAppTs :: TH.Type -> [TH.Type] -> TH.Type
    mkThAppTs fun_ty arg_tys = foldl TH.AppT fun_ty arg_tys

    noTH :: LitString -> SDoc -> TcM a
    noTH s d = failWithTc (hsep [text "Can't represent" <+> ptext s <+>
                                    text "in Template Haskell:",
                                 nest 2 d])

    ppr_th :: TH.Ppr a => a -> SDoc
    ppr_th x = text (TH.pprint x)

    {-
    Note [Reifying field labels]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    When reifying a datatype declared with DuplicateRecordFields enabled, we want
    the reified names of the fields to be labels rather than selector functions.
    That is, we want (reify ''T) and (reify 'foo) to produce

        data T = MkT { foo :: Int }
        foo :: T -> Int

    rather than

        data T = MkT { $sel:foo:MkT :: Int }
        $sel:foo:MkT :: T -> Int

    because otherwise TH code that uses the field names as strings will silently do
    the wrong thing.  Thus we use the field label (e.g. foo) as the OccName, rather
    than the selector (e.g. $sel:foo:MkT).  Since the Orig name M.foo isn't in the
    environment, NameG can't be used to represent such fields.  Instead,
    reifyFieldLabel uses NameQ.

    However, this means that extracting the field name from the output of reify, and
    trying to reify it again, may fail with an ambiguity error if there are multiple
    such fields defined in the module (see the test case
    overloadedrecflds/should_fail/T11103.hs).  The "proper" fix requires changes to
    the TH AST to make it able to represent duplicate record fields.
    -}