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

TcSplice: Template Haskell splices


\begin{code}
{-# OPTIONS -fno-warn-unused-imports -fno-warn-unused-binds #-}
-- The above warning supression flag is a temporary kludge.
-- While working on this module you are encouraged to remove it and fix
-- any warnings in the module. See
--     http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#Warnings
-- for details

module TcSplice( kcSpliceType, tcSpliceExpr, tcSpliceDecls, tcBracket,
                 lookupThName_maybe,
                 runQuasiQuoteExpr, runQuasiQuotePat, 
                 runQuasiQuoteDecl, runQuasiQuoteType,
                 runAnnotation ) where

#include "HsVersions.h"

import HscMain
import TcRnDriver
	-- These imports are the reason that TcSplice 
	-- is very high up the module hierarchy

import HsSyn
import Convert
import RnExpr
import RnEnv
import RdrName
import RnTypes
import TcPat
import TcExpr
import TcHsSyn
import TcSimplify
import TcUnify
import TcType
import TcEnv
import TcMType
import TcHsType
import TcIface
import TypeRep
import InstEnv 
import Name
import NameEnv
import NameSet
import PrelNames
import HscTypes
import OccName
import Var
import Module
import Annotations
import TcRnMonad
import Class
import Inst
import TyCon
import DataCon
import Id
import IdInfo
import TysWiredIn
import DsMeta
import DsExpr
import DsMonad hiding (Splice)
import Serialized
import ErrUtils
import SrcLoc
import Outputable
import Util		( dropList )
import Data.List	( mapAccumL )
import Unique
import Data.Maybe
import BasicTypes
import Panic
import FastString
import Exception
import Control.Monad	( when )

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

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

import GHC.Exts		( unsafeCoerce#, Int#, Int(..) )
import System.IO.Error
\end{code}

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, becuase 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. 

  * After typechecking, the bracket [| |] carries

     a) A mutable list of PendingSplice
          type PendingSplice = (Name, LHsExpr Id)

     b) The quoted expression e, *renamed*: (HsExpr Name)
          The expression e has been typechecked, but the result of
	  that typechecking is discarded.  

  * The brakcet 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 the type checker type-checks a nested splice ${n}(e), it 
	- typechecks e
	- adds the typechecked expression (of type (HsExpr Id))
	  as a pending splice to the enclosing bracket
	- returns something non-committal
    Eg for [| f ${n}(g x) |], the typechecker 
	- attaches the typechecked term (g x) to the pending splices for n
	  in the outer bracket
        - returns a non-committal type \alpha.
	Remember that the bracket discards the typechecked term altogether

  * 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.  

      -----------     $	     ------------   $
      |  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)

* 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 |]

See Note [What is a top-level Id?]

Note [Quoting names]
~~~~~~~~~~~~~~~~~~~~
A quoted name 'n is a bit like a quoted expression [| n |], except that we 
have no cross-stage lifting (c.f. TcExpr.thBrackId).  So, after incrementing
the use-level to account for the brackets, the cases are:

	bind > use			Error
	bind = use			OK
	bind < use	
		Imported things		OK
		Top-level things	OK
		Non-top-level		Error

See Note [What is a top-level Id?] in TcEnv.  Examples:

  f 'map	-- OK; also for top-level defns of this module

  \x. f 'x	-- Not ok (whereas \x. f [| x |] might have been ok, by
		--				 cross-stage lifting

  \y. [| \x. $(f 'y) |]	-- Not ok (same reason)

  [| \x. $(f 'x) |]	-- OK


Note [What is a top-level Id?]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In the level-control criteria above, we need to know what a "top level Id" is.
There are three kinds:
  * Imported from another module		(GlobalId, ExternalName)
  * Bound at the top level of this module	(ExternalName)
  * In GHCi, bound by a previous stmt		(GlobalId)
It's strange that there is no one criterion tht picks out all three, but that's
how it is right now.  (The obvious thing is to give an ExternalName to GHCi Ids 
bound in an earlier Stmt, but what module would you choose?  See 
Note [Interactively-bound Ids in GHCi] in TcRnDriver.)

The predicate we use is TcEnv.thTopLevelId.


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

\begin{code}
tcBracket     :: HsBracket Name -> TcRhoType -> TcM (LHsExpr TcId)
tcSpliceDecls :: LHsExpr Name -> TcM [LHsDecl RdrName]
tcSpliceExpr  :: HsSplice Name -> TcRhoType -> TcM (HsExpr TcId)
kcSpliceType  :: HsSplice Name -> FreeVars -> TcM (HsType Name, TcKind)
	-- None of these functions add constraints to the LIE

lookupThName_maybe :: TH.Name -> TcM (Maybe Name)

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 Name -> TcM Annotation

#ifndef GHCI
tcBracket     x _ = pprPanic "Cant do tcBracket without GHCi"     (ppr x)
tcSpliceExpr  e   = pprPanic "Cant do tcSpliceExpr without GHCi"  (ppr e)
tcSpliceDecls x   = pprPanic "Cant do tcSpliceDecls without GHCi" (ppr x)
kcSpliceType  x fvs = pprPanic "Cant do kcSpliceType without GHCi"  (ppr x)

lookupThName_maybe n = pprPanic "Cant do lookupThName_maybe without GHCi" (ppr n)

runQuasiQuoteExpr q = pprPanic "Cant do runQuasiQuoteExpr without GHCi" (ppr q)
runQuasiQuotePat  q = pprPanic "Cant do runQuasiQuotePat without GHCi" (ppr q)
runQuasiQuoteType q = pprPanic "Cant do runQuasiQuoteType without GHCi" (ppr q)
runQuasiQuoteDecl q = pprPanic "Cant do runQuasiQuoteDecl without GHCi" (ppr q)
runAnnotation   _ q = pprPanic "Cant do runAnnotation without GHCi" (ppr q)
#else
\end{code}

%************************************************************************
%*									*
\subsection{Quoting an expression}
%*									*
%************************************************************************


\begin{code}
-- See Note [How brackets and nested splices are handled]
tcBracket brack res_ty 
  = addErrCtxt (hang (ptext (sLit "In the Template Haskell quotation"))
                   2 (ppr brack)) $
    do { 	-- Check for nested brackets
         cur_stage <- getStage
       ; checkTc (not (isBrackStage cur_stage)) illegalBracket 

	-- Brackets are desugared to code that mentions the TH package
       ; recordThUse

   	-- Typecheck expr to make sure it is valid,
	-- but throw away the results.  We'll type check
	-- it again when we actually use it.
       ; pending_splices <- newMutVar []
       ; lie_var <- getConstraintVar
       ; let brack_stage = Brack cur_stage pending_splices lie_var

       ; (meta_ty, lie) <- setStage brack_stage $
                           captureConstraints $
                           tc_bracket cur_stage brack

       ; simplifyBracket lie

	-- Make the expected type have the right shape
       ; _ <- unifyType meta_ty res_ty

	-- Return the original expression, not the type-decorated one
       ; pendings <- readMutVar pending_splices
       ; return (noLoc (HsBracketOut brack pendings)) }

tc_bracket :: ThStage -> HsBracket Name -> TcM TcType
tc_bracket outer_stage (VarBr name) 	-- Note [Quoting names]
  = do	{ thing <- tcLookup name
	; case thing of
    	    AGlobal _ -> return ()
    	    ATcId { tct_level = bind_lvl, tct_id = id }
		| thTopLevelId id	-- C.f TcExpr.checkCrossStageLifting
		-> keepAliveTc id 	 	
		| otherwise
		-> do { checkTc (thLevel outer_stage + 1 == bind_lvl)
				(quotedNameStageErr name) }
	    _ -> pprPanic "th_bracket" (ppr name)

	; tcMetaTy nameTyConName 	-- Result type is Var (not Q-monadic)
	}

tc_bracket _ (ExpBr expr) 
  = do	{ any_ty <- newFlexiTyVarTy liftedTypeKind
	; _ <- tcMonoExprNC expr any_ty  -- NC for no context; tcBracket does that
	; tcMetaTy expQTyConName }
	-- Result type is ExpQ (= Q Exp)

tc_bracket _ (TypBr typ) 
  = do	{ _ <- tcHsSigTypeNC ThBrackCtxt typ
	; tcMetaTy typeQTyConName }
	-- Result type is Type (= Q Typ)

tc_bracket _ (DecBrG decls)
  = do	{ _ <- tcTopSrcDecls emptyModDetails decls
	       -- Typecheck the declarations, dicarding the result
	       -- We'll get all that stuff later, when we splice it in

	       -- Top-level declarations in the bracket get unqualified names
               -- See Note [Top-level Names in Template Haskell decl quotes] in RnNames

	; tcMetaTy decsQTyConName } -- Result type is Q [Dec]

tc_bracket _ (PatBr pat)
  = do	{ any_ty <- newFlexiTyVarTy liftedTypeKind
	; _ <- tcPat ThPatQuote pat any_ty $ 
               return ()
	; tcMetaTy patQTyConName }
	-- Result type is PatQ (= Q Pat)

tc_bracket _ (DecBrL _)
  = panic "tc_bracket: Unexpected DecBrL"

quotedNameStageErr :: Name -> SDoc
quotedNameStageErr v 
  = sep [ ptext (sLit "Stage error: the non-top-level quoted name") <+> ppr (VarBr v)
	, ptext (sLit "must be used at the same stage at which is is bound")]
\end{code}


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

\begin{code}
tcSpliceExpr (HsSplice name expr) res_ty
  = setSrcSpan (getLoc expr) 	$ do
    { stage <- getStage
    ; case stage of {
	Splice -> tcTopSplice expr res_ty ;
	Comp   -> tcTopSplice expr res_ty ;

	Brack pop_stage ps_var lie_var -> do

        -- See Note [How brackets and nested splices are handled]
	-- A splice inside brackets
  	-- NB: ignore res_ty, apart from zapping it to a mono-type
	-- e.g.   [| reverse $(h 4) |]
	-- Here (h 4) :: Q Exp
	-- but $(h 4) :: forall a.a 	i.e. anything!

     { meta_exp_ty <- tcMetaTy expQTyConName
     ; expr' <- setStage pop_stage $
                setConstraintVar lie_var    $
                tcMonoExpr expr meta_exp_ty

	-- Write the pending splice into the bucket
     ; ps <- readMutVar ps_var
     ; writeMutVar ps_var ((name,expr') : ps)

     ; return (panic "tcSpliceExpr")	-- The returned expression is ignored
     }}}

tcTopSplice :: LHsExpr Name -> TcRhoType -> TcM (HsExpr Id)
-- Note [How top-level splices are handled]
tcTopSplice expr res_ty
  = do { meta_exp_ty <- tcMetaTy expQTyConName

        -- Typecheck the expression
       ; zonked_q_expr <- tcTopSpliceExpr (tcMonoExpr expr meta_exp_ty)

        -- Run the expression
       ; expr2 <- runMetaE zonked_q_expr
       ; showSplice "expression" expr (ppr expr2)

        -- Rename it, but bale out if there are errors
        -- otherwise the type checker just gives more spurious errors
       ; addErrCtxt (spliceResultDoc expr) $ do 
       { (exp3, _fvs) <- checkNoErrs (rnLExpr expr2)

       ; exp4 <- tcMonoExpr exp3 res_ty 
       ; return (unLoc exp4) } }

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

-------------------
tcTopSpliceExpr :: TcM (LHsExpr Id) -> TcM (LHsExpr Id)
-- 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 tcMonoExpr will simply expand the 
-- inner escape before dealing with the outer one

tcTopSpliceExpr tc_action
  = checkNoErrs $  -- checkNoErrs: must not try to run the thing
                   -- if the type checker fails!
    setStage Splice $ 
    do {    -- Typecheck the expression
         (expr', lie) <- captureConstraints tc_action
        
	-- Solve the constraints
	; const_binds <- simplifyTop lie
	
          -- Zonk it and tie the knot of dictionary bindings
       ; zonkTopLExpr (mkHsDictLet (EvBinds const_binds) expr') }
\end{code}


%************************************************************************
%*									*
		Splicing a type
%*									*
%************************************************************************

Very like splicing an expression, but we don't yet share code.

\begin{code}
kcSpliceType splice@(HsSplice name hs_expr) fvs
  = setSrcSpan (getLoc hs_expr) $ do 	
    { stage <- getStage
    ; case stage of {
        Splice -> kcTopSpliceType hs_expr ;
    	Comp   -> kcTopSpliceType hs_expr ;

    	Brack pop_level ps_var lie_var -> do
	   -- See Note [How brackets and nested splices are handled]
	   -- A splice inside brackets
    { meta_ty <- tcMetaTy typeQTyConName
    ; expr' <- setStage pop_level $
    	       setConstraintVar lie_var $
    	       tcMonoExpr hs_expr meta_ty

    	-- Write the pending splice into the bucket
    ; ps <- readMutVar ps_var
    ; writeMutVar ps_var ((name,expr') : ps)

    -- e.g.   [| f (g :: Int -> $(h 4)) |]
    -- Here (h 4) :: Q Type
    -- but $(h 4) :: a 	i.e. any type, of any kind

    ; kind <- newKindVar
    ; return (HsSpliceTy splice fvs kind, kind)	
    }}}

kcTopSpliceType :: LHsExpr Name -> TcM (HsType Name, TcKind)
-- Note [How top-level splices are handled]
kcTopSpliceType expr
  = do	{ meta_ty <- tcMetaTy typeQTyConName

	-- Typecheck the expression
	; zonked_q_expr <- tcTopSpliceExpr (tcMonoExpr expr meta_ty)

	-- Run the expression
	; hs_ty2 <- runMetaT zonked_q_expr
	; showSplice "type" expr (ppr hs_ty2)
  
	-- Rename it, but bale out if there are errors
	-- otherwise the type checker just gives more spurious errors
        ; addErrCtxt (spliceResultDoc expr) $ do 
	{ let doc = ptext (sLit "In the spliced type") <+> ppr hs_ty2
	; hs_ty3 <- checkNoErrs (rnLHsType doc hs_ty2)
	; (ty4, kind) <- kcLHsType hs_ty3
        ; return (unLoc ty4, kind) }}
\end{code}

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

\begin{code}
-- Note [How top-level splices are handled]
-- Always at top level
-- Type sig at top of file:
-- 	tcSpliceDecls :: LHsExpr Name -> TcM [LHsDecl RdrName]
tcSpliceDecls expr
  = do	{ list_q <- tcMetaTy decsQTyConName	-- Q [Dec]
	; zonked_q_expr <- tcTopSpliceExpr (tcMonoExpr expr list_q)

		-- Run the expression
	; decls <- runMetaD zonked_q_expr
	; showSplice "declarations" expr 
		     (ppr (getLoc expr) $$ (vcat (map ppr decls)))

	; return decls }
\end{code}


%************************************************************************
%*									*
	Annotations
%*									*
%************************************************************************

\begin{code}
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 $ 
           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 (HsWrap wrapper (HsVar to_annotation_wrapper_id))
              ; return (L loc (HsApp 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
    flip runMetaAW zonked_wrapped_expr' $ \annotation_wrapper ->
        case 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` Annotation { 
                    ann_target = target,
                    ann_value = serialized
                }
\end{code}


%************************************************************************
%*									*
	Quasi-quoting
%*									*
%************************************************************************

Note [Quasi-quote overview]
~~~~~~~~~~~~~~~~~~~~~~~~~~~
The GHC "quasi-quote" extension is described by Geoff Mainland's paper
"Why it's nice to be quoted: quasiquoting for Haskell" (Haskell
Workshop 2007).

Briefly, one writes
	[p| stuff |]
and the arbitrary string "stuff" gets parsed by the parser 'p', whose
type should be Language.Haskell.TH.Quote.QuasiQuoter.  'p' must be
defined in another module, because we are going to run it here.  It's
a bit like a TH splice:
	$(p "stuff")

However, you can do this in patterns as well as terms.  Becuase of this,
the splice is run by the *renamer* rather than the type checker.

%************************************************************************
%*									*
\subsubsection{Quasiquotation}
%*									*
%************************************************************************

See Note [Quasi-quote overview] in TcSplice.

\begin{code}
runQuasiQuote :: Outputable hs_syn
              => HsQuasiQuote RdrName	-- Contains term of type QuasiQuoter, and the String
              -> Name			-- Of type QuasiQuoter -> String -> Q th_syn
              -> Name			-- Name of th_syn type  
              -> MetaOps th_syn hs_syn 
              -> RnM hs_syn
runQuasiQuote (HsQuasiQuote quoter q_span quote) quote_selector meta_ty meta_ops
  = do	{ quoter' <- lookupOccRn quoter
		-- We use lookupOcc rather than lookupGlobalOcc because in the
		-- erroneous case of \x -> [x| ...|] we get a better error message
		-- (stage restriction rather than out of scope).

        ; when (isUnboundName quoter') failM 
		-- If 'quoter' is not in scope, proceed no further
		-- The error message was generated by lookupOccRn, but it then
		-- succeeds with an "unbound name", which makes the subsequent 
		-- attempt to run the quote fail in a confusing way

          -- Check that the quoter is not locally defined, otherwise the TH
          -- machinery will not be able to run the quasiquote.
   	; this_mod <- getModule
        ; let is_local = nameIsLocalOrFrom this_mod quoter'
        ; checkTc (not is_local) (quoteStageError quoter')

	; traceTc "runQQ" (ppr quoter <+> ppr is_local)

	  -- Build the expression 
      	; let quoterExpr = L q_span $! HsVar $! quoter'
      	; let quoteExpr = L q_span $! HsLit $! HsString quote
      	; let expr = L q_span $
      	             HsApp (L q_span $
      	                    HsApp (L q_span (HsVar quote_selector)) quoterExpr) quoteExpr
      	; meta_exp_ty <- tcMetaTy meta_ty

      	-- Typecheck the expression
      	; zonked_q_expr <- tcTopSpliceExpr (tcMonoExpr expr meta_exp_ty)

      	-- Run the expression
      	; result <- runMetaQ meta_ops zonked_q_expr
      	; showSplice (mt_desc meta_ops) quoteExpr (ppr result)

      	; return result	}

runQuasiQuoteExpr qq = runQuasiQuote qq quoteExpName  expQTyConName  exprMetaOps
runQuasiQuotePat  qq = runQuasiQuote qq quotePatName  patQTyConName  patMetaOps
runQuasiQuoteType qq = runQuasiQuote qq quoteTypeName typeQTyConName typeMetaOps
runQuasiQuoteDecl qq = runQuasiQuote qq quoteDecName  decsQTyConName declMetaOps

quoteStageError :: Name -> SDoc
quoteStageError quoter
  = sep [ptext (sLit "GHC stage restriction:") <+> ppr quoter,
         nest 2 (ptext (sLit "is used in a quasiquote, and must be imported, not defined locally"))]
\end{code}


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

\begin{code}
data MetaOps th_syn hs_syn
  = MT { mt_desc :: String	       -- Type of beast (expression, type etc)
       , mt_show :: th_syn -> String   -- How to show the th_syn thing
       , mt_cvt  :: SrcSpan -> th_syn -> Either Message hs_syn
       	 	    	       	       -- How to convert to hs_syn
    }

exprMetaOps :: MetaOps TH.Exp (LHsExpr RdrName)
exprMetaOps = MT { mt_desc = "expression", mt_show = TH.pprint, mt_cvt = convertToHsExpr }

patMetaOps :: MetaOps TH.Pat (LPat RdrName)
patMetaOps = MT { mt_desc = "pattern", mt_show = TH.pprint, mt_cvt = convertToPat }

typeMetaOps :: MetaOps TH.Type (LHsType RdrName)
typeMetaOps = MT { mt_desc = "type", mt_show = TH.pprint, mt_cvt = convertToHsType }

declMetaOps :: MetaOps [TH.Dec] [LHsDecl RdrName]
declMetaOps = MT { mt_desc = "declarations", mt_show = TH.pprint, mt_cvt = convertToHsDecls }

----------------
runMetaAW :: Outputable output
          => (AnnotationWrapper -> output)
          -> LHsExpr Id         -- Of type AnnotationWrapper
          -> TcM output
runMetaAW k = runMeta False (\_ -> return . Right . k)
    -- We turn off showing the code in meta-level exceptions because doing so exposes
    -- the toAnnotationWrapper function that we slap around the users code

-----------------
runMetaQ :: Outputable hs_syn 
         => MetaOps th_syn hs_syn
	 -> LHsExpr Id
	 -> TcM hs_syn
runMetaQ (MT { mt_show = show_th, mt_cvt = cvt }) expr
  = runMeta True run_and_cvt expr
  where
    run_and_cvt expr_span hval
       = do { th_result <- TH.runQ hval
            ; traceTc "Got TH result:" (text (show_th th_result))
            ; return (cvt expr_span th_result) }

runMetaE :: LHsExpr Id 		-- Of type (Q Exp)
	 -> TcM (LHsExpr RdrName)
runMetaE = runMetaQ exprMetaOps

runMetaT :: LHsExpr Id 		-- Of type (Q Type)
	 -> TcM (LHsType RdrName)	
runMetaT = runMetaQ typeMetaOps

runMetaD :: LHsExpr Id 		-- Of type Q [Dec]
	 -> TcM [LHsDecl RdrName]
runMetaD = runMetaQ declMetaOps

---------------
runMeta :: (Outputable hs_syn)
        => Bool                 -- Whether code should be printed in the exception message
        -> (SrcSpan -> x -> TcM (Either Message hs_syn))	-- How to run x 
	-> LHsExpr Id 		-- Of type x; typically x = Q TH.Exp, or something like that
	-> TcM hs_syn		-- Of type t
runMeta show_code run_and_convert expr
  = do	{ traceTc "About to run" (ppr expr)

	-- Desugar
	; ds_expr <- initDsTc (dsLExpr expr)
	-- Compile and link it; might fail if linking fails
	; hsc_env <- getTopEnv
	; src_span <- getSrcSpanM
	; either_hval <- tryM $ liftIO $
			 HscMain.compileExpr hsc_env src_span ds_expr
	; case either_hval of {
	    Left exn   -> failWithTc (mk_msg "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 (unsafeCoerce# hval)
		; case mb_result of
		    Left err     -> failWithTc err
		    Right result -> do { traceTc "Got HsSyn result:" (ppr 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
                    	 _ -> failWithTc (mk_msg "run" se)	-- Exception
        }}}
  where
    mk_msg s exn = vcat [text "Exception when trying to" <+> text s <+> text "compile-time code:",
			 nest 2 (text (Panic.showException exn)),
			 if show_code then nest 2 (text "Code:" <+> ppr expr) else empty]
\end{code}

Note [Exceptions in TH]
~~~~~~~~~~~~~~~~~~~~~~~
Supppose 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

  * 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


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

\begin{code}
instance TH.Quasi (IOEnv (Env TcGblEnv TcLclEnv)) where
  qNewName s = do { u <- newUnique 
		  ; let i = getKey u
		  ; return (TH.mkNameU s i) }

  qReport True msg  = addErr (text msg)
  qReport False msg = addReport (text msg) empty

  qLocation = do { m <- getModule
		 ; l <- getSrcSpanM
		 ; return (TH.Loc { TH.loc_filename = unpackFS (srcSpanFile l)
				  , TH.loc_module   = moduleNameString (moduleName m)
				  , TH.loc_package  = packageIdString (modulePackageId m)
				  , TH.loc_start = (srcSpanStartLine l, srcSpanStartCol l)
				  , TH.loc_end = (srcSpanEndLine   l, srcSpanEndCol   l) }) }
		
  qReify v = reify v
  qClassInstances = lookupClassInstances

	-- For qRecover, discard error messages if 
	-- the recovery action is chosen.  Otherwise
	-- we'll only fail higher up.  c.f. tryTcLIE_
  qRecover recover main = do { (msgs, mb_res) <- tryTcErrs main
			     ; case mb_res of
		  	         Just val -> do { addMessages msgs	-- There might be warnings
				   	        ; return val }
		  	         Nothing  -> recover			-- Discard all msgs
			  }

  qRunIO io = liftIO io
\end{code}


%************************************************************************
%*									*
\subsection{Errors and contexts}
%*									*
%************************************************************************

\begin{code}
showSplice :: String -> LHsExpr Name -> SDoc -> TcM ()
-- Note that 'before' is *renamed* but not *typechecked*
-- Reason (a) less typechecking crap
--        (b) data constructors after type checking have been
--	      changed to their *wrappers*, and that makes them
--	      print always fully qualified
showSplice what before after
  = do { loc <- getSrcSpanM
       ; traceSplice (vcat [ppr loc <> colon <+> text "Splicing" <+> text what, 
		            nest 2 (sep [nest 2 (ppr before),
				         text "======>",
				         nest 2 after])]) }

illegalBracket :: SDoc
illegalBracket = ptext (sLit "Template Haskell brackets cannot be nested (without intervening splices)")
#endif 	/* GHCI */
\end{code}


%************************************************************************
%*									*
	    Instance Testing
%*									*
%************************************************************************

\begin{code}
lookupClassInstances :: TH.Name -> [TH.Type] -> TcM [TH.Name]
lookupClassInstances c ts
   = do { loc <- getSrcSpanM
        ; case convertToHsPred loc (TH.ClassP c ts) of
            Left msg -> failWithTc msg
            Right rdr_pred -> do
        { rn_pred <- rnLPred doc rdr_pred	-- Rename
        ; kc_pred <- kcHsLPred rn_pred		-- Kind check
        ; ClassP cls tys <- dsHsLPred kc_pred	-- Type check

	-- Now look up instances
        ; inst_envs <- tcGetInstEnvs
        ; let (matches, unifies) = lookupInstEnv inst_envs cls tys
              dfuns = map is_dfun (map fst matches ++ unifies)
        ; return (map reifyName dfuns) } }
  where
    doc = ptext (sLit "TcSplice.classInstances")
\end{code}


%************************************************************************
%*									*
			Reification
%*									*
%************************************************************************


\begin{code}
reify :: TH.Name -> TcM TH.Info
reify th_name
  = do	{ name <- lookupThName th_name
	; thing <- tcLookupTh name
		-- ToDo: this tcLookup could fail, which would give a
		-- 	 rather unhelpful error message
	; traceIf (text "reify" <+> text (show th_name) <+> brackets (ppr_ns th_name) <+> ppr name)
	; reifyThing thing
    }
  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"

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
  =  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, becase 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    -> do
	{ if nameIsLocalOrFrom (tcg_mod gbl_env) name
	  then	-- It's defined in this module
	      case lookupNameEnv (tcg_type_env gbl_env) name of
		Just thing -> return (AGlobal thing)
		Nothing	   -> failWithTc (notInEnv name)
	 
	  else do 		-- It's imported
	{ (eps,hpt) <- getEpsAndHpt
        ; dflags <- getDOpts
	; case lookupType dflags hpt (eps_PTE eps) name of 
	    Just thing -> return (AGlobal thing)
	    Nothing    -> do { thing <- tcImportDecl name
			     ; return (AGlobal thing) }
		-- Imported names should always be findable; 
		-- if not, we fail hard in tcImportDecl
    }}}}

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

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

------------------------------
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)
	; fix <- reifyFixity (idName id)
	; let v = reifyName id
	; case idDetails id of
	    ClassOpId cls -> return (TH.ClassOpI v ty (reifyName cls) fix)
	    _             -> return (TH.VarI     v ty Nothing fix)
    }

reifyThing (AGlobal (ATyCon tc))  = reifyTyCon tc
reifyThing (AGlobal (AClass cls)) = reifyClass cls
reifyThing (AGlobal (ADataCon dc))
  = do	{ let name = dataConName dc
	; ty <- reifyType (idType (dataConWrapId dc))
	; fix <- reifyFixity name
	; return (TH.DataConI (reifyName name) ty 
                              (reifyName (dataConOrigTyCon dc)) fix) 
        }

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
	; fix <- reifyFixity (idName id)
	; return (TH.VarI (reifyName id) ty2 Nothing fix) }

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

reifyThing (AThing {}) = panic "reifyThing AThing"

------------------------------
reifyTyCon :: TyCon -> TcM TH.Info
reifyTyCon tc
  | isFunTyCon tc  
  = return (TH.PrimTyConI (reifyName tc) 2 		  False)
  | isPrimTyCon tc 
  = return (TH.PrimTyConI (reifyName tc) (tyConArity tc) (isUnLiftedTyCon tc))
  | isFamilyTyCon tc
  = let flavour = reifyFamFlavour tc
        tvs     = tyConTyVars tc
        kind    = tyConKind tc
        kind'
          | isLiftedTypeKind kind = Nothing
          | otherwise             = Just $ reifyKind kind
    in
    return (TH.TyConI $
              TH.FamilyD flavour (reifyName tc) (reifyTyVars tvs) kind')
  | isSynTyCon tc
  = do { let (tvs, rhs) = synTyConDefn tc 
       ; rhs' <- reifyType rhs
       ; return (TH.TyConI $ 
		   TH.TySynD (reifyName tc) (reifyTyVars tvs) rhs') 
       }

reifyTyCon tc
  = do 	{ cxt <- reifyCxt (tyConStupidTheta tc)
	; let tvs = tyConTyVars tc
	; cons <- mapM (reifyDataCon (mkTyVarTys tvs)) (tyConDataCons tc)
	; let name = reifyName tc
	      r_tvs  = reifyTyVars tvs
	      deriv = []	-- Don't know about deriving
	      decl | isNewTyCon tc = TH.NewtypeD cxt name r_tvs (head cons) deriv
		   | otherwise	   = TH.DataD    cxt name r_tvs cons 	    deriv
	; return (TH.TyConI decl) }

reifyDataCon :: [Type] -> DataCon -> TcM TH.Con
-- For GADTs etc, see Note [Reifying data constructors]
reifyDataCon tys dc
  = do { let (tvs, theta, arg_tys, _) = dataConSig dc
             subst             = mkTopTvSubst (tvs `zip` tys)	-- Dicard ex_tvs
             (subst', ex_tvs') = mapAccumL substTyVarBndr subst (dropList tys tvs)
             theta'   = substTheta subst' theta
             arg_tys' = substTys subst' arg_tys
             stricts  = map reifyStrict (dataConStrictMarks dc)
	     fields   = dataConFieldLabels dc
	     name     = reifyName dc

       ; r_arg_tys <- reifyTypes arg_tys'

       ; let main_con | not (null fields) 
                      = TH.RecC name (zip3 (map reifyName fields) stricts r_arg_tys)
                      | dataConIsInfix dc
                      = ASSERT( length arg_tys == 2 )
	                TH.InfixC (s1,r_a1) name (s2,r_a2)
                      | otherwise
                      = TH.NormalC name (stricts `zip` r_arg_tys)
	     [r_a1, r_a2] = r_arg_tys
	     [s1,   s2]   = stricts

       ; ASSERT( length arg_tys == length stricts )
         if null ex_tvs' && null theta then
	     return main_con
         else do
         { cxt <- reifyCxt theta'
	 ; return (TH.ForallC (reifyTyVars ex_tvs') cxt main_con) } }

------------------------------
reifyClass :: Class -> TcM TH.Info
reifyClass cls 
  = do	{ cxt <- reifyCxt theta
        ; inst_envs <- tcGetInstEnvs
        ; insts <- mapM reifyClassInstance (InstEnv.classInstances inst_envs cls)
	; ops <- mapM reify_op op_stuff
        ; let dec = TH.ClassD cxt (reifyName cls) (reifyTyVars tvs) fds' ops
	; return (TH.ClassI dec insts ) }
  where
    (tvs, fds, theta, _, _, op_stuff) = classExtraBigSig cls
    fds' = map reifyFunDep fds
    reify_op (op, _) = do { ty <- reifyType (idType op)
			  ; return (TH.SigD (reifyName op) ty) }

------------------------------
reifyClassInstance :: Instance -> TcM TH.ClassInstance
reifyClassInstance i
  = do { cxt <- reifyCxt theta
       ; thtypes <- reifyTypes types
       ; return $ (TH.ClassInstance { 
            TH.ci_tvs = reifyTyVars tvs,
            TH.ci_cxt = cxt,
            TH.ci_tys = thtypes,
            TH.ci_cls = reifyName cls,
            TH.ci_dfun = reifyName (is_dfun i) }) }
  where
     (tvs, theta, cls, types) = instanceHead i

------------------------------
reifyType :: TypeRep.Type -> TcM TH.Type
-- Monadic only because of failure
reifyType ty@(ForAllTy _ _)        = reify_for_all ty
reifyType ty@(PredTy {} `FunTy` _) = reify_for_all ty	        -- Types like ((?x::Int) => Char -> Char)
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 (FunTy t1 t2)     = do { [r1,r2] <- reifyTypes [t1,t2] ; return (TH.ArrowT `TH.AppT` r1 `TH.AppT` r2) }
reifyType ty@(PredTy {})    = pprPanic "reifyType PredTy" (ppr ty)

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

reifyKind :: Kind -> TH.Kind
reifyKind  ki
  = let (kis, ki') = splitKindFunTys ki
        kis_rep    = map reifyKind kis
        ki'_rep    = reifyNonArrowKind ki'
    in
    foldr TH.ArrowK ki'_rep kis_rep
  where
    reifyNonArrowKind k | isLiftedTypeKind k = TH.StarK
                        | otherwise          = pprPanic "Exotic form of kind" 
                                                        (ppr k)

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

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

reifyFamFlavour :: TyCon -> TH.FamFlavour
reifyFamFlavour tc | isSynFamilyTyCon tc = TH.TypeFam
                   | isFamilyTyCon    tc = TH.DataFam
                   | otherwise         
                   = panic "TcSplice.reifyFamFlavour: not a type family"

reifyTyVars :: [TyVar] -> [TH.TyVarBndr]
reifyTyVars = map reifyTyVar
  where
    reifyTyVar tv | isLiftedTypeKind kind = TH.PlainTV  name
                  | otherwise             = TH.KindedTV name (reifyKind kind)
      where
        kind = tyVarKind tv
        name = reifyName tv

reify_tc_app :: TyCon -> [TypeRep.Type] -> TcM TH.Type
reify_tc_app tc tys 
  = do { tys' <- reifyTypes tys 
       ; return (foldl TH.AppT r_tc tys') }
  where
    n_tys = length tys
    r_tc | isTupleTyCon tc          = TH.TupleT n_tys
         | tc `hasKey` listTyConKey = TH.ListT
         | otherwise                = TH.ConT (reifyName tc)

reifyPred :: TypeRep.PredType -> TcM TH.Pred
reifyPred (ClassP cls tys) 
  = do { tys' <- reifyTypes tys 
       ; return $ TH.ClassP (reifyName cls) tys' }

reifyPred p@(IParam _ _)   = noTH (sLit "implicit parameters") (ppr p)
reifyPred (EqPred ty1 ty2) 
  = do { ty1' <- reifyType ty1
       ; ty2' <- reifyType ty2
       ; return $ TH.EqualP ty1' ty2'
       }


------------------------------
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 = packageIdString (modulePackageId 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)

------------------------------
reifyFixity :: Name -> TcM TH.Fixity
reifyFixity name
  = do	{ fix <- lookupFixityRn name
	; return (conv_fix fix) }
    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

reifyStrict :: BasicTypes.HsBang -> TH.Strict
reifyStrict bang | isBanged bang = TH.IsStrict
                 | otherwise     = TH.NotStrict

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

Note [Reifying data constructors]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Template Haskell syntax is rich enough to express even GADTs, 
provided we do so in the equality-predicate form.  So a GADT
like

  data T a where
     MkT1 :: a -> T [a]
     MkT2 :: T Int

will appear in TH syntax like this

  data T a = forall b. (a ~ [b]) => MkT1 b
           | (a ~ Int) => MkT2