module TcRnTypes(
        TcRnIf, TcRn, TcM, RnM, IfM, IfL, IfG, -- The monad is opaque outside this module
        TcRef,

        -- The environment types
        Env(..),
        TcGblEnv(..), TcLclEnv(..),
        IfGblEnv(..), IfLclEnv(..),

        -- Ranamer types
        ErrCtxt, RecFieldEnv(..),
        ImportAvails(..), emptyImportAvails, plusImportAvails,
        WhereFrom(..), mkModDeps,

        -- Typechecker types
        TcTypeEnv, TcIdBinder(..), TcTyThing(..), PromotionErr(..),
        pprTcTyThingCategory, pprPECategory,

        -- Template Haskell
        ThStage(..), PendingStuff(..), topStage, topAnnStage, topSpliceStage,
        ThLevel, impLevel, outerLevel, thLevel,

        -- Arrows
        ArrowCtxt(NoArrowCtxt), newArrowScope, escapeArrowScope,

        -- Canonical constraints
        Xi, Ct(..), Cts, emptyCts, andCts, andManyCts, dropDerivedWC,
        singleCt, listToCts, ctsElts, extendCts, extendCtsList,
        isEmptyCts, isCTyEqCan, isCFunEqCan,
        isCDictCan_Maybe, isCFunEqCan_maybe,
        isCIrredEvCan, isCNonCanonical, isWantedCt, isDerivedCt,
        isGivenCt, isHoleCt,
        ctEvidence, ctLoc, ctPred,
        mkNonCanonical, mkNonCanonicalCt,
        ctEvPred, ctEvTerm, ctEvId, ctEvCheckDepth,

        WantedConstraints(..), insolubleWC, emptyWC, isEmptyWC,
        andWC, unionsWC, addFlats, addImplics, mkFlatWC, addInsols,

        Implication(..),
        SubGoalCounter(..),
        SubGoalDepth, initialSubGoalDepth, maxSubGoalDepth,
        bumpSubGoalDepth, subGoalCounterValue, subGoalDepthExceeded,
        CtLoc(..), ctLocSpan, ctLocEnv, ctLocOrigin,
        ctLocDepth, bumpCtLocDepth,
        setCtLocOrigin, setCtLocEnv,
        CtOrigin(..),
        pushErrCtxt, pushErrCtxtSameOrigin,

        SkolemInfo(..),

        CtEvidence(..),
        mkGivenLoc,
        isWanted, isGiven, isDerived,
        canRewrite, canRewriteOrSame,

        -- Pretty printing
        pprEvVarTheta, pprWantedsWithLocs,
        pprEvVars, pprEvVarWithType,
        pprArising, pprArisingAt,

        -- Misc other types
        TcId, TcIdSet, TcTyVarBind(..), TcTyVarBinds

  ) where

#include "HsVersions.h"

import HsSyn
import HscTypes
import TcEvidence
import Type
import Class    ( Class )
import TyCon    ( TyCon )
import ConLike  ( ConLike(..) )
import DataCon  ( DataCon, dataConUserType, dataConOrigArgTys )
import PatSyn   ( PatSyn, patSynId )
import TcType
import Annotations
import InstEnv
import FamInstEnv
import IOEnv
import RdrName
import Name
import NameEnv
import NameSet
import Avail
import Var
import VarEnv
import Module
import SrcLoc
import VarSet
import ErrUtils
import UniqFM
import UniqSupply
import BasicTypes
import Bag
import DynFlags
import Outputable
import ListSetOps
import FastString

import Data.Set (Set)

#ifdef GHCI
import Data.Map      ( Map )
import Data.Dynamic  ( Dynamic )
import Data.Typeable ( TypeRep )

import qualified Language.Haskell.TH as TH
#endif

type TcRef a     = IORef a
type TcId        = Id
type TcIdSet     = IdSet


type TcRnIf a b = IOEnv (Env a b)
type IfM lcl  = TcRnIf IfGblEnv lcl         -- Iface stuff

type IfG  = IfM ()                          -- Top level
type IfL  = IfM IfLclEnv                    -- Nested
type TcRn = TcRnIf TcGblEnv TcLclEnv
type RnM  = TcRn            -- Historical
type TcM  = TcRn            -- Historical

data TcTyVarBind = TcTyVarBind TcTyVar TcType

type TcTyVarBinds = Bag TcTyVarBind

instance Outputable TcTyVarBind where
  ppr (TcTyVarBind tv ty) = ppr tv <+> text ":=" <+> ppr ty

-- We 'stack' these envs through the Reader like monad infastructure
-- as we move into an expression (although the change is focused in
-- the lcl type).
data Env gbl lcl
  = Env {
        env_top  :: HscEnv,  -- Top-level stuff that never changes
                             -- Includes all info about imported things

        env_us   :: {-# UNPACK #-} !(IORef UniqSupply),
                             -- Unique supply for local varibles

        env_gbl  :: gbl,     -- Info about things defined at the top level
                             -- of the module being compiled

        env_lcl  :: lcl      -- Nested stuff; changes as we go into
    }

instance ContainsDynFlags (Env gbl lcl) where
    extractDynFlags env = hsc_dflags (env_top env)
    replaceDynFlags env dflags
        = env {env_top = replaceDynFlags (env_top env) dflags}

instance ContainsModule gbl => ContainsModule (Env gbl lcl) where
    extractModule env = extractModule (env_gbl env)

-- TcGblEnv describes the top-level of the module at the
-- point at which the typechecker is finished work.
-- It is this structure that is handed on to the desugarer
-- For state that needs to be updated during the typechecking
-- phase and returned at end, use a TcRef (= IORef).

data TcGblEnv
  = TcGblEnv {
        tcg_mod     :: Module,         -- ^ Module being compiled
        tcg_src     :: HscSource,
          -- ^ What kind of module (regular Haskell, hs-boot, ext-core)

        tcg_rdr_env :: GlobalRdrEnv,   -- ^ Top level envt; used during renaming
        tcg_default :: Maybe [Type],
          -- ^ Types used for defaulting. @Nothing@ => no @default@ decl

        tcg_fix_env   :: FixityEnv,     -- ^ Just for things in this module
        tcg_field_env :: RecFieldEnv,   -- ^ Just for things in this module
                                        -- See Note [The interactive package] in HscTypes

        tcg_type_env :: TypeEnv,
          -- ^ Global type env for the module we are compiling now.  All
          -- TyCons and Classes (for this module) end up in here right away,
          -- along with their derived constructors, selectors.
          --
          -- (Ids defined in this module start in the local envt, though they
          --  move to the global envt during zonking)
          --
          -- NB: for what "things in this module" means, see
          -- Note [The interactive package] in HscTypes

        tcg_type_env_var :: TcRef TypeEnv,
                -- Used only to initialise the interface-file
                -- typechecker in initIfaceTcRn, so that it can see stuff
                -- bound in this module when dealing with hi-boot recursions
                -- Updated at intervals (e.g. after dealing with types and classes)

        tcg_inst_env     :: InstEnv,
          -- ^ Instance envt for all /home-package/ modules;
          -- Includes the dfuns in tcg_insts
        tcg_fam_inst_env :: FamInstEnv, -- ^ Ditto for family instances
        tcg_ann_env      :: AnnEnv,     -- ^ And for annotations

                -- Now a bunch of things about this module that are simply
                -- accumulated, but never consulted until the end.
                -- Nevertheless, it's convenient to accumulate them along
                -- with the rest of the info from this module.
        tcg_exports :: [AvailInfo],     -- ^ What is exported
        tcg_imports :: ImportAvails,
          -- ^ Information about what was imported from where, including
          -- things bound in this module. Also store Safe Haskell info
          -- here about transative trusted packaage requirements.

        tcg_dus :: DefUses,   -- ^ What is defined in this module and what is used.
        tcg_used_rdrnames :: TcRef (Set RdrName),
          -- See Note [Tracking unused binding and imports]

        tcg_keep :: TcRef NameSet,
          -- ^ Locally-defined top-level names to keep alive.
          --
          -- "Keep alive" means give them an Exported flag, so that the
          -- simplifier does not discard them as dead code, and so that they
          -- are exposed in the interface file (but not to export to the
          -- user).
          --
          -- Some things, like dict-fun Ids and default-method Ids are "born"
          -- with the Exported flag on, for exactly the above reason, but some
          -- we only discover as we go.  Specifically:
          --
          --   * The to/from functions for generic data types
          --
          --   * Top-level variables appearing free in the RHS of an orphan
          --     rule
          --
          --   * Top-level variables appearing free in a TH bracket

        tcg_th_used :: TcRef Bool,
          -- ^ @True@ <=> Template Haskell syntax used.
          --
          -- We need this so that we can generate a dependency on the
          -- Template Haskell package, because the desugarer is going
          -- to emit loads of references to TH symbols.  The reference
          -- is implicit rather than explicit, so we have to zap a
          -- mutable variable.

        tcg_th_splice_used :: TcRef Bool,
          -- ^ @True@ <=> A Template Haskell splice was used.
          --
          -- Splices disable recompilation avoidance (see #481)

        tcg_dfun_n  :: TcRef OccSet,
          -- ^ Allows us to choose unique DFun names.

        -- The next fields accumulate the payload of the module
        -- The binds, rules and foreign-decl fiels are collected
        -- initially in un-zonked form and are finally zonked in tcRnSrcDecls

        tcg_rn_exports :: Maybe [Located (IE Name)],
        tcg_rn_imports :: [LImportDecl Name],
                -- Keep the renamed imports regardless.  They are not
                -- voluminous and are needed if you want to report unused imports

        tcg_rn_decls :: Maybe (HsGroup Name),
          -- ^ Renamed decls, maybe.  @Nothing@ <=> Don't retain renamed
          -- decls.

        tcg_dependent_files :: TcRef [FilePath], -- ^ dependencies from addDependentFile

#ifdef GHCI
        tcg_th_topdecls :: TcRef [LHsDecl RdrName],
        -- ^ Top-level declarations from addTopDecls

        tcg_th_topnames :: TcRef NameSet,
        -- ^ Exact names bound in top-level declarations in tcg_th_topdecls

        tcg_th_modfinalizers :: TcRef [TH.Q ()],
        -- ^ Template Haskell module finalizers

        tcg_th_state :: TcRef (Map TypeRep Dynamic),
        -- ^ Template Haskell state
#endif /* GHCI */

        tcg_ev_binds  :: Bag EvBind,        -- Top-level evidence bindings
        tcg_binds     :: LHsBinds Id,       -- Value bindings in this module
        tcg_sigs      :: NameSet,           -- ...Top-level names that *lack* a signature
        tcg_imp_specs :: [LTcSpecPrag],     -- ...SPECIALISE prags for imported Ids
        tcg_warns     :: Warnings,          -- ...Warnings and deprecations
        tcg_anns      :: [Annotation],      -- ...Annotations
        tcg_tcs       :: [TyCon],           -- ...TyCons and Classes
        tcg_insts     :: [ClsInst],         -- ...Instances
        tcg_fam_insts :: [FamInst],         -- ...Family instances
        tcg_rules     :: [LRuleDecl Id],    -- ...Rules
        tcg_fords     :: [LForeignDecl Id], -- ...Foreign import & exports
        tcg_vects     :: [LVectDecl Id],    -- ...Vectorisation declarations
        tcg_patsyns   :: [PatSyn],          -- ...Pattern synonyms

        tcg_doc_hdr   :: Maybe LHsDocString, -- ^ Maybe Haddock header docs
        tcg_hpc       :: AnyHpcUsage,        -- ^ @True@ if any part of the
                                             --  prog uses hpc instrumentation.

        tcg_main      :: Maybe Name,         -- ^ The Name of the main
                                             -- function, if this module is
                                             -- the main module.
        tcg_safeInfer :: TcRef Bool          -- Has the typechecker
                                             -- inferred this module
                                             -- as -XSafe (Safe Haskell)
    }

instance ContainsModule TcGblEnv where
    extractModule env = tcg_mod env

data RecFieldEnv
  = RecFields (NameEnv [Name])  -- Maps a constructor name *in this module*
                                -- to the fields for that constructor
              NameSet           -- Set of all fields declared *in this module*;
                                -- used to suppress name-shadowing complaints
                                -- when using record wild cards
                                -- E.g.  let fld = e in C {..}
        -- This is used when dealing with ".." notation in record
        -- construction and pattern matching.
        -- The FieldEnv deals *only* with constructors defined in *this*
        -- module.  For imported modules, we get the same info from the
        -- TypeEnv

data IfGblEnv
  = IfGblEnv {
        -- The type environment for the module being compiled,
        -- in case the interface refers back to it via a reference that
        -- was originally a hi-boot file.
        -- We need the module name so we can test when it's appropriate
        -- to look in this env.
        if_rec_types :: Maybe (Module, IfG TypeEnv)
                -- Allows a read effect, so it can be in a mutable
                -- variable; c.f. handling the external package type env
                -- Nothing => interactive stuff, no loops possible
    }

data IfLclEnv
  = IfLclEnv {
        -- The module for the current IfaceDecl
        -- So if we see   f = \x -> x
        -- it means M.f = \x -> x, where M is the if_mod
        if_mod :: Module,

        -- The field is used only for error reporting
        -- if (say) there's a Lint error in it
        if_loc :: SDoc,
                -- Where the interface came from:
                --      .hi file, or GHCi state, or ext core
                -- plus which bit is currently being examined

        if_tv_env  :: UniqFM TyVar,     -- Nested tyvar bindings
                                        -- (and coercions)
        if_id_env  :: UniqFM Id         -- Nested id binding
    }

data TcLclEnv           -- Changes as we move inside an expression
                        -- Discarded after typecheck/rename; not passed on to desugarer
  = TcLclEnv {
        tcl_loc        :: SrcSpan,         -- Source span
        tcl_ctxt       :: [ErrCtxt],       -- Error context, innermost on top
        tcl_untch      :: Untouchables,    -- Birthplace for new unification variables

        tcl_th_ctxt    :: ThStage,         -- Template Haskell context
        tcl_th_bndrs   :: ThBindEnv,       -- Binding level of in-scope Names
                                           -- defined in this module (not imported)

        tcl_arrow_ctxt :: ArrowCtxt,       -- Arrow-notation context

        tcl_rdr :: LocalRdrEnv,         -- Local name envt
                -- Maintained during renaming, of course, but also during
                -- type checking, solely so that when renaming a Template-Haskell
                -- splice we have the right environment for the renamer.
                --
                --   Does *not* include global name envt; may shadow it
                --   Includes both ordinary variables and type variables;
                --   they are kept distinct because tyvar have a different
                --   occurrence contructor (Name.TvOcc)
                -- We still need the unsullied global name env so that
                --   we can look up record field names

        tcl_env  :: TcTypeEnv,    -- The local type environment:
                                  -- Ids and TyVars defined in this module

        tcl_bndrs :: [TcIdBinder],   -- Stack of locally-bound Ids, innermost on top
                                     -- Used only for error reporting

        tcl_tidy :: TidyEnv,      -- Used for tidying types; contains all
                                  -- in-scope type variables (but not term variables)

        tcl_tyvars :: TcRef TcTyVarSet, -- The "global tyvars"
                        -- Namely, the in-scope TyVars bound in tcl_env,
                        -- plus the tyvars mentioned in the types of Ids bound
                        -- in tcl_lenv.
                        -- Why mutable? see notes with tcGetGlobalTyVars

        tcl_lie  :: TcRef WantedConstraints,    -- Place to accumulate type constraints
        tcl_errs :: TcRef Messages              -- Place to accumulate errors
    }

type TcTypeEnv = NameEnv TcTyThing

type ThBindEnv = NameEnv (TopLevelFlag, ThLevel)
   -- Domain = all Ids bound in this module (ie not imported)
   -- The TopLevelFlag tells if the binding is syntactically top level.
   -- We need to know this, because the cross-stage persistence story allows
   -- cross-stage at arbitrary types if the Id is bound at top level.
   --
   -- Nota bene: a ThLevel of 'outerLevel' is *not* the same as being
   -- bound at top level!  See Note [Template Haskell levels] in TcSplice

data TcIdBinder
  = TcIdBndr
       TcId
       TopLevelFlag    -- Tells whether the bindind is syntactically top-level
                       -- (The monomorphic Ids for a recursive group count
                       --  as not-top-level for this purpose.)

{- Note [Given Insts]
   ~~~~~~~~~~~~~~~~~~
Because of GADTs, we have to pass inwards the Insts provided by type signatures
and existential contexts. Consider
        data T a where { T1 :: b -> b -> T [b] }
        f :: Eq a => T a -> Bool
        f (T1 x y) = [x]==[y]

The constructor T1 binds an existential variable 'b', and we need Eq [b].
Well, we have it, because Eq a refines to Eq [b], but we can only spot that if we
pass it inwards.

-}

---------------------------
-- Template Haskell stages and levels
---------------------------

data ThStage    -- See Note [Template Haskell state diagram] in TcSplice
  = Splice      -- Top-level splicing
                -- This code will be run *at compile time*;
                --   the result replaces the splice
                -- Binding level = 0
      Bool      -- True if in a typed splice, False otherwise

  | Comp        -- Ordinary Haskell code
                -- Binding level = 1

  | Brack                       -- Inside brackets
      ThStage                   --   Enclosing stage
      PendingStuff

data PendingStuff
  = RnPendingUntyped              -- Renaming the inside of an *untyped* bracket
      (TcRef [PendingRnSplice])   -- Pending splices in here

  | RnPendingTyped                -- Renaming the inside of a *typed* bracket

  | TcPending                     -- Typechecking the iniside of a typed bracket
      (TcRef [PendingTcSplice])   --   Accumulate pending splices here
      (TcRef WantedConstraints)   --     and type constraints here

topStage, topAnnStage, topSpliceStage :: ThStage
topStage       = Comp
topAnnStage    = Splice False
topSpliceStage = Splice False

instance Outputable ThStage where
   ppr (Splice _)  = text "Splice"
   ppr Comp        = text "Comp"
   ppr (Brack s _) = text "Brack" <> parens (ppr s)

type ThLevel = Int
    -- NB: see Note [Template Haskell levels] in TcSplice
    -- Incremented when going inside a bracket,
    -- decremented when going inside a splice
    -- NB: ThLevel is one greater than the 'n' in Fig 2 of the
    --     original "Template meta-programming for Haskell" paper

impLevel, outerLevel :: ThLevel
impLevel = 0    -- Imported things; they can be used inside a top level splice
outerLevel = 1  -- Things defined outside brackets

thLevel :: ThStage -> ThLevel
thLevel (Splice _)  = 0
thLevel Comp        = 1
thLevel (Brack s _) = thLevel s + 1

---------------------------
-- Arrow-notation context
---------------------------

{- Note [Escaping the arrow scope]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In arrow notation, a variable bound by a proc (or enclosed let/kappa)
is not in scope to the left of an arrow tail (-<) or the head of (|..|).
For example

        proc x -> (e1 -< e2)

Here, x is not in scope in e1, but it is in scope in e2.  This can get
a bit complicated:

        let x = 3 in
        proc y -> (proc z -> e1) -< e2

Here, x and z are in scope in e1, but y is not.

We implement this by
recording the environment when passing a proc (using newArrowScope),
and returning to that (using escapeArrowScope) on the left of -< and the
head of (|..|).

All this can be dealt with by the *renamer*; by the time we get to
the *type checker* we have sorted out the scopes
-}

data ArrowCtxt
  = NoArrowCtxt
  | ArrowCtxt (Env TcGblEnv TcLclEnv)

-- Record the current environment (outside a proc)
newArrowScope :: TcM a -> TcM a
newArrowScope
  = updEnv $ \env ->
        env { env_lcl = (env_lcl env) { tcl_arrow_ctxt = ArrowCtxt env } }

-- Return to the stored environment (from the enclosing proc)
escapeArrowScope :: TcM a -> TcM a
escapeArrowScope
  = updEnv $ \ env -> case tcl_arrow_ctxt (env_lcl env) of
        NoArrowCtxt -> env
        ArrowCtxt env' -> env'

---------------------------
-- TcTyThing
---------------------------

data TcTyThing
  = AGlobal TyThing             -- Used only in the return type of a lookup

  | ATcId   {           -- Ids defined in this module; may not be fully zonked
        tct_id     :: TcId,
        tct_closed :: TopLevelFlag }   -- See Note [Bindings with closed types]

  | ATyVar  Name TcTyVar        -- The type variable to which the lexically scoped type
                                -- variable is bound. We only need the Name
                                -- for error-message purposes; it is the corresponding
                                -- Name in the domain of the envt

  | AThing  TcKind   -- Used temporarily, during kind checking, for the
                     -- tycons and clases in this recursive group
                     -- Can be a mono-kind or a poly-kind; in TcTyClsDcls see
                     -- Note [Type checking recursive type and class declarations]

  | APromotionErr PromotionErr

data PromotionErr
  = TyConPE          -- TyCon used in a kind before we are ready
                     --     data T :: T -> * where ...
  | ClassPE          -- Ditto Class

  | FamDataConPE     -- Data constructor for a data family
                     -- See Note [AFamDataCon: not promoting data family constructors] in TcRnDriver

  | RecDataConPE     -- Data constructor in a recursive loop
                     -- See Note [ARecDataCon: recusion and promoting data constructors] in TcTyClsDecls
  | NoDataKinds      -- -XDataKinds not enabled

instance Outputable TcTyThing where     -- Debugging only
   ppr (AGlobal g)      = pprTyThing g
   ppr elt@(ATcId {})   = text "Identifier" <>
                          brackets (ppr (tct_id elt) <> dcolon
                                 <> ppr (varType (tct_id elt)) <> comma
                                 <+> ppr (tct_closed elt))
   ppr (ATyVar n tv)    = text "Type variable" <+> quotes (ppr n) <+> equals <+> ppr tv
   ppr (AThing k)       = text "AThing" <+> ppr k
   ppr (APromotionErr err) = text "APromotionErr" <+> ppr err

instance Outputable PromotionErr where
  ppr ClassPE      = text "ClassPE"
  ppr TyConPE      = text "TyConPE"
  ppr FamDataConPE = text "FamDataConPE"
  ppr RecDataConPE = text "RecDataConPE"
  ppr NoDataKinds  = text "NoDataKinds"

pprTcTyThingCategory :: TcTyThing -> SDoc
pprTcTyThingCategory (AGlobal thing)    = pprTyThingCategory thing
pprTcTyThingCategory (ATyVar {})        = ptext (sLit "Type variable")
pprTcTyThingCategory (ATcId {})         = ptext (sLit "Local identifier")
pprTcTyThingCategory (AThing {})        = ptext (sLit "Kinded thing")
pprTcTyThingCategory (APromotionErr pe) = pprPECategory pe

pprPECategory :: PromotionErr -> SDoc
pprPECategory ClassPE      = ptext (sLit "Class")
pprPECategory TyConPE      = ptext (sLit "Type constructor")
pprPECategory FamDataConPE = ptext (sLit "Data constructor")
pprPECategory RecDataConPE = ptext (sLit "Data constructor")
pprPECategory NoDataKinds  = ptext (sLit "Data constructor")

type ErrCtxt = (Bool, TidyEnv -> TcM (TidyEnv, MsgDoc))
        -- Monadic so that we have a chance
        -- to deal with bound type variables just before error
        -- message construction

        -- Bool:  True <=> this is a landmark context; do not
        --                 discard it when trimming for display

-- | 'ImportAvails' summarises what was imported from where, irrespective of
-- whether the imported things are actually used or not.  It is used:
--
--  * when processing the export list,
--
--  * when constructing usage info for the interface file,
--
--  * to identify the list of directly imported modules for initialisation
--    purposes and for optimised overlap checking of family instances,
--
--  * when figuring out what things are really unused
--
data ImportAvails
   = ImportAvails {
        imp_mods :: ImportedMods,
          --      = ModuleEnv [(ModuleName, Bool, SrcSpan, Bool)],
          -- ^ Domain is all directly-imported modules
          -- The 'ModuleName' is what the module was imported as, e.g. in
          -- @
          --     import Foo as Bar
          -- @
          -- it is @Bar@.
          --
          -- The 'Bool' means:
          --
          --  - @True@ => import was @import Foo ()@
          --
          --  - @False@ => import was some other form
          --
          -- Used
          --
          --   (a) to help construct the usage information in the interface
          --       file; if we import something we need to recompile if the
          --       export version changes
          --
          --   (b) to specify what child modules to initialise
          --
          -- We need a full ModuleEnv rather than a ModuleNameEnv here,
          -- because we might be importing modules of the same name from
          -- different packages. (currently not the case, but might be in the
          -- future).

        imp_dep_mods :: ModuleNameEnv (ModuleName, IsBootInterface),
          -- ^ Home-package modules needed by the module being compiled
          --
          -- It doesn't matter whether any of these dependencies
          -- are actually /used/ when compiling the module; they
          -- are listed if they are below it at all.  For
          -- example, suppose M imports A which imports X.  Then
          -- compiling M might not need to consult X.hi, but X
          -- is still listed in M's dependencies.

        imp_dep_pkgs :: [PackageId],
          -- ^ Packages needed by the module being compiled, whether directly,
          -- or via other modules in this package, or via modules imported
          -- from other packages.

        imp_trust_pkgs :: [PackageId],
          -- ^ This is strictly a subset of imp_dep_pkgs and records the
          -- packages the current module needs to trust for Safe Haskell
          -- compilation to succeed. A package is required to be trusted if
          -- we are dependent on a trustworthy module in that package.
          -- While perhaps making imp_dep_pkgs a tuple of (PackageId, Bool)
          -- where True for the bool indicates the package is required to be
          -- trusted is the more logical  design, doing so complicates a lot
          -- of code not concerned with Safe Haskell.
          -- See Note [RnNames . Tracking Trust Transitively]

        imp_trust_own_pkg :: Bool,
          -- ^ Do we require that our own package is trusted?
          -- This is to handle efficiently the case where a Safe module imports
          -- a Trustworthy module that resides in the same package as it.
          -- See Note [RnNames . Trust Own Package]

        imp_orphs :: [Module],
          -- ^ Orphan modules below us in the import tree (and maybe including
          -- us for imported modules)

        imp_finsts :: [Module]
          -- ^ Family instance modules below us in the import tree (and maybe
          -- including us for imported modules)
      }

mkModDeps :: [(ModuleName, IsBootInterface)]
          -> ModuleNameEnv (ModuleName, IsBootInterface)
mkModDeps deps = foldl add emptyUFM deps
               where
                 add env elt@(m,_) = addToUFM env m elt

emptyImportAvails :: ImportAvails
emptyImportAvails = ImportAvails { imp_mods          = emptyModuleEnv,
                                   imp_dep_mods      = emptyUFM,
                                   imp_dep_pkgs      = [],
                                   imp_trust_pkgs    = [],
                                   imp_trust_own_pkg = False,
                                   imp_orphs         = [],
                                   imp_finsts        = [] }

-- | Union two ImportAvails
--
-- This function is a key part of Import handling, basically
-- for each import we create a separate ImportAvails structure
-- and then union them all together with this function.
plusImportAvails ::  ImportAvails ->  ImportAvails ->  ImportAvails
plusImportAvails
  (ImportAvails { imp_mods = mods1,
                  imp_dep_mods = dmods1, imp_dep_pkgs = dpkgs1,
                  imp_trust_pkgs = tpkgs1, imp_trust_own_pkg = tself1,
                  imp_orphs = orphs1, imp_finsts = finsts1 })
  (ImportAvails { imp_mods = mods2,
                  imp_dep_mods = dmods2, imp_dep_pkgs = dpkgs2,
                  imp_trust_pkgs = tpkgs2, imp_trust_own_pkg = tself2,
                  imp_orphs = orphs2, imp_finsts = finsts2 })
  = ImportAvails { imp_mods          = plusModuleEnv_C (++) mods1 mods2,
                   imp_dep_mods      = plusUFM_C plus_mod_dep dmods1 dmods2,
                   imp_dep_pkgs      = dpkgs1 `unionLists` dpkgs2,
                   imp_trust_pkgs    = tpkgs1 `unionLists` tpkgs2,
                   imp_trust_own_pkg = tself1 || tself2,
                   imp_orphs         = orphs1 `unionLists` orphs2,
                   imp_finsts        = finsts1 `unionLists` finsts2 }
  where
    plus_mod_dep (m1, boot1) (m2, boot2)
        = WARN( not (m1 == m2), (ppr m1 <+> ppr m2) $$ (ppr boot1 <+> ppr boot2) )
                -- Check mod-names match
          (m1, boot1 && boot2) -- If either side can "see" a non-hi-boot interface, use that

data WhereFrom
  = ImportByUser IsBootInterface        -- Ordinary user import (perhaps {-# SOURCE #-})
  | ImportBySystem                      -- Non user import.
  | ImportByPlugin                      -- Importing a plugin;
                                        -- See Note [Care with plugin imports] in LoadIface

instance Outputable WhereFrom where
  ppr (ImportByUser is_boot) | is_boot     = ptext (sLit "{- SOURCE -}")
                             | otherwise   = empty
  ppr ImportBySystem                       = ptext (sLit "{- SYSTEM -}")
  ppr ImportByPlugin                       = ptext (sLit "{- PLUGIN -}")

-- The syntax of xi types:
-- xi ::= a | T xis | xis -> xis | ... | forall a. tau
-- Two important notes:
--      (i) No type families, unless we are under a ForAll
--      (ii) Note that xi types can contain unexpanded type synonyms;
--           however, the (transitive) expansions of those type synonyms
--           will not contain any type functions, unless we are under a ForAll.
-- We enforce the structure of Xi types when we flatten (TcCanonical)

type Xi = Type       -- In many comments, "xi" ranges over Xi

type Cts = Bag Ct

data Ct
  -- Atomic canonical constraints
  = CDictCan {  -- e.g.  Num xi
      cc_ev :: CtEvidence,   -- See Note [Ct/evidence invariant]
      cc_class  :: Class,
      cc_tyargs :: [Xi]
    }

  | CIrredEvCan {  -- These stand for yet-unusable predicates
      cc_ev :: CtEvidence   -- See Note [Ct/evidence invariant]
        -- The ctev_pred of the evidence is
        -- of form   (tv xi1 xi2 ... xin)
        --      or   (tv1 ~ ty2)   where the CTyEqCan  kind invariant fails
        --      or   (F tys ~ ty)  where the CFunEqCan kind invariant fails
        -- See Note [CIrredEvCan constraints]
    }

  | CTyEqCan {  -- tv ~ xi      (recall xi means function free)
       -- Invariant:
       --   * tv not in tvs(xi)   (occurs check)
       --   * typeKind xi `subKind` typeKind tv
       --       See Note [Kind orientation for CTyEqCan]
       --   * We prefer unification variables on the left *JUST* for efficiency
      cc_ev :: CtEvidence,    -- See Note [Ct/evidence invariant]
      cc_tyvar  :: TcTyVar,
      cc_rhs    :: Xi
    }

  | CFunEqCan {  -- F xis ~ xi
       -- Invariant: * isSynFamilyTyCon cc_fun
       --            * typeKind (F xis) `subKind` typeKind xi
       --       See Note [Kind orientation for CFunEqCan]
      cc_ev     :: CtEvidence,  -- See Note [Ct/evidence invariant]
      cc_fun    :: TyCon,       -- A type function
      cc_tyargs :: [Xi],        -- Either under-saturated or exactly saturated
      cc_rhs    :: Xi           --    *never* over-saturated (because if so
                                --    we should have decomposed)
    }

  | CNonCanonical {        -- See Note [NonCanonical Semantics]
      cc_ev  :: CtEvidence
    }

  | CHoleCan {             -- Treated as an "insoluble" constraint
                           -- See Note [Insoluble constraints]
      cc_ev  :: CtEvidence,
      cc_occ :: OccName    -- The name of this hole
    }

mkNonCanonical :: CtEvidence -> Ct
mkNonCanonical ev = CNonCanonical { cc_ev = ev }

mkNonCanonicalCt :: Ct -> Ct
mkNonCanonicalCt ct = CNonCanonical { cc_ev = cc_ev ct }

ctEvidence :: Ct -> CtEvidence
ctEvidence = cc_ev

ctLoc :: Ct -> CtLoc
ctLoc = ctev_loc . cc_ev

ctPred :: Ct -> PredType
-- See Note [Ct/evidence invariant]
ctPred ct = ctEvPred (cc_ev ct)

dropDerivedWC :: WantedConstraints -> WantedConstraints
-- See Note [Dropping derived constraints]
dropDerivedWC wc@(WC { wc_flat = flats, wc_insol = insols })
  = wc { wc_flat  = filterBag isWantedCt          flats
       , wc_insol = filterBag (not . isDerivedCt) insols  }
    -- Keep Givens from insols because they indicate unreachable code
    -- The implications are (recursively) already filtered

isWantedCt :: Ct -> Bool
isWantedCt = isWanted . cc_ev

isGivenCt :: Ct -> Bool
isGivenCt = isGiven . cc_ev

isDerivedCt :: Ct -> Bool
isDerivedCt = isDerived . cc_ev

isCTyEqCan :: Ct -> Bool
isCTyEqCan (CTyEqCan {})  = True
isCTyEqCan (CFunEqCan {}) = False
isCTyEqCan _              = False

isCDictCan_Maybe :: Ct -> Maybe Class
isCDictCan_Maybe (CDictCan {cc_class = cls })  = Just cls
isCDictCan_Maybe _              = Nothing

isCIrredEvCan :: Ct -> Bool
isCIrredEvCan (CIrredEvCan {}) = True
isCIrredEvCan _                = False

isCFunEqCan_maybe :: Ct -> Maybe (TyCon, [Type])
isCFunEqCan_maybe (CFunEqCan { cc_fun = tc, cc_tyargs = xis }) = Just (tc, xis)
isCFunEqCan_maybe _ = Nothing

isCFunEqCan :: Ct -> Bool
isCFunEqCan (CFunEqCan {}) = True
isCFunEqCan _ = False

isCNonCanonical :: Ct -> Bool
isCNonCanonical (CNonCanonical {}) = True
isCNonCanonical _ = False

isHoleCt:: Ct -> Bool
isHoleCt (CHoleCan {}) = True
isHoleCt _ = False

instance Outputable Ct where
  ppr ct = ppr (cc_ev ct) <+> parens (text ct_sort)
         where ct_sort = case ct of
                           CTyEqCan {}      -> "CTyEqCan"
                           CFunEqCan {}     -> "CFunEqCan"
                           CNonCanonical {} -> "CNonCanonical"
                           CDictCan {}      -> "CDictCan"
                           CIrredEvCan {}   -> "CIrredEvCan"
                           CHoleCan {}      -> "CHoleCan"

singleCt :: Ct -> Cts
singleCt = unitBag

andCts :: Cts -> Cts -> Cts
andCts = unionBags

listToCts :: [Ct] -> Cts
listToCts = listToBag

ctsElts :: Cts -> [Ct]
ctsElts = bagToList

extendCts :: Cts -> Ct -> Cts
extendCts = snocBag

extendCtsList :: Cts -> [Ct] -> Cts
extendCtsList cts xs | null xs   = cts
                     | otherwise = cts `unionBags` listToBag xs

andManyCts :: [Cts] -> Cts
andManyCts = unionManyBags

emptyCts :: Cts
emptyCts = emptyBag

isEmptyCts :: Cts -> Bool
isEmptyCts = isEmptyBag

data WantedConstraints
  = WC { wc_flat  :: Cts               -- Unsolved constraints, all wanted
       , wc_impl  :: Bag Implication
       , wc_insol :: Cts               -- Insoluble constraints, can be
                                       -- wanted, given, or derived
                                       -- See Note [Insoluble constraints]
    }

emptyWC :: WantedConstraints
emptyWC = WC { wc_flat = emptyBag, wc_impl = emptyBag, wc_insol = emptyBag }

mkFlatWC :: [Ct] -> WantedConstraints
mkFlatWC cts
  = WC { wc_flat = listToBag cts, wc_impl = emptyBag, wc_insol = emptyBag }

isEmptyWC :: WantedConstraints -> Bool
isEmptyWC (WC { wc_flat = f, wc_impl = i, wc_insol = n })
  = isEmptyBag f && isEmptyBag i && isEmptyBag n

insolubleWC :: WantedConstraints -> Bool
-- True if there are any insoluble constraints in the wanted bag
insolubleWC wc = not (isEmptyBag (wc_insol wc))
               || anyBag ic_insol (wc_impl wc)

andWC :: WantedConstraints -> WantedConstraints -> WantedConstraints
andWC (WC { wc_flat = f1, wc_impl = i1, wc_insol = n1 })
      (WC { wc_flat = f2, wc_impl = i2, wc_insol = n2 })
  = WC { wc_flat  = f1 `unionBags` f2
       , wc_impl  = i1 `unionBags` i2
       , wc_insol = n1 `unionBags` n2 }

unionsWC :: [WantedConstraints] -> WantedConstraints
unionsWC = foldr andWC emptyWC

addFlats :: WantedConstraints -> Bag Ct -> WantedConstraints
addFlats wc cts
  = wc { wc_flat = wc_flat wc `unionBags` cts }

addImplics :: WantedConstraints -> Bag Implication -> WantedConstraints
addImplics wc implic = wc { wc_impl = wc_impl wc `unionBags` implic }

addInsols :: WantedConstraints -> Bag Ct -> WantedConstraints
addInsols wc cts
  = wc { wc_insol = wc_insol wc `unionBags` cts }

instance Outputable WantedConstraints where
  ppr (WC {wc_flat = f, wc_impl = i, wc_insol = n})
   = ptext (sLit "WC") <+> braces (vcat
        [ if isEmptyBag f then empty else
          ptext (sLit "wc_flat =")  <+> pprBag ppr f
        , if isEmptyBag i then empty else
          ptext (sLit "wc_impl =")  <+> pprBag ppr i
        , if isEmptyBag n then empty else
          ptext (sLit "wc_insol =") <+> pprBag ppr n ])

pprBag :: (a -> SDoc) -> Bag a -> SDoc
pprBag pp b = foldrBag (($$) . pp) empty b

data Implication
  = Implic {
      ic_untch :: Untouchables, -- Untouchables: unification variables
                                -- free in the environment

      ic_skols  :: [TcTyVar],    -- Introduced skolems
      ic_info  :: SkolemInfo,    -- See Note [Skolems in an implication]
                                 -- See Note [Shadowing in a constraint]

      ic_given  :: [EvVar],      -- Given evidence variables
                                 --   (order does not matter)
                                 -- See Invariant (GivenInv) in TcType

      ic_fsks  :: [TcTyVar],     -- Extra flatten-skolems introduced by
                                 -- by flattening the givens
      ic_no_eqs :: Bool,         -- True  <=> ic_givens have no equalities, for sure
                                 -- False <=> ic_givens might have equalities

      ic_env   :: TcLclEnv,      -- Gives the source location and error context
                                 -- for the implicatdion, and hence for all the
                                 -- given evidence variables

      ic_wanted :: WantedConstraints,  -- The wanted
      ic_insol  :: Bool,               -- True iff insolubleWC ic_wanted is true

      ic_binds  :: EvBindsVar   -- Points to the place to fill in the
                                -- abstraction and bindings
    }

instance Outputable Implication where
  ppr (Implic { ic_untch = untch, ic_skols = skols, ic_fsks = fsks
              , ic_given = given, ic_no_eqs = no_eqs
              , ic_wanted = wanted
              , ic_binds = binds, ic_info = info })
   = ptext (sLit "Implic") <+> braces
     (sep [ ptext (sLit "Untouchables =") <+> ppr untch
          , ptext (sLit "Skolems =") <+> pprTvBndrs skols
          , ptext (sLit "Flatten-skolems =") <+> pprTvBndrs fsks
          , ptext (sLit "No-eqs =") <+> ppr no_eqs
          , ptext (sLit "Given =") <+> pprEvVars given
          , ptext (sLit "Wanted =") <+> ppr wanted
          , ptext (sLit "Binds =") <+> ppr binds
          , pprSkolInfo info ])

pprEvVars :: [EvVar] -> SDoc    -- Print with their types
pprEvVars ev_vars = vcat (map pprEvVarWithType ev_vars)

pprEvVarTheta :: [EvVar] -> SDoc
pprEvVarTheta ev_vars = pprTheta (map evVarPred ev_vars)

pprEvVarWithType :: EvVar -> SDoc
pprEvVarWithType v = ppr v <+> dcolon <+> pprType (evVarPred v)

pprWantedsWithLocs :: WantedConstraints -> SDoc
pprWantedsWithLocs wcs
  =  vcat [ pprBag ppr (wc_flat wcs)
          , pprBag ppr (wc_impl wcs)
          , pprBag ppr (wc_insol wcs) ]

data CtEvidence
  = CtGiven { ctev_pred :: TcPredType      -- See Note [Ct/evidence invariant]
            , ctev_evtm :: EvTerm          -- See Note [Evidence field of CtEvidence]
            , ctev_loc  :: CtLoc }
    -- Truly given, not depending on subgoals
    -- NB: Spontaneous unifications belong here

  | CtWanted { ctev_pred :: TcPredType     -- See Note [Ct/evidence invariant]
             , ctev_evar :: EvVar          -- See Note [Evidence field of CtEvidence]
             , ctev_loc  :: CtLoc }
    -- Wanted goal

  | CtDerived { ctev_pred :: TcPredType
              , ctev_loc  :: CtLoc }
    -- A goal that we don't really have to solve and can't immediately
    -- rewrite anything other than a derived (there's no evidence!)
    -- but if we do manage to solve it may help in solving other goals.

ctEvPred :: CtEvidence -> TcPredType
-- The predicate of a flavor
ctEvPred = ctev_pred

ctEvTerm :: CtEvidence -> EvTerm
ctEvTerm (CtGiven   { ctev_evtm = tm }) = tm
ctEvTerm (CtWanted  { ctev_evar = ev }) = EvId ev
ctEvTerm ctev@(CtDerived {}) = pprPanic "ctEvTerm: derived constraint cannot have id"
                                      (ppr ctev)

-- | Checks whether the evidence can be used to solve a goal with the given minimum depth
ctEvCheckDepth :: SubGoalDepth -> CtEvidence -> Bool
ctEvCheckDepth _      (CtGiven {})   = True -- Given evidence has infinite depth
ctEvCheckDepth min ev@(CtWanted {})  = min <= ctLocDepth (ctev_loc ev)
ctEvCheckDepth _   ev@(CtDerived {}) = pprPanic "ctEvCheckDepth: cannot consider derived evidence" (ppr ev)

ctEvId :: CtEvidence -> TcId
ctEvId (CtWanted  { ctev_evar = ev }) = ev
ctEvId ctev = pprPanic "ctEvId:" (ppr ctev)

instance Outputable CtEvidence where
  ppr fl = case fl of
             CtGiven {}   -> ptext (sLit "[G]") <+> ppr (ctev_evtm fl) <+> ppr_pty
             CtWanted {}  -> ptext (sLit "[W]") <+> ppr (ctev_evar fl) <+> ppr_pty
             CtDerived {} -> ptext (sLit "[D]") <+> text "_" <+> ppr_pty
         where ppr_pty = dcolon <+> ppr (ctEvPred fl)

isWanted :: CtEvidence -> Bool
isWanted (CtWanted {}) = True
isWanted _ = False

isGiven :: CtEvidence -> Bool
isGiven (CtGiven {})  = True
isGiven _ = False

isDerived :: CtEvidence -> Bool
isDerived (CtDerived {}) = True
isDerived _              = False

-----------------------------------------
canRewrite :: CtEvidence -> CtEvidence -> Bool
-- Very important function!
-- See Note [canRewrite and canRewriteOrSame]
canRewrite (CtGiven {})   _              = True
canRewrite (CtWanted {})  (CtDerived {}) = True
canRewrite (CtDerived {}) (CtDerived {}) = True  -- Derived can't solve wanted/given
canRewrite _ _ = False             -- No evidence for a derived, anyway

canRewriteOrSame :: CtEvidence -> CtEvidence -> Bool
canRewriteOrSame (CtGiven {})   _              = True
canRewriteOrSame (CtWanted {})  (CtWanted {})  = True
canRewriteOrSame (CtWanted {})  (CtDerived {}) = True
canRewriteOrSame (CtDerived {}) (CtDerived {}) = True
canRewriteOrSame _ _ = False

data SubGoalCounter = CountConstraints | CountTyFunApps

data SubGoalDepth  -- See Note [SubGoalDepth]
   = SubGoalDepth
         {-# UNPACK #-} !Int      -- Dictionary constraints
         {-# UNPACK #-} !Int      -- Type function reductions
  deriving (Eq, Ord)

instance Outputable SubGoalDepth where
 ppr (SubGoalDepth c f) =  angleBrackets $
        char 'C' <> colon <> int c <> comma <>
        char 'F' <> colon <> int f

initialSubGoalDepth :: SubGoalDepth
initialSubGoalDepth = SubGoalDepth 0 0

maxSubGoalDepth :: DynFlags -> SubGoalDepth
maxSubGoalDepth dflags = SubGoalDepth (ctxtStkDepth dflags) (tyFunStkDepth dflags)

bumpSubGoalDepth :: SubGoalCounter -> SubGoalDepth -> SubGoalDepth
bumpSubGoalDepth CountConstraints (SubGoalDepth c f) = SubGoalDepth (c+1) f
bumpSubGoalDepth CountTyFunApps   (SubGoalDepth c f) = SubGoalDepth c (f+1)

subGoalCounterValue :: SubGoalCounter -> SubGoalDepth -> Int
subGoalCounterValue CountConstraints (SubGoalDepth c _) = c
subGoalCounterValue CountTyFunApps   (SubGoalDepth _ f) = f

subGoalDepthExceeded :: SubGoalDepth -> SubGoalDepth -> Maybe SubGoalCounter
subGoalDepthExceeded (SubGoalDepth mc mf) (SubGoalDepth c f)
        | c > mc    = Just CountConstraints
        | f > mf    = Just CountTyFunApps
        | otherwise = Nothing

data CtLoc = CtLoc { ctl_origin :: CtOrigin
                   , ctl_env    :: TcLclEnv
                   , ctl_depth  :: !SubGoalDepth }
  -- The TcLclEnv includes particularly
  --    source location:  tcl_loc   :: SrcSpan
  --    context:          tcl_ctxt  :: [ErrCtxt]
  --    binder stack:     tcl_bndrs :: [TcIdBinders]

mkGivenLoc :: SkolemInfo -> TcLclEnv -> CtLoc
mkGivenLoc skol_info env = CtLoc { ctl_origin = GivenOrigin skol_info
                                 , ctl_env = env
                                 , ctl_depth = initialSubGoalDepth }

ctLocEnv :: CtLoc -> TcLclEnv
ctLocEnv = ctl_env

ctLocDepth :: CtLoc -> SubGoalDepth
ctLocDepth = ctl_depth

ctLocOrigin :: CtLoc -> CtOrigin
ctLocOrigin = ctl_origin

ctLocSpan :: CtLoc -> SrcSpan
ctLocSpan (CtLoc { ctl_env = lcl}) = tcl_loc lcl

bumpCtLocDepth :: SubGoalCounter -> CtLoc -> CtLoc
bumpCtLocDepth cnt loc@(CtLoc { ctl_depth = d }) = loc { ctl_depth = bumpSubGoalDepth cnt d }

setCtLocOrigin :: CtLoc -> CtOrigin -> CtLoc
setCtLocOrigin ctl orig = ctl { ctl_origin = orig }

setCtLocEnv :: CtLoc -> TcLclEnv -> CtLoc
setCtLocEnv ctl env = ctl { ctl_env = env }

pushErrCtxt :: CtOrigin -> ErrCtxt -> CtLoc -> CtLoc
pushErrCtxt o err loc@(CtLoc { ctl_env = lcl })
  = loc { ctl_origin = o, ctl_env = lcl { tcl_ctxt = err : tcl_ctxt lcl } }

pushErrCtxtSameOrigin :: ErrCtxt -> CtLoc -> CtLoc
-- Just add information w/o updating the origin!
pushErrCtxtSameOrigin err loc@(CtLoc { ctl_env = lcl })
  = loc { ctl_env = lcl { tcl_ctxt = err : tcl_ctxt lcl } }

pprArising :: CtOrigin -> SDoc
-- Used for the main, top-level error message
-- We've done special processing for TypeEq and FunDep origins
pprArising (TypeEqOrigin {}) = empty
pprArising FunDepOrigin      = empty
pprArising orig              = text "arising from" <+> ppr orig

pprArisingAt :: CtLoc -> SDoc
pprArisingAt (CtLoc { ctl_origin = o, ctl_env = lcl})
  = sep [ text "arising from" <+> ppr o
        , text "at" <+> ppr (tcl_loc lcl)]

-- SkolemInfo gives the origin of *given* constraints
--   a) type variables are skolemised
--   b) an implication constraint is generated
data SkolemInfo
  = SigSkol UserTypeCtxt        -- A skolem that is created by instantiating
            Type                -- a programmer-supplied type signature
                                -- Location of the binding site is on the TyVar

        -- The rest are for non-scoped skolems
  | ClsSkol Class       -- Bound at a class decl
  | InstSkol            -- Bound at an instance decl
  | DataSkol            -- Bound at a data type declaration
  | FamInstSkol         -- Bound at a family instance decl
  | PatSkol             -- An existential type variable bound by a pattern for
      ConLike           -- a data constructor with an existential type.
      (HsMatchContext Name)
             -- e.g.   data T = forall a. Eq a => MkT a
             --        f (MkT x) = ...
             -- The pattern MkT x will allocate an existential type
             -- variable for 'a'.

  | ArrowSkol           -- An arrow form (see TcArrows)

  | IPSkol [HsIPName]   -- Binding site of an implicit parameter

  | RuleSkol RuleName   -- The LHS of a RULE

  | InferSkol [(Name,TcType)]
                        -- We have inferred a type for these (mutually-recursivive)
                        -- polymorphic Ids, and are now checking that their RHS
                        -- constraints are satisfied.

  | BracketSkol         -- Template Haskell bracket

  | UnifyForAllSkol     -- We are unifying two for-all types
       [TcTyVar]        -- The instantiated skolem variables
       TcType           -- The instantiated type *inside* the forall

  | UnkSkol             -- Unhelpful info (until I improve it)

instance Outputable SkolemInfo where
  ppr = pprSkolInfo

pprSkolInfo :: SkolemInfo -> SDoc
-- Complete the sentence "is a rigid type variable bound by..."
pprSkolInfo (SigSkol (FunSigCtxt f) ty)
                            = hang (ptext (sLit "the type signature for"))
                                 2 (pprPrefixOcc f <+> dcolon <+> ppr ty)
pprSkolInfo (SigSkol cx ty) = hang (pprUserTypeCtxt cx <> colon)
                                 2 (ppr ty)
pprSkolInfo (IPSkol ips)    = ptext (sLit "the implicit-parameter bindings for")
                              <+> pprWithCommas ppr ips
pprSkolInfo (ClsSkol cls)   = ptext (sLit "the class declaration for") <+> quotes (ppr cls)
pprSkolInfo InstSkol        = ptext (sLit "the instance declaration")
pprSkolInfo DataSkol        = ptext (sLit "the data type declaration")
pprSkolInfo FamInstSkol     = ptext (sLit "the family instance declaration")
pprSkolInfo BracketSkol     = ptext (sLit "a Template Haskell bracket")
pprSkolInfo (RuleSkol name) = ptext (sLit "the RULE") <+> doubleQuotes (ftext name)
pprSkolInfo ArrowSkol       = ptext (sLit "the arrow form")
pprSkolInfo (PatSkol cl mc) = case cl of
    RealDataCon dc -> sep [ ptext (sLit "a pattern with constructor")
                          , nest 2 $ ppr dc <+> dcolon
                            <+> ppr (dataConUserType dc) <> comma
                          , ptext (sLit "in") <+> pprMatchContext mc ]
    PatSynCon ps -> sep [ ptext (sLit "a pattern with pattern synonym")
                        , nest 2 $ ppr ps <+> dcolon
                          <+> ppr (varType (patSynId ps)) <> comma
                        , ptext (sLit "in") <+> pprMatchContext mc ]
pprSkolInfo (InferSkol ids) = sep [ ptext (sLit "the inferred type of")
                                  , vcat [ ppr name <+> dcolon <+> ppr ty
                                         | (name,ty) <- ids ]]
pprSkolInfo (UnifyForAllSkol tvs ty) = ptext (sLit "the type") <+> ppr (mkForAllTys tvs ty)

-- UnkSkol
-- For type variables the others are dealt with by pprSkolTvBinding.
-- For Insts, these cases should not happen
pprSkolInfo UnkSkol = WARN( True, text "pprSkolInfo: UnkSkol" ) ptext (sLit "UnkSkol")

data CtOrigin
  = GivenOrigin SkolemInfo
  | FlatSkolOrigin              -- Flatten-skolems created for Givens
                                -- Note [When does an implication have given equalities?]
                                -- in TcSimplify

  -- All the others are for *wanted* constraints
  | OccurrenceOf Name           -- Occurrence of an overloaded identifier
  | AppOrigin                   -- An application of some kind

  | SpecPragOrigin Name         -- Specialisation pragma for identifier

  | TypeEqOrigin { uo_actual   :: TcType
                 , uo_expected :: TcType }
  | KindEqOrigin
      TcType TcType             -- A kind equality arising from unifying these two types
      CtOrigin                  -- originally arising from this

  | IPOccOrigin  HsIPName       -- Occurrence of an implicit parameter

  | LiteralOrigin (HsOverLit Name)      -- Occurrence of a literal
  | NegateOrigin                        -- Occurrence of syntactic negation

  | ArithSeqOrigin (ArithSeqInfo Name) -- [x..], [x..y] etc
  | PArrSeqOrigin  (ArithSeqInfo Name) -- [:x..y:] and [:x,y..z:]
  | SectionOrigin
  | TupleOrigin                        -- (..,..)
  | AmbigOrigin UserTypeCtxt    -- Will be FunSigCtxt, InstDeclCtxt, or SpecInstCtxt
  | ExprSigOrigin       -- e :: ty
  | PatSigOrigin        -- p :: ty
  | PatOrigin           -- Instantiating a polytyped pattern at a constructor
  | RecordUpdOrigin
  | ViewPatOrigin

  | ScOrigin            -- Typechecking superclasses of an instance declaration
  | DerivOrigin         -- Typechecking deriving
  | DerivOriginDC DataCon Int
                        -- Checking constraints arising from this data con and field index
  | DerivOriginCoerce Id Type Type
                        -- DerivOriginCoerce id ty1 ty2: Trying to coerce class method `id` from
                        -- `ty1` to `ty2`.
  | StandAloneDerivOrigin -- Typechecking stand-alone deriving
  | DefaultOrigin       -- Typechecking a default decl
  | DoOrigin            -- Arising from a do expression
  | MCompOrigin         -- Arising from a monad comprehension
  | IfOrigin            -- Arising from an if statement
  | ProcOrigin          -- Arising from a proc expression
  | AnnOrigin           -- An annotation
  | FunDepOrigin
  | HoleOrigin
  | UnboundOccurrenceOf RdrName
  | ListOrigin          -- An overloaded list

pprO :: CtOrigin -> SDoc
pprO (GivenOrigin sk)      = ppr sk
pprO FlatSkolOrigin        = ptext (sLit "a given flatten-skolem")
pprO (OccurrenceOf name)   = hsep [ptext (sLit "a use of"), quotes (ppr name)]
pprO AppOrigin             = ptext (sLit "an application")
pprO (SpecPragOrigin name) = hsep [ptext (sLit "a specialisation pragma for"), quotes (ppr name)]
pprO (IPOccOrigin name)    = hsep [ptext (sLit "a use of implicit parameter"), quotes (ppr name)]
pprO RecordUpdOrigin       = ptext (sLit "a record update")
pprO (AmbigOrigin ctxt)    = ptext (sLit "the ambiguity check for")
                             <+> case ctxt of
                                    FunSigCtxt name -> quotes (ppr name)
                                    InfSigCtxt name -> quotes (ppr name)
                                    _               -> pprUserTypeCtxt ctxt
pprO ExprSigOrigin         = ptext (sLit "an expression type signature")
pprO PatSigOrigin          = ptext (sLit "a pattern type signature")
pprO PatOrigin             = ptext (sLit "a pattern")
pprO ViewPatOrigin         = ptext (sLit "a view pattern")
pprO IfOrigin              = ptext (sLit "an if statement")
pprO (LiteralOrigin lit)   = hsep [ptext (sLit "the literal"), quotes (ppr lit)]
pprO (ArithSeqOrigin seq)  = hsep [ptext (sLit "the arithmetic sequence"), quotes (ppr seq)]
pprO (PArrSeqOrigin seq)   = hsep [ptext (sLit "the parallel array sequence"), quotes (ppr seq)]
pprO SectionOrigin         = ptext (sLit "an operator section")
pprO TupleOrigin           = ptext (sLit "a tuple")
pprO NegateOrigin          = ptext (sLit "a use of syntactic negation")
pprO ScOrigin              = ptext (sLit "the superclasses of an instance declaration")
pprO DerivOrigin           = ptext (sLit "the 'deriving' clause of a data type declaration")
pprO (DerivOriginDC dc n)  = hsep [ ptext (sLit "the"), speakNth n,
                                    ptext (sLit "field of"), quotes (ppr dc),
                                    parens (ptext (sLit "type") <+> quotes (ppr ty)) ]
    where ty = dataConOrigArgTys dc !! (n-1)
pprO (DerivOriginCoerce meth ty1 ty2)
                           = fsep [ ptext (sLit "the coercion"), ptext (sLit "of the method")
                                  , quotes (ppr meth), ptext (sLit "from type"), quotes (ppr ty1)
                                  , ptext (sLit "to type"), quotes (ppr ty2) ]
pprO StandAloneDerivOrigin = ptext (sLit "a 'deriving' declaration")
pprO DefaultOrigin         = ptext (sLit "a 'default' declaration")
pprO DoOrigin              = ptext (sLit "a do statement")
pprO MCompOrigin           = ptext (sLit "a statement in a monad comprehension")
pprO ProcOrigin            = ptext (sLit "a proc expression")
pprO (TypeEqOrigin t1 t2)  = ptext (sLit "a type equality") <+> sep [ppr t1, char '~', ppr t2]
pprO (KindEqOrigin t1 t2 _) = ptext (sLit "a kind equality arising from") <+> sep [ppr t1, char '~', ppr t2]
pprO AnnOrigin             = ptext (sLit "an annotation")
pprO FunDepOrigin          = ptext (sLit "a functional dependency")
pprO HoleOrigin            = ptext (sLit "a use of") <+> quotes (ptext $ sLit "_")
pprO (UnboundOccurrenceOf name) = hsep [ptext (sLit "an undeclared identifier"), quotes (ppr name)]
pprO ListOrigin            = ptext (sLit "an overloaded list")

instance Outputable CtOrigin where
  ppr = pprO