TcRnTypes.hs 144 KB
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{-
(c) The University of Glasgow 2006-2012
(c) The GRASP Project, Glasgow University, 1992-2002
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Various types used during typechecking, please see TcRnMonad as well for
operations on these types. You probably want to import it, instead of this
module.

All the monads exported here are built on top of the same IOEnv monad. The
monad functions like a Reader monad in the way it passes the environment
around. This is done to allow the environment to be manipulated in a stack
like fashion when entering expressions... ect.

For state that is global and should be returned at the end (e.g not part
of the stack mechanism), you should use an TcRef (= IORef) to store them.
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-}
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{-# LANGUAGE CPP, ExistentialQuantification, GeneralizedNewtypeDeriving,
             ViewPatterns #-}
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module TcRnTypes(
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        TcRnIf, TcRn, TcM, RnM, IfM, IfL, IfG, -- The monad is opaque outside this module
        TcRef,
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        -- The environment types
        Env(..),
        TcGblEnv(..), TcLclEnv(..),
        IfGblEnv(..), IfLclEnv(..),
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        tcVisibleOrphanMods,
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        -- Frontend types (shouldn't really be here)
        FrontendResult(..),

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        -- Renamer types
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        ErrCtxt, RecFieldEnv,
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        ImportAvails(..), emptyImportAvails, plusImportAvails,
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        WhereFrom(..), mkModDeps, modDepsElts,
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        -- Typechecker types
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        TcTypeEnv, TcIdBinderStack, TcIdBinder(..),
        TcTyThing(..), PromotionErr(..),
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        IdBindingInfo(..), ClosedTypeId, RhsNames,
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        IsGroupClosed(..),
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        SelfBootInfo(..),
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        pprTcTyThingCategory, pprPECategory, CompleteMatch(..),
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        -- Desugaring types
        DsM, DsLclEnv(..), DsGblEnv(..), PArrBuiltin(..),
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        DsMetaEnv, DsMetaVal(..), CompleteMatchMap,
        mkCompleteMatchMap, extendCompleteMatchMap,
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        -- Template Haskell
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        ThStage(..), SpliceType(..), PendingStuff(..),
        topStage, topAnnStage, topSpliceStage,
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        ThLevel, impLevel, outerLevel, thLevel,
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        ForeignSrcLang(..),
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        -- Arrows
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        ArrowCtxt(..),
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        -- TcSigInfo
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        TcSigFun, TcSigInfo(..), TcIdSigInfo(..),
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        TcIdSigInst(..), TcPatSynInfo(..),
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        isPartialSig, hasCompleteSig,
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        -- Canonical constraints
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        Xi, Ct(..), Cts, emptyCts, andCts, andManyCts, pprCts,
        singleCt, listToCts, ctsElts, consCts, snocCts, extendCtsList,
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        isEmptyCts, isCTyEqCan, isCFunEqCan,
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        isPendingScDict, superClassesMightHelp,
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        isCDictCan_Maybe, isCFunEqCan_maybe,
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        isCIrredEvCan, isCNonCanonical, isWantedCt, isDerivedCt,
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        isGivenCt, isHoleCt, isOutOfScopeCt, isExprHoleCt, isTypeHoleCt,
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        isUserTypeErrorCt, getUserTypeErrorMsg,
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        ctEvidence, ctLoc, setCtLoc, ctPred, ctFlavour, ctEqRel, ctOrigin,
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        mkTcEqPredLikeEv,
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        mkNonCanonical, mkNonCanonicalCt, mkGivens,
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        ctEvPred, ctEvLoc, ctEvOrigin, ctEvEqRel,
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        ctEvTerm, ctEvCoercion, ctEvId,
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        tyCoVarsOfCt, tyCoVarsOfCts,
        tyCoVarsOfCtList, tyCoVarsOfCtsList,
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        WantedConstraints(..), insolubleWC, emptyWC, isEmptyWC,
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        andWC, unionsWC, mkSimpleWC, mkImplicWC,
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        addInsols, getInsolubles, insolublesOnly, addSimples, addImplics,
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        tyCoVarsOfWC, dropDerivedWC, dropDerivedSimples, dropDerivedInsols,
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        tyCoVarsOfWCList, trulyInsoluble,
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        isDroppableDerivedLoc, isDroppableDerivedCt, insolubleImplic,
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        arisesFromGivens,
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        Implication(..), ImplicStatus(..), isInsolubleStatus, isSolvedStatus,
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        SubGoalDepth, initialSubGoalDepth, maxSubGoalDepth,
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        bumpSubGoalDepth, subGoalDepthExceeded,
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        CtLoc(..), ctLocSpan, ctLocEnv, ctLocLevel, ctLocOrigin,
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        ctLocTypeOrKind_maybe,
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        ctLocDepth, bumpCtLocDepth,
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        setCtLocOrigin, updateCtLocOrigin, setCtLocEnv, setCtLocSpan,
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        CtOrigin(..), exprCtOrigin, lexprCtOrigin, matchesCtOrigin, grhssCtOrigin,
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        isVisibleOrigin, toInvisibleOrigin,
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        TypeOrKind(..), isTypeLevel, isKindLevel,
        pprCtOrigin, pprCtLoc,
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        pushErrCtxt, pushErrCtxtSameOrigin,
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        SkolemInfo(..), pprSigSkolInfo, pprSkolInfo,
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        termEvidenceAllowed,
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        CtEvidence(..), TcEvDest(..),
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        mkKindLoc, toKindLoc, mkGivenLoc,
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        isWanted, isGiven, isDerived, isGivenOrWDeriv,
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        ctEvRole,
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        -- Constraint solver plugins
        TcPlugin(..), TcPluginResult(..), TcPluginSolver,
        TcPluginM, runTcPluginM, unsafeTcPluginTcM,
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        getEvBindsTcPluginM,
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        CtFlavour(..), ShadowInfo(..), ctEvFlavour,
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        CtFlavourRole, ctEvFlavourRole, ctFlavourRole,
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        eqCanRewriteFR, eqMayRewriteFR,
        eqCanDischarge,
        funEqCanDischarge, funEqCanDischargeF,
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        -- Pretty printing
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        pprEvVarTheta,
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        pprEvVars, pprEvVarWithType,
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        -- Misc other types
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        TcId, TcIdSet,
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        Hole(..), holeOcc,
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        NameShape(..),

        -- Role annotations
        RoleAnnotEnv, emptyRoleAnnotEnv, mkRoleAnnotEnv,
        lookupRoleAnnot, getRoleAnnots,
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  ) where

#include "HsVersions.h"

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import HsSyn
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import CoreSyn
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import HscTypes
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import TcEvidence
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import Type
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import Class    ( Class )
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import TyCon    ( TyCon, tyConKind )
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import Coercion ( Coercion, mkHoleCo )
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import ConLike  ( ConLike(..) )
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import DataCon  ( DataCon, dataConUserType, dataConOrigArgTys )
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import PatSyn   ( PatSyn, pprPatSynType )
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import Id       ( idType, idName )
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import FieldLabel ( FieldLabel )
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import TcType
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import Annotations
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import InstEnv
import FamInstEnv
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import PmExpr
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import IOEnv
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import RdrName
import Name
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import NameEnv
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import NameSet
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import Avail
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import Var
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import FV
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import VarEnv
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import Module
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import SrcLoc
import VarSet
import ErrUtils
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import UniqFM
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import UniqSupply
import BasicTypes
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import Bag
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import DynFlags
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import Outputable
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import ListSetOps
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import FastString
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import qualified GHC.LanguageExtensions as LangExt
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import Fingerprint
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import Util
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import PrelNames ( isUnboundName )
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import Control.Monad (ap, liftM, msum)
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import qualified Control.Monad.Fail as MonadFail
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import Data.Set      ( Set )
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import qualified Data.Set as S
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import Data.List ( sort )
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import Data.Map ( Map )
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import Data.Dynamic  ( Dynamic )
import Data.Typeable ( TypeRep )
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import Data.Maybe    ( mapMaybe )
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import GHCi.Message
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import GHCi.RemoteTypes
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import qualified Language.Haskell.TH as TH
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-- | A 'NameShape' is a substitution on 'Name's that can be used
-- to refine the identities of a hole while we are renaming interfaces
-- (see 'RnModIface').  Specifically, a 'NameShape' for
-- 'ns_module_name' @A@, defines a mapping from @{A.T}@
-- (for some 'OccName' @T@) to some arbitrary other 'Name'.
--
-- The most intruiging thing about a 'NameShape', however, is
-- how it's constructed.  A 'NameShape' is *implied* by the
-- exported 'AvailInfo's of the implementor of an interface:
-- if an implementor of signature @<H>@ exports @M.T@, you implicitly
-- define a substitution from @{H.T}@ to @M.T@.  So a 'NameShape'
-- is computed from the list of 'AvailInfo's that are exported
-- by the implementation of a module, or successively merged
-- together by the export lists of signatures which are joining
-- together.
--
-- It's not the most obvious way to go about doing this, but it
-- does seem to work!
--
-- NB: Can't boot this and put it in NameShape because then we
-- start pulling in too many DynFlags things.
data NameShape = NameShape {
        ns_mod_name :: ModuleName,
        ns_exports :: [AvailInfo],
        ns_map :: OccEnv Name
    }


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{-
************************************************************************
*                                                                      *
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               Standard monad definition for TcRn
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    All the combinators for the monad can be found in TcRnMonad
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*                                                                      *
************************************************************************
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The monad itself has to be defined here, because it is mentioned by ErrCtxt
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-}
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type TcRnIf a b = IOEnv (Env a b)
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type TcRn       = TcRnIf TcGblEnv TcLclEnv    -- Type inference
type IfM lcl    = TcRnIf IfGblEnv lcl         -- Iface stuff
type IfG        = IfM ()                      --    Top level
type IfL        = IfM IfLclEnv                --    Nested
type DsM        = TcRnIf DsGblEnv DsLclEnv    -- Desugaring

-- TcRn is the type-checking and renaming monad: the main monad that
-- most type-checking takes place in.  The global environment is
-- 'TcGblEnv', which tracks all of the top-level type-checking
-- information we've accumulated while checking a module, while the
-- local environment is 'TcLclEnv', which tracks local information as
-- we move inside expressions.
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-- | Historical "renaming monad" (now it's just 'TcRn').
type RnM  = TcRn

-- | Historical "type-checking monad" (now it's just 'TcRn').
type TcM  = TcRn
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-- We 'stack' these envs through the Reader like monad infrastructure
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-- as we move into an expression (although the change is focused in
-- the lcl type).
data Env gbl lcl
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  = Env {
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        env_top  :: HscEnv,  -- Top-level stuff that never changes
                             -- Includes all info about imported things
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        env_us   :: {-# UNPACK #-} !(IORef UniqSupply),
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                             -- Unique supply for local variables
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        env_gbl  :: gbl,     -- Info about things defined at the top level
                             -- of the module being compiled
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        env_lcl  :: lcl      -- Nested stuff; changes as we go into
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    }
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instance ContainsDynFlags (Env gbl lcl) where
    extractDynFlags env = hsc_dflags (env_top env)

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

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{-
************************************************************************
*                                                                      *
                The interface environments
              Used when dealing with IfaceDecls
*                                                                      *
************************************************************************
-}

data IfGblEnv
  = IfGblEnv {
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        -- Some information about where this environment came from;
        -- useful for debugging.
        if_doc :: SDoc,
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        -- 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.
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        -- See Note [Tying the knot] in TcIface
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        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
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        -- NB: This is a semantic module, see
        -- Note [Identity versus semantic module]
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        if_mod :: Module,

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        -- Whether or not the IfaceDecl came from a boot
        -- file or not; we'll use this to choose between
        -- NoUnfolding and BootUnfolding
        if_boot :: Bool,

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        -- 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

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        if_nsubst :: Maybe NameShape,

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        -- This field is used to make sure "implicit" declarations
        -- (anything that cannot be exported in mi_exports) get
        -- wired up correctly in typecheckIfacesForMerging.  Most
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        -- of the time it's @Nothing@.  See Note [Resolving never-exported Names in TcIface]
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        -- in TcIface.
        if_implicits_env :: Maybe TypeEnv,

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        if_tv_env  :: FastStringEnv TyVar,     -- Nested tyvar bindings
        if_id_env  :: FastStringEnv Id         -- Nested id binding
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    }

{-
************************************************************************
*                                                                      *
                Desugarer monad
*                                                                      *
************************************************************************

Now the mondo monad magic (yes, @DsM@ is a silly name)---carry around
a @UniqueSupply@ and some annotations, which
presumably include source-file location information:
-}

-- If '-XParallelArrays' is given, the desugarer populates this table with the corresponding
-- variables found in 'Data.Array.Parallel'.
--
data PArrBuiltin
        = PArrBuiltin
        { lengthPVar         :: Var     -- ^ lengthP
        , replicatePVar      :: Var     -- ^ replicateP
        , singletonPVar      :: Var     -- ^ singletonP
        , mapPVar            :: Var     -- ^ mapP
        , filterPVar         :: Var     -- ^ filterP
        , zipPVar            :: Var     -- ^ zipP
        , crossMapPVar       :: Var     -- ^ crossMapP
        , indexPVar          :: Var     -- ^ (!:)
        , emptyPVar          :: Var     -- ^ emptyP
        , appPVar            :: Var     -- ^ (+:+)
        , enumFromToPVar     :: Var     -- ^ enumFromToP
        , enumFromThenToPVar :: Var     -- ^ enumFromThenToP
        }

data DsGblEnv
        = DsGblEnv
        { ds_mod          :: Module             -- For SCC profiling
        , ds_fam_inst_env :: FamInstEnv         -- Like tcg_fam_inst_env
        , ds_unqual  :: PrintUnqualified
        , ds_msgs    :: IORef Messages          -- Warning messages
        , ds_if_env  :: (IfGblEnv, IfLclEnv)    -- Used for looking up global,
                                                -- possibly-imported things
        , ds_dph_env :: GlobalRdrEnv            -- exported entities of 'Data.Array.Parallel.Prim'
                                                -- iff '-fvectorise' flag was given as well as
                                                -- exported entities of 'Data.Array.Parallel' iff
                                                -- '-XParallelArrays' was given; otherwise, empty
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        , ds_parr_bi :: PArrBuiltin             -- desugarer names for '-XParallelArrays'
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        , ds_complete_matches :: CompleteMatchMap
           -- Additional complete pattern matches
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        }

instance ContainsModule DsGblEnv where
    extractModule = ds_mod

data DsLclEnv = DsLclEnv {
        dsl_meta    :: DsMetaEnv,        -- Template Haskell bindings
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        dsl_loc     :: RealSrcSpan,      -- To put in pattern-matching error msgs
        dsl_dicts   :: Bag EvVar,        -- Constraints from GADT pattern-matching
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        dsl_tm_cs   :: Bag SimpleEq,
        dsl_pm_iter :: IORef Int         -- no iterations for pmcheck
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     }

-- Inside [| |] brackets, the desugarer looks
-- up variables in the DsMetaEnv
type DsMetaEnv = NameEnv DsMetaVal

data DsMetaVal
   = DsBound Id         -- Bound by a pattern inside the [| |].
                        -- Will be dynamically alpha renamed.
                        -- The Id has type THSyntax.Var

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   | DsSplice (HsExpr GhcTc) -- These bindings are introduced by
                             -- the PendingSplices on a HsBracketOut
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{-
************************************************************************
*                                                                      *
                Global typechecker environment
*                                                                      *
************************************************************************
-}

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-- | 'FrontendResult' describes the result of running the
-- frontend of a Haskell module.  Usually, you'll get
-- a 'FrontendTypecheck', since running the frontend involves
-- typechecking a program, but for an hs-boot merge you'll
-- just get a ModIface, since no actual typechecking occurred.
--
-- This data type really should be in HscTypes, but it needs
-- to have a TcGblEnv which is only defined here.
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data FrontendResult
        = FrontendTypecheck TcGblEnv

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-- Note [Identity versus semantic module]
-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-- When typechecking an hsig file, it is convenient to keep track
-- of two different "this module" identifiers:
--
--      - The IDENTITY module is simply thisPackage + the module
--        name; i.e. it uniquely *identifies* the interface file
--        we're compiling.  For example, p[A=<A>]:A is an
--        identity module identifying the requirement named A
--        from library p.
--
--      - The SEMANTIC module, which is the actual module that
--        this signature is intended to represent (e.g. if
--        we have a identity module p[A=base:Data.IORef]:A,
--        then the semantic module is base:Data.IORef)
--
-- Which one should you use?
--
--      - In the desugarer and later phases of compilation,
--        identity and semantic modules coincide, since we never compile
--        signatures (we just generate blank object files for
--        hsig files.)
--
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--        A corrolary of this is that the following invariant holds at any point
--        past desugaring,
--
--            if I have a Module, this_mod, in hand representing the module
--            currently being compiled,
--            then moduleUnitId this_mod == thisPackage dflags
--
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--      - For any code involving Names, we want semantic modules.
--        See lookupIfaceTop in IfaceEnv, mkIface and addFingerprints
--        in MkIface, and tcLookupGlobal in TcEnv
--
--      - When reading interfaces, we want the identity module to
--        identify the specific interface we want (such interfaces
--        should never be loaded into the EPS).  However, if a
--        hole module <A> is requested, we look for A.hi
--        in the home library we are compiling.  (See LoadIface.)
--        Similarly, in RnNames we check for self-imports using
--        identity modules, to allow signatures to import their implementor.
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--
--      - For recompilation avoidance, you want the identity module,
--        since that will actually say the specific interface you
--        want to track (and recompile if it changes)
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-- | 'TcGblEnv' describes the top-level of the module at the
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-- point at which the typechecker is finished work.
-- It is this structure that is handed on to the desugarer
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-- For state that needs to be updated during the typechecking
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-- phase and returned at end, use a 'TcRef' (= 'IORef').
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data TcGblEnv
  = TcGblEnv {
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        tcg_mod     :: Module,         -- ^ Module being compiled
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        tcg_semantic_mod :: Module,    -- ^ If a signature, the backing module
            -- See also Note [Identity versus semantic module]
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        tcg_src     :: HscSource,
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          -- ^ What kind of module (regular Haskell, hs-boot, hsig)
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        tcg_rdr_env :: GlobalRdrEnv,   -- ^ Top level envt; used during renaming
        tcg_default :: Maybe [Type],
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          -- ^ Types used for defaulting. @Nothing@ => no @default@ decl
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        tcg_fix_env   :: FixityEnv,     -- ^ Just for things in this module
        tcg_field_env :: RecFieldEnv,   -- ^ Just for things in this module
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                                        -- See Note [The interactive package] in HscTypes
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        tcg_type_env :: TypeEnv,
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          -- ^ Global type env for the module we are compiling now.  All
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          -- 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)
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          --
          -- NB: for what "things in this module" means, see
          -- Note [The interactive package] in HscTypes
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        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;
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          -- Includes the dfuns in tcg_insts
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        tcg_fam_inst_env :: FamInstEnv, -- ^ Ditto for family instances
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        tcg_ann_env      :: AnnEnv,     -- ^ And for annotations
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        -- | Family instances we have to check for consistency.
        -- Invariant: each FamInst in the list's fi_fam matches the
        -- key of the entry in the 'NameEnv'.  This gets consumed
        -- by 'checkRecFamInstConsistency'.
        -- See Note [Don't check hs-boot type family instances too early]
        tcg_pending_fam_checks :: NameEnv [([FamInst], FamInstEnv)],

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                -- 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,
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          -- ^ Information about what was imported from where, including
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          -- things bound in this module. Also store Safe Haskell info
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          -- here about transitive trusted package requirements.
          --
          -- There are not many uses of this field, so you can grep for
          -- all them.
          --
          -- The ImportAvails records information about the following
          -- things:
          --
          --    1. All of the modules you directly imported (tcRnImports)
          --    2. The orphans (only!) of all imported modules in a GHCi
          --       session (runTcInteractive)
          --    3. The module that instantiated a signature
          --    4. Each of the signatures that merged in
          --
          -- It is used in the following ways:
          --    - imp_orphs is used to determine what orphan modules should be
          --      visible in the context (tcVisibleOrphanMods)
          --    - imp_finsts is used to determine what family instances should
          --      be visible (tcExtendLocalFamInstEnv)
          --    - To resolve the meaning of the export list of a module
          --      (tcRnExports)
          --    - imp_mods is used to compute usage info (mkIfaceTc, deSugar)
          --    - imp_trust_own_pkg is used for Safe Haskell in interfaces
          --      (mkIfaceTc, as well as in HscMain)
          --    - To create the Dependencies field in interface (mkDependencies)
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        tcg_dus       :: DefUses,   -- ^ What is defined in this module and what is used.
        tcg_used_gres :: TcRef [GlobalRdrElt],  -- ^ Records occurrences of imported entities
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          -- See Note [Tracking unused binding and imports]
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        tcg_keep :: TcRef NameSet,
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          -- ^ 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

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

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

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        tcg_th_top_level_locs :: TcRef (Set RealSrcSpan),
          -- ^ Locations of the top-level splices; used for providing details on
          -- scope in error messages for out-of-scope variables

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        tcg_dfun_n  :: TcRef OccSet,
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          -- ^ Allows us to choose unique DFun names.
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        tcg_merged :: [(Module, Fingerprint)],
          -- ^ The requirements we merged with; we always have to recompile
          -- if any of these changed.

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        -- The next fields accumulate the payload of the module
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        -- The binds, rules and foreign-decl fields are collected
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        -- initially in un-zonked form and are finally zonked in tcRnSrcDecls
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        tcg_rn_exports :: Maybe [(Located (IE GhcRn), Avails)],
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                -- Nothing <=> no explicit export list
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                -- Is always Nothing if we don't want to retain renamed
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                -- exports.
                -- If present contains each renamed export list item
                -- together with its exported names.
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        tcg_rn_imports :: [LImportDecl GhcRn],
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                -- Keep the renamed imports regardless.  They are not
                -- voluminous and are needed if you want to report unused imports
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        tcg_rn_decls :: Maybe (HsGroup GhcRn),
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          -- ^ Renamed decls, maybe.  @Nothing@ <=> Don't retain renamed
          -- decls.
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        tcg_dependent_files :: TcRef [FilePath], -- ^ dependencies from addDependentFile
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        tcg_th_topdecls :: TcRef [LHsDecl GhcPs],
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        -- ^ Top-level declarations from addTopDecls

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        tcg_th_foreign_files :: TcRef [(ForeignSrcLang, String)],
        -- ^ Foreign files emitted from TH.
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        tcg_th_topnames :: TcRef NameSet,
        -- ^ Exact names bound in top-level declarations in tcg_th_topdecls
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        tcg_th_modfinalizers :: TcRef [TcM ()],
        -- ^ Template Haskell module finalizers.
        --
        -- They are computations in the @TcM@ monad rather than @Q@ because we
        -- set them to use particular local environments.
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        tcg_th_state :: TcRef (Map TypeRep Dynamic),
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        tcg_th_remote_state :: TcRef (Maybe (ForeignRef (IORef QState))),
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        -- ^ Template Haskell state
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        tcg_ev_binds  :: Bag EvBind,        -- Top-level evidence bindings
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        -- Things defined in this module, or (in GHCi)
        -- in the declarations for a single GHCi command.
        -- For the latter, see Note [The interactive package] in HscTypes
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        tcg_tr_module :: Maybe Id,   -- Id for $trModule :: GHC.Types.Module
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                                             -- for which every module has a top-level defn
                                             -- except in GHCi in which case we have Nothing
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        tcg_binds     :: LHsBinds GhcTc,     -- Value bindings in this module
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        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
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        tcg_rules     :: [LRuleDecl GhcTc],  -- ...Rules
        tcg_fords     :: [LForeignDecl GhcTc], -- ...Foreign import & exports
        tcg_vects     :: [LVectDecl GhcTc],   -- ...Vectorisation declarations
        tcg_patsyns   :: [PatSyn],            -- ...Pattern synonyms
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        tcg_doc_hdr   :: Maybe LHsDocString, -- ^ Maybe Haddock header docs
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        tcg_hpc       :: AnyHpcUsage,        -- ^ @True@ if any part of the
                                             --  prog uses hpc instrumentation.

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        tcg_self_boot :: SelfBootInfo,       -- ^ Whether this module has a
                                             -- corresponding hi-boot file

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        tcg_main      :: Maybe Name,         -- ^ The Name of the main
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                                             -- function, if this module is
                                             -- the main module.
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        tcg_safeInfer :: TcRef (Bool, WarningMessages),
        -- ^ Has the typechecker inferred this module as -XSafe (Safe Haskell)
        -- See Note [Safe Haskell Overlapping Instances Implementation],
        -- although this is used for more than just that failure case.

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        tcg_tc_plugins :: [TcPluginSolver],
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        -- ^ A list of user-defined plugins for the constraint solver.
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        tcg_top_loc :: RealSrcSpan,
        -- ^ The RealSrcSpan this module came from

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        tcg_static_wc :: TcRef WantedConstraints,
          -- ^ Wanted constraints of static forms.
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        -- See Note [Constraints in static forms].
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        tcg_complete_matches :: [CompleteMatch]
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    }
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-- NB: topModIdentity, not topModSemantic!
-- Definition sites of orphan identities will be identity modules, not semantic
-- modules.
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-- Note [Constraints in static forms]
-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
--
-- When a static form produces constraints like
--
-- f :: StaticPtr (Bool -> String)
-- f = static show
--
-- we collect them in tcg_static_wc and resolve them at the end
-- of type checking. They need to be resolved separately because
-- we don't want to resolve them in the context of the enclosing
-- expression. Consider
--
-- g :: Show a => StaticPtr (a -> String)
-- g = static show
--
-- If the @Show a0@ constraint that the body of the static form produces was
-- resolved in the context of the enclosing expression, then the body of the
-- static form wouldn't be closed because the Show dictionary would come from
-- g's context instead of coming from the top level.

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tcVisibleOrphanMods :: TcGblEnv -> ModuleSet
tcVisibleOrphanMods tcg_env
    = mkModuleSet (tcg_mod tcg_env : imp_orphs (tcg_imports tcg_env))

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instance ContainsModule TcGblEnv where
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    extractModule env = tcg_semantic_mod env
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type RecFieldEnv = NameEnv [FieldLabel]
        -- Maps a constructor name *in this module*
        -- to the fields for that constructor.
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        -- 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
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data SelfBootInfo
  = NoSelfBoot    -- No corresponding hi-boot file
  | SelfBoot
       { sb_mds :: ModDetails   -- There was a hi-boot file,
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       , sb_tcs :: NameSet }    -- defining these TyCons,
-- What is sb_tcs used for?  See Note [Extra dependencies from .hs-boot files]
-- in RnSource

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{- Note [Tracking unused binding and imports]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We gather two sorts of usage information

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 * tcg_dus (defs/uses)
      Records *defined* Names (local, top-level)
          and *used*    Names (local or imported)

      Used (a) to report "defined but not used"
               (see RnNames.reportUnusedNames)
           (b) to generate version-tracking usage info in interface
               files (see MkIface.mkUsedNames)
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   This usage info is mainly gathered by the renamer's
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   gathering of free-variables

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 * tcg_used_gres
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      Used only to report unused import declarations
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      Records each *occurrence* an *imported* (not locally-defined) entity.
      The occurrence is recorded by keeping a GlobalRdrElt for it.
      These is not the GRE that is in the GlobalRdrEnv; rather it
      is recorded *after* the filtering done by pickGREs.  So it reflect
      /how that occurrence is in scope/.   See Note [GRE filtering] in
      RdrName.
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************************************************************************
*                                                                      *
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                The local typechecker environment
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*                                                                      *
************************************************************************
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Note [The Global-Env/Local-Env story]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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During type checking, we keep in the tcg_type_env
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        * All types and classes
        * All Ids derived from types and classes (constructors, selectors)
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At the end of type checking, we zonk the local bindings,
and as we do so we add to the tcg_type_env
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        * Locally defined top-level Ids
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Why?  Because they are now Ids not TcIds.  This final GlobalEnv is
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        a) fed back (via the knot) to typechecking the
           unfoldings of interface signatures
        b) used in the ModDetails of this module
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-}
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data TcLclEnv           -- Changes as we move inside an expression
                        -- Discarded after typecheck/rename; not passed on to desugarer
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  = TcLclEnv {
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        tcl_loc        :: RealSrcSpan,     -- Source span
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        tcl_ctxt       :: [ErrCtxt],       -- Error context, innermost on top
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        tcl_tclvl      :: TcLevel,         -- Birthplace for new unification variables
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        tcl_th_ctxt    :: ThStage,         -- Template Haskell context
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        tcl_th_bndrs   :: ThBindEnv,       -- and binder info
            -- The ThBindEnv records the TH binding level of in-scope Names
            -- defined in this module (not imported)
            -- We can't put this info in the TypeEnv because it's needed
            -- (and extended) in the renamer, for untyed splices
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        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
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                --   occurrence constructor (Name.TvOcc)
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                -- 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
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        tcl_bndrs :: TcIdBinderStack,   -- Used for reporting relevant bindings
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        tcl_tidy :: TidyEnv,      -- Used for tidying types; contains all
                                  -- in-scope type variables (but not term variables)
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        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.
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                        -- Why mutable? see notes with tcGetGlobalTyCoVars
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        tcl_lie  :: TcRef WantedConstraints,    -- Place to accumulate type constraints
        tcl_errs :: TcRef Messages              -- Place to accumulate errors
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    }

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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

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type TcTypeEnv = NameEnv TcTyThing
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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

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{- Note [Given Insts]
   ~~~~~~~~~~~~~~~~~~
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and existential contexts. Consider
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        data T a where { T1 :: b -> b -> T [b] }
        f :: Eq a => T a -> Bool
        f (T1 x y) = [x]==[y]
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The constructor T1 binds an existential variable 'b', and we need Eq [b].
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Well, we have it, because Eq a refines to Eq [b], but we can only spot that if we
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pass it inwards.

-}
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-- | Type alias for 'IORef'; the convention is we'll use this for mutable
-- bits of data in 'TcGblEnv' which are updated during typechecking and
-- returned at the end.
type TcRef a     = IORef a
-- ToDo: when should I refer to it as a 'TcId' instead of an 'Id'?
type TcId        = Id
type TcIdSet     = IdSet

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---------------------------
-- The TcIdBinderStack
---------------------------

type TcIdBinderStack = [TcIdBinder]
   -- This is a stack of locally-bound ids, innermost on top
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   -- Used only in error reporting (relevantBindings in TcError)
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   -- We can't use the tcl_env type environment, because it doesn't
   --   keep track of the nesting order
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data TcIdBinder
  = TcIdBndr
       TcId
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       TopLevelFlag    -- Tells whether the binding is syntactically top-level