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


Type checking of type signatures in interface files
-}

{-# LANGUAGE CPP #-}
{-# LANGUAGE NondecreasingIndentation #-}

module TcIface (
        tcLookupImported_maybe,
        importDecl, checkWiredInTyCon, tcHiBootIface, typecheckIface,
        typecheckIfacesForMerging,
        typecheckIfaceForInstantiate,
        tcIfaceDecl, tcIfaceInst, tcIfaceFamInst, tcIfaceRules,
        tcIfaceVectInfo, tcIfaceAnnotations, tcIfaceCompleteSigs,
        tcIfaceExpr,    -- Desired by HERMIT (Trac #7683)
        tcIfaceGlobal
 ) where

#include "HsVersions.h"

import TcTypeNats(typeNatCoAxiomRules)
import IfaceSyn
import LoadIface
import IfaceEnv
import BuildTyCl
import TcRnMonad
import TcType
import Type
import Coercion
import CoAxiom
import TyCoRep    -- needs to build types & coercions in a knot
import HscTypes
import Annotations
import InstEnv
import FamInstEnv
import CoreSyn
import CoreUtils
import CoreUnfold
import CoreLint
import MkCore
import Id
import MkId
import IdInfo
import Class
import TyCon
import ConLike
import DataCon
import PrelNames
import TysWiredIn
import Literal
import Var
import VarEnv
import VarSet
import Name
import NameEnv
import NameSet
import OccurAnal        ( occurAnalyseExpr )
import Demand
import Module
import UniqFM
import UniqSupply
import Outputable
import Maybes
import SrcLoc
import DynFlags
import Util
import FastString
import BasicTypes hiding ( SuccessFlag(..) )
import ListSetOps
import GHC.Fingerprint
import qualified BooleanFormula as BF

import Data.List
import Control.Monad
import qualified Data.Map as Map

{-
This module takes

        IfaceDecl -> TyThing
        IfaceType -> Type
        etc

An IfaceDecl is populated with RdrNames, and these are not renamed to
Names before typechecking, because there should be no scope errors etc.

        -- For (b) consider: f = \$(...h....)
        -- where h is imported, and calls f via an hi-boot file.
        -- This is bad!  But it is not seen as a staging error, because h
        -- is indeed imported.  We don't want the type-checker to black-hole
        -- when simplifying and compiling the splice!
        --
        -- Simple solution: discard any unfolding that mentions a variable
        -- bound in this module (and hence not yet processed).
        -- The discarding happens when forkM finds a type error.


************************************************************************
*                                                                      *
                Type-checking a complete interface
*                                                                      *
************************************************************************

Suppose we discover we don't need to recompile.  Then we must type
check the old interface file.  This is a bit different to the
incremental type checking we do as we suck in interface files.  Instead
we do things similarly as when we are typechecking source decls: we
bring into scope the type envt for the interface all at once, using a
knot.  Remember, the decls aren't necessarily in dependency order --
and even if they were, the type decls might be mutually recursive.

Note [Knot-tying typecheckIface]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Suppose we are typechecking an interface A.hi, and we come across
a Name for another entity defined in A.hi.  How do we get the
'TyCon', in this case?  There are three cases:

    1) tcHiBootIface in TcIface: We're typechecking an hi-boot file in
    preparation of checking if the hs file we're building
    is compatible.  In this case, we want all of the internal
    TyCons to MATCH the ones that we just constructed during
    typechecking: the knot is thus tied through if_rec_types.

    2) retypecheckLoop in GhcMake: We are retypechecking a
    mutually recursive cluster of hi files, in order to ensure
    that all of the references refer to each other correctly.
    In this case, the knot is tied through the HPT passed in,
    which contains all of the interfaces we are in the process
    of typechecking.

    3) genModDetails in HscMain: We are typechecking an
    old interface to generate the ModDetails.  In this case,
    we do the same thing as (2) and pass in an HPT with
    the HomeModInfo being generated to tie knots.

The upshot is that the CLIENT of this function is responsible
for making sure that the knot is tied correctly.  If you don't,
then you'll get a message saying that we couldn't load the
declaration you wanted.

BTW, in one-shot mode we never call typecheckIface; instead,
loadInterface handles type-checking interface.  In that case,
knots are tied through the EPS.  No problem!
-}

-- Clients of this function be careful, see Note [Knot-tying typecheckIface]
typecheckIface :: ModIface      -- Get the decls from here
               -> IfG ModDetails
typecheckIface iface
  = initIfaceLcl (mi_semantic_module iface) (text "typecheckIface") (mi_boot iface) $ do
        {       -- Get the right set of decls and rules.  If we are compiling without -O
                -- we discard pragmas before typechecking, so that we don't "see"
                -- information that we shouldn't.  From a versioning point of view
                -- It's not actually *wrong* to do so, but in fact GHCi is unable
                -- to handle unboxed tuples, so it must not see unfoldings.
          ignore_prags <- goptM Opt_IgnoreInterfacePragmas

                -- Typecheck the decls.  This is done lazily, so that the knot-tying
                -- within this single module works out right.  It's the callers
                -- job to make sure the knot is tied.
        ; names_w_things <- loadDecls ignore_prags (mi_decls iface)
        ; let type_env = mkNameEnv names_w_things

                -- Now do those rules, instances and annotations
        ; insts     <- mapM tcIfaceInst (mi_insts iface)
        ; fam_insts <- mapM tcIfaceFamInst (mi_fam_insts iface)
        ; rules     <- tcIfaceRules ignore_prags (mi_rules iface)
        ; anns      <- tcIfaceAnnotations (mi_anns iface)

                -- Vectorisation information
        ; vect_info <- tcIfaceVectInfo (mi_semantic_module iface) type_env (mi_vect_info iface)

                -- Exports
        ; exports <- ifaceExportNames (mi_exports iface)

                -- Complete Sigs
        ; complete_sigs <- tcIfaceCompleteSigs (mi_complete_sigs iface)

                -- Finished
        ; traceIf (vcat [text "Finished typechecking interface for" <+> ppr (mi_module iface),
                         -- Careful! If we tug on the TyThing thunks too early
                         -- we'll infinite loop with hs-boot.  See #10083 for
                         -- an example where this would cause non-termination.
                         text "Type envt:" <+> ppr (map fst names_w_things)])
        ; return $ ModDetails { md_types     = type_env
                              , md_insts     = insts
                              , md_fam_insts = fam_insts
                              , md_rules     = rules
                              , md_anns      = anns
                              , md_vect_info = vect_info
                              , md_exports   = exports
                              , md_complete_sigs = complete_sigs
                              }
    }

{-
************************************************************************
*                                                                      *
                Typechecking for merging
*                                                                      *
************************************************************************
-}

-- | Returns true if an 'IfaceDecl' is for @data T@ (an abstract data type)
isAbstractIfaceDecl :: IfaceDecl -> Bool
isAbstractIfaceDecl IfaceData{ ifCons = IfAbstractTyCon } = True
isAbstractIfaceDecl IfaceClass{ ifBody = IfAbstractClass } = True
isAbstractIfaceDecl IfaceFamily{ ifFamFlav = IfaceAbstractClosedSynFamilyTyCon } = True
isAbstractIfaceDecl _ = False

ifMaybeRoles :: IfaceDecl -> Maybe [Role]
ifMaybeRoles IfaceData    { ifRoles = rs } = Just rs
ifMaybeRoles IfaceSynonym { ifRoles = rs } = Just rs
ifMaybeRoles IfaceClass   { ifRoles = rs } = Just rs
ifMaybeRoles _ = Nothing

-- | Merge two 'IfaceDecl's together, preferring a non-abstract one.  If
-- both are non-abstract we pick one arbitrarily (and check for consistency
-- later.)
mergeIfaceDecl :: IfaceDecl -> IfaceDecl -> IfaceDecl
mergeIfaceDecl d1 d2
    | isAbstractIfaceDecl d1 = d2 `withRolesFrom` d1
    | isAbstractIfaceDecl d2 = d1 `withRolesFrom` d2
    | IfaceClass{ ifBody = IfConcreteClass { ifSigs = ops1, ifMinDef = bf1 } } <- d1
    , IfaceClass{ ifBody = IfConcreteClass { ifSigs = ops2, ifMinDef = bf2 } } <- d2
    = let ops = nameEnvElts $
                  plusNameEnv_C mergeIfaceClassOp
                    (mkNameEnv [ (n, op) | op@(IfaceClassOp n _ _) <- ops1 ])
                    (mkNameEnv [ (n, op) | op@(IfaceClassOp n _ _) <- ops2 ])
      in d1 { ifBody = (ifBody d1) {
                ifSigs  = ops,
                ifMinDef = BF.mkOr [noLoc bf1, noLoc bf2]
                }
            } `withRolesFrom` d2
    -- It doesn't matter; we'll check for consistency later when
    -- we merge, see 'mergeSignatures'
    | otherwise              = d1 `withRolesFrom` d2

-- Note [Role merging]
-- ~~~~~~~~~~~~~~~~~~~
-- First, why might it be necessary to do a non-trivial role
-- merge?  It may rescue a merge that might otherwise fail:
--
--      signature A where
--          type role T nominal representational
--          data T a b
--
--      signature A where
--          type role T representational nominal
--          data T a b
--
-- A module that defines T as representational in both arguments
-- would successfully fill both signatures, so it would be better
-- if we merged the roles of these types in some nontrivial
-- way.
--
-- However, we have to be very careful about how we go about
-- doing this, because role subtyping is *conditional* on
-- the supertype being NOT representationally injective, e.g.,
-- if we have instead:
--
--      signature A where
--          type role T nominal representational
--          data T a b = T a b
--
--      signature A where
--          type role T representational nominal
--          data T a b = T a b
--
-- Should we merge the definitions of T so that the roles are R/R (or N/N)?
-- Absolutely not: neither resulting type is a subtype of the original
-- types (see Note [Role subtyping]), because data is not representationally
-- injective.
--
-- Thus, merging only occurs when BOTH TyCons in question are
-- representationally injective.  If they're not, no merge.

withRolesFrom :: IfaceDecl -> IfaceDecl -> IfaceDecl
d1 `withRolesFrom` d2
    | Just roles1 <- ifMaybeRoles d1
    , Just roles2 <- ifMaybeRoles d2
    , not (isRepInjectiveIfaceDecl d1 || isRepInjectiveIfaceDecl d2)
    = d1 { ifRoles = mergeRoles roles1 roles2 }
    | otherwise = d1
  where
    mergeRoles roles1 roles2 = zipWith max roles1 roles2

isRepInjectiveIfaceDecl :: IfaceDecl -> Bool
isRepInjectiveIfaceDecl IfaceData{ ifCons = IfDataTyCon _ } = True
isRepInjectiveIfaceDecl IfaceFamily{ ifFamFlav = IfaceDataFamilyTyCon } = True
isRepInjectiveIfaceDecl _ = False

mergeIfaceClassOp :: IfaceClassOp -> IfaceClassOp -> IfaceClassOp
mergeIfaceClassOp op1@(IfaceClassOp _ _ (Just _)) _ = op1
mergeIfaceClassOp _ op2 = op2

-- | Merge two 'OccEnv's of 'IfaceDecl's by 'OccName'.
mergeIfaceDecls :: OccEnv IfaceDecl -> OccEnv IfaceDecl -> OccEnv IfaceDecl
mergeIfaceDecls = plusOccEnv_C mergeIfaceDecl

-- | This is a very interesting function.  Like typecheckIface, we want
-- to type check an interface file into a ModDetails.  However, the use-case
-- for these ModDetails is different: we want to compare all of the
-- ModDetails to ensure they define compatible declarations, and then
-- merge them together.  So in particular, we have to take a different
-- strategy for knot-tying: we first speculatively merge the declarations
-- to get the "base" truth for what we believe the types will be
-- (this is "type computation.")  Then we read everything in relative
-- to this truth and check for compatibility.
--
-- During the merge process, we may need to nondeterministically
-- pick a particular declaration to use, if multiple signatures define
-- the declaration ('mergeIfaceDecl').  If, for all choices, there
-- are no type synonym cycles in the resulting merged graph, then
-- we can show that our choice cannot matter. Consider the
-- set of entities which the declarations depend on: by assumption
-- of acyclicity, we can assume that these have already been shown to be equal
-- to each other (otherwise merging will fail).  Then it must
-- be the case that all candidate declarations here are type-equal
-- (the choice doesn't matter) or there is an inequality (in which
-- case merging will fail.)
--
-- Unfortunately, the choice can matter if there is a cycle.  Consider the
-- following merge:
--
--      signature H where { type A = C;  type B = A; data C      }
--      signature H where { type A = (); data B;     type C = B  }
--
-- If we pick @type A = C@ as our representative, there will be
-- a cycle and merging will fail. But if we pick @type A = ()@ as
-- our representative, no cycle occurs, and we instead conclude
-- that all of the types are unit.  So it seems that we either
-- (a) need a stronger acyclicity check which considers *all*
-- possible choices from a merge, or (b) we must find a selection
-- of declarations which is acyclic, and show that this is always
-- the "best" choice we could have made (ezyang conjectures this
-- is the case but does not have a proof).  For now this is
-- not implemented.
--
-- It's worth noting that at the moment, a data constructor and a
-- type synonym are never compatible.  Consider:
--
--      signature H where { type Int=C;         type B = Int; data C = Int}
--      signature H where { export Prelude.Int; data B;       type C = B; }
--
-- This will be rejected, because the reexported Int in the second
-- signature (a proper data type) is never considered equal to a
-- type synonym.  Perhaps this should be relaxed, where a type synonym
-- in a signature is considered implemented by a data type declaration
-- which matches the reference of the type synonym.
typecheckIfacesForMerging :: Module -> [ModIface] -> IORef TypeEnv -> IfM lcl (TypeEnv, [ModDetails])
typecheckIfacesForMerging mod ifaces tc_env_var =
  -- cannot be boot (False)
  initIfaceLcl mod (text "typecheckIfacesForMerging") False $ do
    ignore_prags <- goptM Opt_IgnoreInterfacePragmas
    -- Build the initial environment
    -- NB: Don't include dfuns here, because we don't want to
    -- serialize them out.  See Note [rnIfaceNeverExported] in RnModIface
    -- NB: But coercions are OK, because they will have the right OccName.
    let mk_decl_env decls
            = mkOccEnv [ (getOccName decl, decl)
                       | decl <- decls
                       , case decl of
                            IfaceId { ifIdDetails = IfDFunId } -> False -- exclude DFuns
                            _ -> True ]
        decl_envs = map (mk_decl_env . map snd . mi_decls) ifaces
                        :: [OccEnv IfaceDecl]
        decl_env = foldl' mergeIfaceDecls emptyOccEnv decl_envs
                        ::  OccEnv IfaceDecl
    -- TODO: change loadDecls to accept w/o Fingerprint
    names_w_things <- loadDecls ignore_prags (map (\x -> (fingerprint0, x))
                                                  (occEnvElts decl_env))
    let global_type_env = mkNameEnv names_w_things
    writeMutVar tc_env_var global_type_env

    -- OK, now typecheck each ModIface using this environment
    details <- forM ifaces $ \iface -> do
        -- See Note [Resolving never-exported Names in TcIface]
        type_env <- fixM $ \type_env -> do
            setImplicitEnvM type_env $ do
                decls <- loadDecls ignore_prags (mi_decls iface)
                return (mkNameEnv decls)
        -- But note that we use this type_env to typecheck references to DFun
        -- in 'IfaceInst'
        setImplicitEnvM type_env $ do
        insts     <- mapM tcIfaceInst (mi_insts iface)
        fam_insts <- mapM tcIfaceFamInst (mi_fam_insts iface)
        rules     <- tcIfaceRules ignore_prags (mi_rules iface)
        anns      <- tcIfaceAnnotations (mi_anns iface)
        vect_info <- tcIfaceVectInfo (mi_semantic_module iface) type_env (mi_vect_info iface)
        exports   <- ifaceExportNames (mi_exports iface)
        complete_sigs <- tcIfaceCompleteSigs (mi_complete_sigs iface)
        return $ ModDetails { md_types     = type_env
                            , md_insts     = insts
                            , md_fam_insts = fam_insts
                            , md_rules     = rules
                            , md_anns      = anns
                            , md_vect_info = vect_info
                            , md_exports   = exports
                            , md_complete_sigs = complete_sigs
                            }
    return (global_type_env, details)

-- | Typecheck a signature 'ModIface' under the assumption that we have
-- instantiated it under some implementation (recorded in 'mi_semantic_module')
-- and want to check if the implementation fills the signature.
--
-- This needs to operate slightly differently than 'typecheckIface'
-- because (1) we have a 'NameShape', from the exports of the
-- implementing module, which we will use to give our top-level
-- declarations the correct 'Name's even when the implementor
-- provided them with a reexport, and (2) we have to deal with
-- DFun silliness (see Note [rnIfaceNeverExported])
typecheckIfaceForInstantiate :: NameShape -> ModIface -> IfM lcl ModDetails
typecheckIfaceForInstantiate nsubst iface =
  initIfaceLclWithSubst (mi_semantic_module iface)
                        (text "typecheckIfaceForInstantiate")
                        (mi_boot iface) nsubst $ do
    ignore_prags <- goptM Opt_IgnoreInterfacePragmas
    -- See Note [Resolving never-exported Names in TcIface]
    type_env <- fixM $ \type_env -> do
        setImplicitEnvM type_env $ do
            decls     <- loadDecls ignore_prags (mi_decls iface)
            return (mkNameEnv decls)
    -- See Note [rnIfaceNeverExported]
    setImplicitEnvM type_env $ do
    insts     <- mapM tcIfaceInst (mi_insts iface)
    fam_insts <- mapM tcIfaceFamInst (mi_fam_insts iface)
    rules     <- tcIfaceRules ignore_prags (mi_rules iface)
    anns      <- tcIfaceAnnotations (mi_anns iface)
    vect_info <- tcIfaceVectInfo (mi_semantic_module iface) type_env (mi_vect_info iface)
    exports   <- ifaceExportNames (mi_exports iface)
    complete_sigs <- tcIfaceCompleteSigs (mi_complete_sigs iface)
    return $ ModDetails { md_types     = type_env
                        , md_insts     = insts
                        , md_fam_insts = fam_insts
                        , md_rules     = rules
                        , md_anns      = anns
                        , md_vect_info = vect_info
                        , md_exports   = exports
                        , md_complete_sigs = complete_sigs
                        }

-- Note [Resolving never-exported Names in TcIface]
-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-- For the high-level overview, see
-- Note [Handling never-exported TyThings under Backpack]
--
-- As described in 'typecheckIfacesForMerging', the splendid innovation
-- of signature merging is to rewrite all Names in each of the signatures
-- we are merging together to a pre-merged structure; this is the key
-- ingredient that lets us solve some problems when merging type
-- synonyms.
--
-- However, when a 'Name' refers to a NON-exported entity, as is the
-- case with the DFun of a ClsInst, or a CoAxiom of a type family,
-- this strategy causes problems: if we pick one and rewrite all
-- references to a shared 'Name', we will accidentally fail to check
-- if the DFun or CoAxioms are compatible, as they will never be
-- checked--only exported entities are checked for compatibility,
-- and a non-exported TyThing is checked WHEN we are checking the
-- ClsInst or type family for compatibility in checkBootDeclM.
-- By virtue of the fact that everything's been pointed to the merged
-- declaration, you'll never notice there's a difference even if there
-- is one.
--
-- Fortunately, there are only a few places in the interface declarations
-- where this can occur, so we replace those calls with 'tcIfaceImplicit',
-- which will consult a local TypeEnv that records any never-exported
-- TyThings which we should wire up with.
--
-- Note that we actually knot-tie this local TypeEnv (the 'fixM'), because a
-- type family can refer to a coercion axiom, all of which are done in one go
-- when we typecheck 'mi_decls'.  An alternate strategy would be to typecheck
-- coercions first before type families, but that seemed more fragile.
--

{-
************************************************************************
*                                                                      *
                Type and class declarations
*                                                                      *
************************************************************************
-}

tcHiBootIface :: HscSource -> Module -> TcRn SelfBootInfo
-- Load the hi-boot iface for the module being compiled,
-- if it indeed exists in the transitive closure of imports
-- Return the ModDetails; Nothing if no hi-boot iface
tcHiBootIface hsc_src mod
  | HsBootFile <- hsc_src            -- Already compiling a hs-boot file
  = return NoSelfBoot
  | otherwise
  = do  { traceIf (text "loadHiBootInterface" <+> ppr mod)

        ; mode <- getGhcMode
        ; if not (isOneShot mode)
                -- In --make and interactive mode, if this module has an hs-boot file
                -- we'll have compiled it already, and it'll be in the HPT
                --
                -- We check wheher the interface is a *boot* interface.
                -- It can happen (when using GHC from Visual Studio) that we
                -- compile a module in TypecheckOnly mode, with a stable,
                -- fully-populated HPT.  In that case the boot interface isn't there
                -- (it's been replaced by the mother module) so we can't check it.
                -- And that's fine, because if M's ModInfo is in the HPT, then
                -- it's been compiled once, and we don't need to check the boot iface
          then do { hpt <- getHpt
                 ; case lookupHpt hpt (moduleName mod) of
                      Just info | mi_boot (hm_iface info)
                                -> mkSelfBootInfo (hm_iface info) (hm_details info)
                      _ -> return NoSelfBoot }
          else do

        -- OK, so we're in one-shot mode.
        -- Re #9245, we always check if there is an hi-boot interface
        -- to check consistency against, rather than just when we notice
        -- that an hi-boot is necessary due to a circular import.
        { read_result <- findAndReadIface
                                need (fst (splitModuleInsts mod)) mod
                                True    -- Hi-boot file

        ; case read_result of {
            Succeeded (iface, _path) -> do { tc_iface <- initIfaceTcRn $ typecheckIface iface
                                           ; mkSelfBootInfo iface tc_iface } ;
            Failed err               ->

        -- There was no hi-boot file. But if there is circularity in
        -- the module graph, there really should have been one.
        -- Since we've read all the direct imports by now,
        -- eps_is_boot will record if any of our imports mention the
        -- current module, which either means a module loop (not
        -- a SOURCE import) or that our hi-boot file has mysteriously
        -- disappeared.
    do  { eps <- getEps
        ; case lookupUFM (eps_is_boot eps) (moduleName mod) of
            Nothing -> return NoSelfBoot -- The typical case

            Just (_, False) -> failWithTc moduleLoop
                -- Someone below us imported us!
                -- This is a loop with no hi-boot in the way

            Just (_mod, True) -> failWithTc (elaborate err)
                -- The hi-boot file has mysteriously disappeared.
    }}}}
  where
    need = text "Need the hi-boot interface for" <+> ppr mod
                 <+> text "to compare against the Real Thing"

    moduleLoop = text "Circular imports: module" <+> quotes (ppr mod)
                     <+> text "depends on itself"

    elaborate err = hang (text "Could not find hi-boot interface for" <+>
                          quotes (ppr mod) <> colon) 4 err


mkSelfBootInfo :: ModIface -> ModDetails -> TcRn SelfBootInfo
mkSelfBootInfo iface mds
  = do -- NB: This is computed DIRECTLY from the ModIface rather
       -- than from the ModDetails, so that we can query 'sb_tcs'
       -- WITHOUT forcing the contents of the interface.
       let tcs = map ifName
                 . filter isIfaceTyCon
                 . map snd
                 $ mi_decls iface
       return $ SelfBoot { sb_mds = mds
                         , sb_tcs = mkNameSet tcs }
  where
    -- | Retuerns @True@ if, when you call 'tcIfaceDecl' on
    -- this 'IfaceDecl', an ATyCon would be returned.
    -- NB: This code assumes that a TyCon cannot be implicit.
    isIfaceTyCon IfaceId{}      = False
    isIfaceTyCon IfaceData{}    = True
    isIfaceTyCon IfaceSynonym{} = True
    isIfaceTyCon IfaceFamily{}  = True
    isIfaceTyCon IfaceClass{}   = True
    isIfaceTyCon IfaceAxiom{}   = False
    isIfaceTyCon IfacePatSyn{}  = False

{-
************************************************************************
*                                                                      *
                Type and class declarations
*                                                                      *
************************************************************************

When typechecking a data type decl, we *lazily* (via forkM) typecheck
the constructor argument types.  This is in the hope that we may never
poke on those argument types, and hence may never need to load the
interface files for types mentioned in the arg types.

E.g.
        data Foo.S = MkS Baz.T
Maybe we can get away without even loading the interface for Baz!

This is not just a performance thing.  Suppose we have
        data Foo.S = MkS Baz.T
        data Baz.T = MkT Foo.S
(in different interface files, of course).
Now, first we load and typecheck Foo.S, and add it to the type envt.
If we do explore MkS's argument, we'll load and typecheck Baz.T.
If we explore MkT's argument we'll find Foo.S already in the envt.

If we typechecked constructor args eagerly, when loading Foo.S we'd try to
typecheck the type Baz.T.  So we'd fault in Baz.T... and then need Foo.S...
which isn't done yet.

All very cunning. However, there is a rather subtle gotcha which bit
me when developing this stuff.  When we typecheck the decl for S, we
extend the type envt with S, MkS, and all its implicit Ids.  Suppose
(a bug, but it happened) that the list of implicit Ids depended in
turn on the constructor arg types.  Then the following sequence of
events takes place:
        * we build a thunk <t> for the constructor arg tys
        * we build a thunk for the extended type environment (depends on <t>)
        * we write the extended type envt into the global EPS mutvar

Now we look something up in the type envt
        * that pulls on <t>
        * which reads the global type envt out of the global EPS mutvar
        * but that depends in turn on <t>

It's subtle, because, it'd work fine if we typechecked the constructor args
eagerly -- they don't need the extended type envt.  They just get the extended
type envt by accident, because they look at it later.

What this means is that the implicitTyThings MUST NOT DEPEND on any of
the forkM stuff.
-}

tcIfaceDecl :: Bool     -- ^ True <=> discard IdInfo on IfaceId bindings
            -> IfaceDecl
            -> IfL TyThing
tcIfaceDecl = tc_iface_decl Nothing

tc_iface_decl :: Maybe Class  -- ^ For associated type/data family declarations
              -> Bool         -- ^ True <=> discard IdInfo on IfaceId bindings
              -> IfaceDecl
              -> IfL TyThing
tc_iface_decl _ ignore_prags (IfaceId {ifName = name, ifType = iface_type,
                                       ifIdDetails = details, ifIdInfo = info})
  = do  { ty <- tcIfaceType iface_type
        ; details <- tcIdDetails ty details
        ; info <- tcIdInfo ignore_prags name ty info
        ; return (AnId (mkGlobalId details name ty info)) }

tc_iface_decl _ _ (IfaceData {ifName = tc_name,
                          ifCType = cType,
                          ifBinders = binders,
                          ifResKind = res_kind,
                          ifRoles = roles,
                          ifCtxt = ctxt, ifGadtSyntax = gadt_syn,
                          ifCons = rdr_cons,
                          ifParent = mb_parent })
  = bindIfaceTyConBinders_AT binders $ \ binders' -> do
    { res_kind' <- tcIfaceType res_kind

    ; tycon <- fixM $ \ tycon -> do
            { stupid_theta <- tcIfaceCtxt ctxt
            ; parent' <- tc_parent tc_name mb_parent
            ; cons <- tcIfaceDataCons tc_name tycon binders' rdr_cons
            ; return (mkAlgTyCon tc_name binders' res_kind'
                                 roles cType stupid_theta
                                 cons parent' gadt_syn) }
    ; traceIf (text "tcIfaceDecl4" <+> ppr tycon)
    ; return (ATyCon tycon) }
  where
    tc_parent :: Name -> IfaceTyConParent -> IfL AlgTyConFlav
    tc_parent tc_name IfNoParent
      = do { tc_rep_name <- newTyConRepName tc_name
           ; return (VanillaAlgTyCon tc_rep_name) }
    tc_parent _ (IfDataInstance ax_name _ arg_tys)
      = do { ax <- tcIfaceCoAxiom ax_name
           ; let fam_tc  = coAxiomTyCon ax
                 ax_unbr = toUnbranchedAxiom ax
           ; lhs_tys <- tcIfaceTcArgs arg_tys
           ; return (DataFamInstTyCon ax_unbr fam_tc lhs_tys) }

tc_iface_decl _ _ (IfaceSynonym {ifName = tc_name,
                                      ifRoles = roles,
                                      ifSynRhs = rhs_ty,
                                      ifBinders = binders,
                                      ifResKind = res_kind })
   = bindIfaceTyConBinders_AT binders $ \ binders' -> do
     { res_kind' <- tcIfaceType res_kind     -- Note [Synonym kind loop]
     ; rhs      <- forkM (mk_doc tc_name) $
                   tcIfaceType rhs_ty
     ; let tycon = buildSynTyCon tc_name binders' res_kind' roles rhs
     ; return (ATyCon tycon) }
   where
     mk_doc n = text "Type synonym" <+> ppr n

tc_iface_decl parent _ (IfaceFamily {ifName = tc_name,
                                     ifFamFlav = fam_flav,
                                     ifBinders = binders,
                                     ifResKind = res_kind,
                                     ifResVar = res, ifFamInj = inj })
   = bindIfaceTyConBinders_AT binders $ \ binders' -> do
     { res_kind' <- tcIfaceType res_kind    -- Note [Synonym kind loop]
     ; rhs      <- forkM (mk_doc tc_name) $
                   tc_fam_flav tc_name fam_flav
     ; res_name <- traverse (newIfaceName . mkTyVarOccFS) res
     ; let tycon = mkFamilyTyCon tc_name binders' res_kind' res_name rhs parent inj
     ; return (ATyCon tycon) }
   where
     mk_doc n = text "Type synonym" <+> ppr n

     tc_fam_flav :: Name -> IfaceFamTyConFlav -> IfL FamTyConFlav
     tc_fam_flav tc_name IfaceDataFamilyTyCon
       = do { tc_rep_name <- newTyConRepName tc_name
            ; return (DataFamilyTyCon tc_rep_name) }
     tc_fam_flav _ IfaceOpenSynFamilyTyCon= return OpenSynFamilyTyCon
     tc_fam_flav _ (IfaceClosedSynFamilyTyCon mb_ax_name_branches)
       = do { ax <- traverse (tcIfaceCoAxiom . fst) mb_ax_name_branches
            ; return (ClosedSynFamilyTyCon ax) }
     tc_fam_flav _ IfaceAbstractClosedSynFamilyTyCon
         = return AbstractClosedSynFamilyTyCon
     tc_fam_flav _ IfaceBuiltInSynFamTyCon
         = pprPanic "tc_iface_decl"
                    (text "IfaceBuiltInSynFamTyCon in interface file")

tc_iface_decl _parent _ignore_prags
            (IfaceClass {ifName = tc_name,
                         ifRoles = roles,
                         ifBinders = binders,
                         ifFDs = rdr_fds,
                         ifBody = IfAbstractClass})
  = bindIfaceTyConBinders binders $ \ binders' -> do
    { fds  <- mapM tc_fd rdr_fds
    ; cls  <- buildClass tc_name binders' roles fds Nothing
    ; return (ATyCon (classTyCon cls)) }

tc_iface_decl _parent ignore_prags
            (IfaceClass {ifName = tc_name,
                         ifRoles = roles,
                         ifBinders = binders,
                         ifFDs = rdr_fds,
                         ifBody = IfConcreteClass {
                             ifClassCtxt = rdr_ctxt,
                             ifATs = rdr_ats, ifSigs = rdr_sigs,
                             ifMinDef = mindef_occ
                         }})
  = bindIfaceTyConBinders binders $ \ binders' -> do
    { traceIf (text "tc-iface-class1" <+> ppr tc_name)
    ; ctxt <- mapM tc_sc rdr_ctxt
    ; traceIf (text "tc-iface-class2" <+> ppr tc_name)
    ; sigs <- mapM tc_sig rdr_sigs
    ; fds  <- mapM tc_fd rdr_fds
    ; traceIf (text "tc-iface-class3" <+> ppr tc_name)
    ; mindef <- traverse (lookupIfaceTop . mkVarOccFS) mindef_occ
    ; cls  <- fixM $ \ cls -> do
              { ats  <- mapM (tc_at cls) rdr_ats
              ; traceIf (text "tc-iface-class4" <+> ppr tc_name)
              ; buildClass tc_name binders' roles fds (Just (ctxt, ats, sigs, mindef)) }
    ; return (ATyCon (classTyCon cls)) }
  where
   tc_sc pred = forkM (mk_sc_doc pred) (tcIfaceType pred)
        -- The *length* of the superclasses is used by buildClass, and hence must
        -- not be inside the thunk.  But the *content* maybe recursive and hence
        -- must be lazy (via forkM).  Example:
        --     class C (T a) => D a where
        --       data T a
        -- Here the associated type T is knot-tied with the class, and
        -- so we must not pull on T too eagerly.  See Trac #5970

   tc_sig :: IfaceClassOp -> IfL TcMethInfo
   tc_sig (IfaceClassOp op_name rdr_ty dm)
     = do { let doc = mk_op_doc op_name rdr_ty
          ; op_ty <- forkM (doc <+> text "ty") $ tcIfaceType rdr_ty
                -- Must be done lazily for just the same reason as the
                -- type of a data con; to avoid sucking in types that
                -- it mentions unless it's necessary to do so
          ; dm'   <- tc_dm doc dm
          ; return (op_name, op_ty, dm') }

   tc_dm :: SDoc
         -> Maybe (DefMethSpec IfaceType)
         -> IfL (Maybe (DefMethSpec (SrcSpan, Type)))
   tc_dm _   Nothing               = return Nothing
   tc_dm _   (Just VanillaDM)      = return (Just VanillaDM)
   tc_dm doc (Just (GenericDM ty))
        = do { -- Must be done lazily to avoid sucking in types
             ; ty' <- forkM (doc <+> text "dm") $ tcIfaceType ty
             ; return (Just (GenericDM (noSrcSpan, ty'))) }

   tc_at cls (IfaceAT tc_decl if_def)
     = do ATyCon tc <- tc_iface_decl (Just cls) ignore_prags tc_decl
          mb_def <- case if_def of
                      Nothing  -> return Nothing
                      Just def -> forkM (mk_at_doc tc)                 $
                                  extendIfaceTyVarEnv (tyConTyVars tc) $
                                  do { tc_def <- tcIfaceType def
                                     ; return (Just (tc_def, noSrcSpan)) }
                  -- Must be done lazily in case the RHS of the defaults mention
                  -- the type constructor being defined here
                  -- e.g.   type AT a; type AT b = AT [b]   Trac #8002
          return (ATI tc mb_def)

   mk_sc_doc pred = text "Superclass" <+> ppr pred
   mk_at_doc tc = text "Associated type" <+> ppr tc
   mk_op_doc op_name op_ty = text "Class op" <+> sep [ppr op_name, ppr op_ty]

tc_iface_decl _ _ (IfaceAxiom { ifName = tc_name, ifTyCon = tc
                              , ifAxBranches = branches, ifRole = role })
  = do { tc_tycon    <- tcIfaceTyCon tc
       -- Must be done lazily, because axioms are forced when checking
       -- for family instance consistency, and the RHS may mention
       -- a hs-boot declared type constructor that is going to be
       -- defined by this module.
       -- e.g. type instance F Int = ToBeDefined
       -- See Trac #13803
       ; tc_branches <- forkM (text "Axiom branches" <+> ppr tc_name)
                      $ tc_ax_branches branches
       ; let axiom = CoAxiom { co_ax_unique   = nameUnique tc_name
                             , co_ax_name     = tc_name
                             , co_ax_tc       = tc_tycon
                             , co_ax_role     = role
                             , co_ax_branches = manyBranches tc_branches
                             , co_ax_implicit = False }
       ; return (ACoAxiom axiom) }

tc_iface_decl _ _ (IfacePatSyn{ ifName = name
                              , ifPatMatcher = if_matcher
                              , ifPatBuilder = if_builder
                              , ifPatIsInfix = is_infix
                              , ifPatUnivBndrs = univ_bndrs
                              , ifPatExBndrs = ex_bndrs
                              , ifPatProvCtxt = prov_ctxt
                              , ifPatReqCtxt = req_ctxt
                              , ifPatArgs = args
                              , ifPatTy = pat_ty
                              , ifFieldLabels = field_labels })
  = do { traceIf (text "tc_iface_decl" <+> ppr name)
       ; matcher <- tc_pr if_matcher
       ; builder <- fmapMaybeM tc_pr if_builder
       ; bindIfaceForAllBndrs univ_bndrs $ \univ_tvs -> do
       { bindIfaceForAllBndrs ex_bndrs $ \ex_tvs -> do
       { patsyn <- forkM (mk_doc name) $
             do { prov_theta <- tcIfaceCtxt prov_ctxt
                ; req_theta  <- tcIfaceCtxt req_ctxt
                ; pat_ty     <- tcIfaceType pat_ty
                ; arg_tys    <- mapM tcIfaceType args
                ; return $ buildPatSyn name is_infix matcher builder
                                       (univ_tvs, req_theta)
                                       (ex_tvs, prov_theta)
                                       arg_tys pat_ty field_labels }
       ; return $ AConLike . PatSynCon $ patsyn }}}
  where
     mk_doc n = text "Pattern synonym" <+> ppr n
     tc_pr :: (IfExtName, Bool) -> IfL (Id, Bool)
     tc_pr (nm, b) = do { id <- forkM (ppr nm) (tcIfaceExtId nm)
                        ; return (id, b) }

tc_fd :: FunDep IfLclName -> IfL (FunDep TyVar)
tc_fd (tvs1, tvs2) = do { tvs1' <- mapM tcIfaceTyVar tvs1
                        ; tvs2' <- mapM tcIfaceTyVar tvs2
                        ; return (tvs1', tvs2') }

tc_ax_branches :: [IfaceAxBranch] -> IfL [CoAxBranch]
tc_ax_branches if_branches = foldlM tc_ax_branch [] if_branches

tc_ax_branch :: [CoAxBranch] -> IfaceAxBranch -> IfL [CoAxBranch]
tc_ax_branch prev_branches
             (IfaceAxBranch { ifaxbTyVars = tv_bndrs, ifaxbCoVars = cv_bndrs
                            , ifaxbLHS = lhs, ifaxbRHS = rhs
                            , ifaxbRoles = roles, ifaxbIncomps = incomps })
  = bindIfaceTyConBinders_AT
      (map (\b -> TvBndr b (NamedTCB Inferred)) tv_bndrs) $ \ tvs ->
         -- The _AT variant is needed here; see Note [CoAxBranch type variables] in CoAxiom
    bindIfaceIds cv_bndrs $ \ cvs -> do
    { tc_lhs <- tcIfaceTcArgs lhs
    ; tc_rhs <- tcIfaceType rhs
    ; let br = CoAxBranch { cab_loc     = noSrcSpan
                          , cab_tvs     = binderVars tvs
                          , cab_cvs     = cvs
                          , cab_lhs     = tc_lhs
                          , cab_roles   = roles
                          , cab_rhs     = tc_rhs
                          , cab_incomps = map (prev_branches `getNth`) incomps }
    ; return (prev_branches ++ [br]) }

tcIfaceDataCons :: Name -> TyCon -> [TyConBinder] -> IfaceConDecls -> IfL AlgTyConRhs
tcIfaceDataCons tycon_name tycon tc_tybinders if_cons
  = case if_cons of
        IfAbstractTyCon  -> return AbstractTyCon
        IfDataTyCon cons -> do  { data_cons  <- mapM tc_con_decl cons
                                ; return (mkDataTyConRhs data_cons) }
        IfNewTyCon  con  -> do  { data_con  <- tc_con_decl con
                                ; mkNewTyConRhs tycon_name tycon data_con }
  where
    univ_tv_bndrs :: [TyVarBinder]
    univ_tv_bndrs = mkDataConUnivTyVarBinders tc_tybinders

    tc_con_decl (IfCon { ifConInfix = is_infix,
                         ifConExTvs = ex_bndrs,
                         ifConName = dc_name,
                         ifConCtxt = ctxt, ifConEqSpec = spec,
                         ifConArgTys = args, ifConFields = lbl_names,
                         ifConStricts = if_stricts,
                         ifConSrcStricts = if_src_stricts})
     = -- Universally-quantified tyvars are shared with
       -- parent TyCon, and are already in scope
       bindIfaceForAllBndrs ex_bndrs    $ \ ex_tv_bndrs -> do
        { traceIf (text "Start interface-file tc_con_decl" <+> ppr dc_name)

        -- Read the context and argument types, but lazily for two reasons
        -- (a) to avoid looking tugging on a recursive use of
        --     the type itself, which is knot-tied
        -- (b) to avoid faulting in the component types unless
        --     they are really needed
        ; ~(eq_spec, theta, arg_tys, stricts) <- forkM (mk_doc dc_name) $
             do { eq_spec <- tcIfaceEqSpec spec
                ; theta   <- tcIfaceCtxt ctxt
                ; arg_tys <- mapM tcIfaceType args
                ; stricts <- mapM tc_strict if_stricts
                        -- The IfBang field can mention
                        -- the type itself; hence inside forkM
                ; return (eq_spec, theta, arg_tys, stricts) }

        -- Remember, tycon is the representation tycon
        ; let orig_res_ty = mkFamilyTyConApp tycon
                                (substTyVars (mkTvSubstPrs (map eqSpecPair eq_spec))
                                             (binderVars tc_tybinders))

        ; prom_rep_name <- newTyConRepName dc_name

        ; con <- buildDataCon (pprPanic "tcIfaceDataCons: FamInstEnvs" (ppr dc_name))
                       dc_name is_infix prom_rep_name
                       (map src_strict if_src_stricts)
                       (Just stricts)
                       -- Pass the HsImplBangs (i.e. final
                       -- decisions) to buildDataCon; it'll use
                       -- these to guide the construction of a
                       -- worker.
                       -- See Note [Bangs on imported data constructors] in MkId
                       lbl_names
                       univ_tv_bndrs ex_tv_bndrs
                       eq_spec theta
                       arg_tys orig_res_ty tycon
        ; traceIf (text "Done interface-file tc_con_decl" <+> ppr dc_name)
        ; return con }
    mk_doc con_name = text "Constructor" <+> ppr con_name

    tc_strict :: IfaceBang -> IfL HsImplBang
    tc_strict IfNoBang = return (HsLazy)
    tc_strict IfStrict = return (HsStrict)
    tc_strict IfUnpack = return (HsUnpack Nothing)
    tc_strict (IfUnpackCo if_co) = do { co <- tcIfaceCo if_co
                                      ; return (HsUnpack (Just co)) }

    src_strict :: IfaceSrcBang -> HsSrcBang
    src_strict (IfSrcBang unpk bang) = HsSrcBang NoSourceText unpk bang

tcIfaceEqSpec :: IfaceEqSpec -> IfL [EqSpec]
tcIfaceEqSpec spec
  = mapM do_item spec
  where
    do_item (occ, if_ty) = do { tv <- tcIfaceTyVar occ
                              ; ty <- tcIfaceType if_ty
                              ; return (mkEqSpec tv ty) }

{-
Note [Synonym kind loop]
~~~~~~~~~~~~~~~~~~~~~~~~
Notice that we eagerly grab the *kind* from the interface file, but
build a forkM thunk for the *rhs* (and family stuff).  To see why,
consider this (Trac #2412)

M.hs:       module M where { import X; data T = MkT S }
X.hs:       module X where { import {-# SOURCE #-} M; type S = T }
M.hs-boot:  module M where { data T }

When kind-checking M.hs we need S's kind.  But we do not want to
find S's kind from (typeKind S-rhs), because we don't want to look at
S-rhs yet!  Since S is imported from X.hi, S gets just one chance to
be defined, and we must not do that until we've finished with M.T.

Solution: record S's kind in the interface file; now we can safely
look at it.

************************************************************************
*                                                                      *
                Instances
*                                                                      *
************************************************************************
-}

tcIfaceInst :: IfaceClsInst -> IfL ClsInst
tcIfaceInst (IfaceClsInst { ifDFun = dfun_name, ifOFlag = oflag
                          , ifInstCls = cls, ifInstTys = mb_tcs
                          , ifInstOrph = orph })
  = do { dfun <- forkM (text "Dict fun" <+> ppr dfun_name) $
                    fmap tyThingId (tcIfaceImplicit dfun_name)
       ; let mb_tcs' = map (fmap ifaceTyConName) mb_tcs
       ; return (mkImportedInstance cls mb_tcs' dfun_name dfun oflag orph) }

tcIfaceFamInst :: IfaceFamInst -> IfL FamInst
tcIfaceFamInst (IfaceFamInst { ifFamInstFam = fam, ifFamInstTys = mb_tcs
                             , ifFamInstAxiom = axiom_name } )
    = do { axiom' <- forkM (text "Axiom" <+> ppr axiom_name) $
                     tcIfaceCoAxiom axiom_name
             -- will panic if branched, but that's OK
         ; let axiom'' = toUnbranchedAxiom axiom'
               mb_tcs' = map (fmap ifaceTyConName) mb_tcs
         ; return (mkImportedFamInst fam mb_tcs' axiom'') }

{-
************************************************************************
*                                                                      *
                Rules
*                                                                      *
************************************************************************

We move a IfaceRule from eps_rules to eps_rule_base when all its LHS free vars
are in the type environment.  However, remember that typechecking a Rule may
(as a side effect) augment the type envt, and so we may need to iterate the process.
-}

tcIfaceRules :: Bool            -- True <=> ignore rules
             -> [IfaceRule]
             -> IfL [CoreRule]
tcIfaceRules ignore_prags if_rules
  | ignore_prags = return []
  | otherwise    = mapM tcIfaceRule if_rules

tcIfaceRule :: IfaceRule -> IfL CoreRule
tcIfaceRule (IfaceRule {ifRuleName = name, ifActivation = act, ifRuleBndrs = bndrs,
                        ifRuleHead = fn, ifRuleArgs = args, ifRuleRhs = rhs,
                        ifRuleAuto = auto, ifRuleOrph = orph })
  = do  { ~(bndrs', args', rhs') <-
                -- Typecheck the payload lazily, in the hope it'll never be looked at
                forkM (text "Rule" <+> pprRuleName name) $
                bindIfaceBndrs bndrs                      $ \ bndrs' ->
                do { args' <- mapM tcIfaceExpr args
                   ; rhs'  <- tcIfaceExpr rhs
                   ; return (bndrs', args', rhs') }
        ; let mb_tcs = map ifTopFreeName args
        ; this_mod <- getIfModule
        ; return (Rule { ru_name = name, ru_fn = fn, ru_act = act,
                          ru_bndrs = bndrs', ru_args = args',
                          ru_rhs = occurAnalyseExpr rhs',
                          ru_rough = mb_tcs,
                          ru_origin = this_mod,
                          ru_orphan = orph,
                          ru_auto = auto,
                          ru_local = False }) } -- An imported RULE is never for a local Id
                                                -- or, even if it is (module loop, perhaps)
                                                -- we'll just leave it in the non-local set
  where
        -- This function *must* mirror exactly what Rules.roughTopNames does
        -- We could have stored the ru_rough field in the iface file
        -- but that would be redundant, I think.
        -- The only wrinkle is that we must not be deceived by
        -- type synonyms at the top of a type arg.  Since
        -- we can't tell at this point, we are careful not
        -- to write them out in coreRuleToIfaceRule
    ifTopFreeName :: IfaceExpr -> Maybe Name
    ifTopFreeName (IfaceType (IfaceTyConApp tc _ )) = Just (ifaceTyConName tc)
    ifTopFreeName (IfaceType (IfaceTupleTy s _ ts)) = Just (tupleTyConName s (length (tcArgsIfaceTypes ts)))
    ifTopFreeName (IfaceApp f _)                    = ifTopFreeName f
    ifTopFreeName (IfaceExt n)                      = Just n
    ifTopFreeName _                                 = Nothing

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

tcIfaceAnnotations :: [IfaceAnnotation] -> IfL [Annotation]
tcIfaceAnnotations = mapM tcIfaceAnnotation

tcIfaceAnnotation :: IfaceAnnotation -> IfL Annotation
tcIfaceAnnotation (IfaceAnnotation target serialized) = do
    target' <- tcIfaceAnnTarget target
    return $ Annotation {
        ann_target = target',
        ann_value = serialized
    }

tcIfaceAnnTarget :: IfaceAnnTarget -> IfL (AnnTarget Name)
tcIfaceAnnTarget (NamedTarget occ) = do
    name <- lookupIfaceTop occ
    return $ NamedTarget name
tcIfaceAnnTarget (ModuleTarget mod) = do
    return $ ModuleTarget mod

{-
************************************************************************
*                                                                      *
                Complete Match Pragmas
*                                                                      *
************************************************************************
-}

tcIfaceCompleteSigs :: [IfaceCompleteMatch] -> IfL [CompleteMatch]
tcIfaceCompleteSigs = mapM tcIfaceCompleteSig

tcIfaceCompleteSig :: IfaceCompleteMatch -> IfL CompleteMatch
tcIfaceCompleteSig (IfaceCompleteMatch ms t) = return (CompleteMatch ms t)

{-
************************************************************************
*                                                                      *
                Vectorisation information
*                                                                      *
************************************************************************
-}

-- We need access to the type environment as we need to look up information about type constructors
-- (i.e., their data constructors and whether they are class type constructors).  If a vectorised
-- type constructor or class is defined in the same module as where it is vectorised, we cannot
-- look that information up from the type constructor that we obtained via a 'forkM'ed
-- 'tcIfaceTyCon' without recursively loading the interface that we are already type checking again
-- and again and again...
--
tcIfaceVectInfo :: Module -> TypeEnv -> IfaceVectInfo -> IfL VectInfo
tcIfaceVectInfo mod typeEnv (IfaceVectInfo
                             { ifaceVectInfoVar            = vars
                             , ifaceVectInfoTyCon          = tycons
                             , ifaceVectInfoTyConReuse     = tyconsReuse
                             , ifaceVectInfoParallelVars   = parallelVars
                             , ifaceVectInfoParallelTyCons = parallelTyCons
                             })
  = do { let parallelTyConsSet = mkNameSet parallelTyCons
       ; vVars         <- mapM vectVarMapping                  vars
       ; let varsSet = mkVarSet (map fst vVars)
       ; tyConRes1     <- mapM (vectTyConVectMapping varsSet)  tycons
       ; tyConRes2     <- mapM (vectTyConReuseMapping varsSet) tyconsReuse
       ; vParallelVars <- mapM vectVar                         parallelVars
       ; let (vTyCons, vDataCons, vScSels) = unzip3 (tyConRes1 ++ tyConRes2)
       ; return $ VectInfo
                  { vectInfoVar            = mkDVarEnv vVars `extendDVarEnvList` concat vScSels
                  , vectInfoTyCon          = mkNameEnv vTyCons
                  , vectInfoDataCon        = mkNameEnv (concat vDataCons)
                  , vectInfoParallelVars   = mkDVarSet vParallelVars
                  , vectInfoParallelTyCons = parallelTyConsSet
                  }
       }
  where
    vectVarMapping name
      = do { vName <- lookupIfaceTop (mkLocalisedOccName mod mkVectOcc name)
           ; var   <- forkM (text "vect var"  <+> ppr name)  $
                        tcIfaceExtId name
           ; vVar  <- forkM (text "vect vVar [mod =" <+>
                             ppr mod <> text "; nameModule =" <+>
                             ppr (nameModule name) <> text "]" <+> ppr vName) $
                       tcIfaceExtId vName
           ; return (var, (var, vVar))
           }
      -- where
      --   lookupLocalOrExternalId name
      --     = do { let mb_id = lookupTypeEnv typeEnv name
      --          ; case mb_id of
      --                -- id is local
      --              Just (AnId id) -> return id
      --                -- name is not an Id => internal inconsistency
      --              Just _         -> notAnIdErr
      --                -- Id is external
      --              Nothing        -> tcIfaceExtId name
      --          }
      --
      --   notAnIdErr = pprPanic "TcIface.tcIfaceVectInfo: not an id" (ppr name)

    vectVar name
      = forkM (text "vect scalar var"  <+> ppr name)  $
          tcIfaceExtId name

    vectTyConVectMapping vars name
      = do { vName  <- lookupIfaceTop (mkLocalisedOccName mod mkVectTyConOcc name)
           ; vectTyConMapping vars name vName
           }

    vectTyConReuseMapping vars name
      = vectTyConMapping vars name name

    vectTyConMapping vars name vName
      = do { tycon  <- lookupLocalOrExternalTyCon name
           ; vTycon <- forkM (text "vTycon of" <+> ppr vName) $
                         lookupLocalOrExternalTyCon vName

               -- Map the data constructors of the original type constructor to those of the
               -- vectorised type constructor /unless/ the type constructor was vectorised
               -- abstractly; if it was vectorised abstractly, the workers of its data constructors
               -- do not appear in the set of vectorised variables.
               --
               -- NB: This is lazy!  We don't pull at the type constructors before we actually use
               --     the data constructor mapping.
           ; let isAbstract | isClassTyCon tycon = False
                            | datacon:_ <- tyConDataCons tycon
                                                 = not $ dataConWrapId datacon `elemVarSet` vars
                            | otherwise          = True
                 vDataCons  | isAbstract = []
                            | otherwise  = [ (dataConName datacon, (datacon, vDatacon))
                                           | (datacon, vDatacon) <- zip (tyConDataCons tycon)
                                                                        (tyConDataCons vTycon)
                                           ]

                   -- Map the (implicit) superclass and methods selectors as they don't occur in
                   -- the var map.
                 vScSels    | Just cls  <- tyConClass_maybe tycon
                            , Just vCls <- tyConClass_maybe vTycon
                            = [ (sel, (sel, vSel))
                              | (sel, vSel) <- zip (classAllSelIds cls) (classAllSelIds vCls)
                              ]
                            | otherwise
                            = []

           ; return ( (name, (tycon, vTycon))          -- (T, T_v)
                    , vDataCons                        -- list of (Ci, Ci_v)
                    , vScSels                          -- list of (seli, seli_v)
                    )
           }
      where
          -- we need a fully defined version of the type constructor to be able to extract
          -- its data constructors etc.
        lookupLocalOrExternalTyCon name
          = do { let mb_tycon = lookupTypeEnv typeEnv name
               ; case mb_tycon of
                     -- tycon is local
                   Just (ATyCon tycon) -> return tycon
                     -- name is not a tycon => internal inconsistency
                   Just _              -> notATyConErr
                     -- tycon is external
                   Nothing             -> tcIfaceTyConByName name
               }

        notATyConErr = pprPanic "TcIface.tcIfaceVectInfo: not a tycon" (ppr name)

{-
************************************************************************
*                                                                      *
                        Types
*                                                                      *
************************************************************************
-}

tcIfaceType :: IfaceType -> IfL Type
tcIfaceType = go
  where
    go (IfaceTyVar n)         = TyVarTy <$> tcIfaceTyVar n
    go (IfaceFreeTyVar n)     = pprPanic "tcIfaceType:IfaceFreeTyVar" (ppr n)
    go (IfaceAppTy t1 t2)     = AppTy <$> go t1 <*> go t2
    go (IfaceLitTy l)         = LitTy <$> tcIfaceTyLit l
    go (IfaceFunTy t1 t2)     = FunTy <$> go t1 <*> go t2
    go (IfaceDFunTy t1 t2)    = FunTy <$> go t1 <*> go t2
    go (IfaceTupleTy s i tks) = tcIfaceTupleTy s i tks
    go (IfaceTyConApp tc tks)
      = do { tc' <- tcIfaceTyCon tc
           ; tks' <- mapM go (tcArgsIfaceTypes tks)
           ; return (mkTyConApp tc' tks') }
    go (IfaceForAllTy bndr t)
      = bindIfaceForAllBndr bndr $ \ tv' vis ->
        ForAllTy (TvBndr tv' vis) <$> go t
    go (IfaceCastTy ty co)   = CastTy <$> go ty <*> tcIfaceCo co
    go (IfaceCoercionTy co)  = CoercionTy <$> tcIfaceCo co

tcIfaceTupleTy :: TupleSort -> IsPromoted -> IfaceTcArgs -> IfL Type
tcIfaceTupleTy sort is_promoted args
 = do { args' <- tcIfaceTcArgs args
      ; let arity = length args'
      ; base_tc <- tcTupleTyCon True sort arity
      ; case is_promoted of
          IsNotPromoted
            -> return (mkTyConApp base_tc args')

          IsPromoted
            -> do { let tc        = promoteDataCon (tyConSingleDataCon base_tc)
                        kind_args = map typeKind args'
                  ; return (mkTyConApp tc (kind_args ++ args')) } }

-- See Note [Unboxed tuple RuntimeRep vars] in TyCon
tcTupleTyCon :: Bool    -- True <=> typechecking a *type* (vs. an expr)
             -> TupleSort
             -> Arity   -- the number of args. *not* the tuple arity.
             -> IfL TyCon
tcTupleTyCon in_type sort arity
  = case sort of
      ConstraintTuple -> do { thing <- tcIfaceGlobal (cTupleTyConName arity)
                            ; return (tyThingTyCon thing) }
      BoxedTuple   -> return (tupleTyCon Boxed   arity)
      UnboxedTuple -> return (tupleTyCon Unboxed arity')
        where arity' | in_type   = arity `div` 2
                     | otherwise = arity
                      -- in expressions, we only have term args

tcIfaceTcArgs :: IfaceTcArgs -> IfL [Type]
tcIfaceTcArgs = mapM tcIfaceType . tcArgsIfaceTypes

-----------------------------------------
tcIfaceCtxt :: IfaceContext -> IfL ThetaType
tcIfaceCtxt sts = mapM tcIfaceType sts

-----------------------------------------
tcIfaceTyLit :: IfaceTyLit -> IfL TyLit
tcIfaceTyLit (IfaceNumTyLit n) = return (NumTyLit n)
tcIfaceTyLit (IfaceStrTyLit n) = return (StrTyLit n)

{-
%************************************************************************
%*                                                                      *
                        Coercions
*                                                                      *
************************************************************************
-}

tcIfaceCo :: IfaceCoercion -> IfL Coercion
tcIfaceCo = go
  where
    go (IfaceReflCo r t)         = Refl r <$> tcIfaceType t
    go (IfaceFunCo r c1 c2)      = mkFunCo r <$> go c1 <*> go c2
    go (IfaceTyConAppCo r tc cs)
      = TyConAppCo r <$> tcIfaceTyCon tc <*> mapM go cs
    go (IfaceAppCo c1 c2)        = AppCo <$> go c1 <*> go c2
    go (IfaceForAllCo tv k c)  = do { k' <- go k
                                      ; bindIfaceTyVar tv $ \ tv' ->
                                        ForAllCo tv' k' <$> go c }
    go (IfaceCoVarCo n)          = CoVarCo <$> go_var n
    go (IfaceAxiomInstCo n i cs) = AxiomInstCo <$> tcIfaceCoAxiom n <*> pure i <*> mapM go cs
    go (IfaceUnivCo p r t1 t2)   = UnivCo <$> tcIfaceUnivCoProv p <*> pure r
                                          <*> tcIfaceType t1 <*> tcIfaceType t2
    go (IfaceSymCo c)            = SymCo    <$> go c
    go (IfaceTransCo c1 c2)      = TransCo  <$> go c1
                                            <*> go c2
    go (IfaceInstCo c1 t2)       = InstCo   <$> go c1
                                            <*> go t2
    go (IfaceNthCo d c)          = NthCo d  <$> go c
    go (IfaceLRCo lr c)          = LRCo lr  <$> go c
    go (IfaceCoherenceCo c1 c2)  = CoherenceCo <$> go c1
                                               <*> go c2
    go (IfaceKindCo c)           = KindCo   <$> go c
    go (IfaceSubCo c)            = SubCo    <$> go c
    go (IfaceAxiomRuleCo ax cos) = AxiomRuleCo <$> go_axiom_rule ax
                                               <*> mapM go cos

    go_var :: FastString -> IfL CoVar
    go_var = tcIfaceLclId

    go_axiom_rule :: FastString -> IfL CoAxiomRule
    go_axiom_rule n =
      case Map.lookup n typeNatCoAxiomRules of
        Just ax -> return ax
        _  -> pprPanic "go_axiom_rule" (ppr n)

tcIfaceUnivCoProv :: IfaceUnivCoProv -> IfL UnivCoProvenance
tcIfaceUnivCoProv IfaceUnsafeCoerceProv     = return UnsafeCoerceProv
tcIfaceUnivCoProv (IfacePhantomProv kco)    = PhantomProv <$> tcIfaceCo kco
tcIfaceUnivCoProv (IfaceProofIrrelProv kco) = ProofIrrelProv <$> tcIfaceCo kco
tcIfaceUnivCoProv (IfacePluginProv str)     = return $ PluginProv str
tcIfaceUnivCoProv (IfaceHoleProv _)         =
    pprPanic "tcIfaceUnivCoProv" (text "holes can't occur in interface files")

{-
************************************************************************
*                                                                      *
                        Core
*                                                                      *
************************************************************************
-}

tcIfaceExpr :: IfaceExpr -> IfL CoreExpr
tcIfaceExpr (IfaceType ty)
  = Type <$> tcIfaceType ty

tcIfaceExpr (IfaceCo co)
  = Coercion <$> tcIfaceCo co

tcIfaceExpr (IfaceCast expr co)
  = Cast <$> tcIfaceExpr expr <*> tcIfaceCo co

tcIfaceExpr (IfaceLcl name)
  = Var <$> tcIfaceLclId name

tcIfaceExpr (IfaceExt gbl)
  = Var <$> tcIfaceExtId gbl

tcIfaceExpr (IfaceLit lit)
  = do lit' <- tcIfaceLit lit
       return (Lit lit')

tcIfaceExpr (IfaceFCall cc ty) = do
    ty' <- tcIfaceType ty
    u <- newUnique
    dflags <- getDynFlags
    return (Var (mkFCallId dflags u cc ty'))

tcIfaceExpr (IfaceTuple sort args)
  = do { args' <- mapM tcIfaceExpr args
       ; tc <- tcTupleTyCon False sort arity
       ; let con_tys = map exprType args'
             some_con_args = map Type con_tys ++ args'
             con_args = case sort of
               UnboxedTuple -> map (Type . getRuntimeRep "tcIfaceExpr") con_tys ++ some_con_args
               _            -> some_con_args
                        -- Put the missing type arguments back in
             con_id   = dataConWorkId (tyConSingleDataCon tc)
       ; return (mkApps (Var con_id) con_args) }
  where
    arity = length args

tcIfaceExpr (IfaceLam (bndr, os) body)
  = bindIfaceBndr bndr $ \bndr' ->
    Lam (tcIfaceOneShot os bndr') <$> tcIfaceExpr body
  where
    tcIfaceOneShot IfaceOneShot b = setOneShotLambda b
    tcIfaceOneShot _            b = b

tcIfaceExpr (IfaceApp fun arg)
  = App <$> tcIfaceExpr fun <*> tcIfaceExpr arg

tcIfaceExpr (IfaceECase scrut ty)
  = do { scrut' <- tcIfaceExpr scrut
       ; ty' <- tcIfaceType ty
       ; return (castBottomExpr scrut' ty') }

tcIfaceExpr (IfaceCase scrut case_bndr alts)  = do
    scrut' <- tcIfaceExpr scrut
    case_bndr_name <- newIfaceName (mkVarOccFS case_bndr)
    let
        scrut_ty   = exprType scrut'
        case_bndr' = mkLocalIdOrCoVar case_bndr_name scrut_ty
        tc_app     = splitTyConApp scrut_ty
                -- NB: Won't always succeed (polymorphic case)
                --     but won't be demanded in those cases
                -- NB: not tcSplitTyConApp; we are looking at Core here
                --     look through non-rec newtypes to find the tycon that
                --     corresponds to the datacon in this case alternative

    extendIfaceIdEnv [case_bndr'] $ do
     alts' <- mapM (tcIfaceAlt scrut' tc_app) alts
     return (Case scrut' case_bndr' (coreAltsType alts') alts')

tcIfaceExpr (IfaceLet (IfaceNonRec (IfLetBndr fs ty info ji) rhs) body)
  = do  { name    <- newIfaceName (mkVarOccFS fs)
        ; ty'     <- tcIfaceType ty
        ; id_info <- tcIdInfo False {- Don't ignore prags; we are inside one! -}
                              name ty' info
        ; let id = mkLocalIdOrCoVarWithInfo name ty' id_info
                     `asJoinId_maybe` tcJoinInfo ji
        ; rhs' <- tcIfaceExpr rhs
        ; body' <- extendIfaceIdEnv [id] (tcIfaceExpr body)
        ; return (Let (NonRec id rhs') body') }

tcIfaceExpr (IfaceLet (IfaceRec pairs) body)
  = do { ids <- mapM tc_rec_bndr (map fst pairs)
       ; extendIfaceIdEnv ids $ do
       { pairs' <- zipWithM tc_pair pairs ids
       ; body' <- tcIfaceExpr body
       ; return (Let (Rec pairs') body') } }
 where
   tc_rec_bndr (IfLetBndr fs ty _ ji)
     = do { name <- newIfaceName (mkVarOccFS fs)
          ; ty'  <- tcIfaceType ty
          ; return (mkLocalIdOrCoVar name ty' `asJoinId_maybe` tcJoinInfo ji) }
   tc_pair (IfLetBndr _ _ info _, rhs) id
     = do { rhs' <- tcIfaceExpr rhs
          ; id_info <- tcIdInfo False {- Don't ignore prags; we are inside one! -}
                                (idName id) (idType id) info
          ; return (setIdInfo id id_info, rhs') }

tcIfaceExpr (IfaceTick tickish expr) = do
    expr' <- tcIfaceExpr expr
    -- If debug flag is not set: Ignore source notes
    dbgLvl <- fmap debugLevel getDynFlags
    case tickish of
      IfaceSource{} | dbgLvl > 0
                    -> return expr'
      _otherwise    -> do
        tickish' <- tcIfaceTickish tickish
        return (Tick tickish' expr')

-------------------------
tcIfaceTickish :: IfaceTickish -> IfM lcl (Tickish Id)
tcIfaceTickish (IfaceHpcTick modl ix)   = return (HpcTick modl ix)
tcIfaceTickish (IfaceSCC  cc tick push) = return (ProfNote cc tick push)
tcIfaceTickish (IfaceSource src name)   = return (SourceNote src name)

-------------------------
tcIfaceLit :: Literal -> IfL Literal
-- Integer literals deserialise to (LitInteger i <error thunk>)
-- so tcIfaceLit just fills in the type.
-- See Note [Integer literals] in Literal
tcIfaceLit (LitInteger i _)
  = do t <- tcIfaceTyConByName integerTyConName
       return (mkLitInteger i (mkTyConTy t))
tcIfaceLit lit = return lit

-------------------------
tcIfaceAlt :: CoreExpr -> (TyCon, [Type])
           -> (IfaceConAlt, [FastString], IfaceExpr)
           -> IfL (AltCon, [TyVar], CoreExpr)
tcIfaceAlt _ _ (IfaceDefault, names, rhs)
  = ASSERT( null names ) do
    rhs' <- tcIfaceExpr rhs
    return (DEFAULT, [], rhs')

tcIfaceAlt _ _ (IfaceLitAlt lit, names, rhs)
  = ASSERT( null names ) do
    lit' <- tcIfaceLit lit
    rhs' <- tcIfaceExpr rhs
    return (LitAlt lit', [], rhs')

-- A case alternative is made quite a bit more complicated
-- by the fact that we omit type annotations because we can
-- work them out.  True enough, but its not that easy!
tcIfaceAlt scrut (tycon, inst_tys) (IfaceDataAlt data_occ, arg_strs, rhs)
  = do  { con <- tcIfaceDataCon data_occ
        ; when (debugIsOn && not (con `elem` tyConDataCons tycon))
               (failIfM (ppr scrut $$ ppr con $$ ppr tycon $$ ppr (tyConDataCons tycon)))
        ; tcIfaceDataAlt con inst_tys arg_strs rhs }

tcIfaceDataAlt :: DataCon -> [Type] -> [FastString] -> IfaceExpr
               -> IfL (AltCon, [TyVar], CoreExpr)
tcIfaceDataAlt con inst_tys arg_strs rhs
  = do  { us <- newUniqueSupply
        ; let uniqs = uniqsFromSupply us
        ; let (ex_tvs, arg_ids)
                      = dataConRepFSInstPat arg_strs uniqs con inst_tys

        ; rhs' <- extendIfaceEnvs  ex_tvs       $
                  extendIfaceIdEnv arg_ids      $
                  tcIfaceExpr rhs
        ; return (DataAlt con, ex_tvs ++ arg_ids, rhs') }

{-
************************************************************************
*                                                                      *
                IdInfo
*                                                                      *
************************************************************************
-}

tcIdDetails :: Type -> IfaceIdDetails -> IfL IdDetails
tcIdDetails _  IfVanillaId = return VanillaId
tcIdDetails ty IfDFunId
  = return (DFunId (isNewTyCon (classTyCon cls)))
  where
    (_, _, cls, _) = tcSplitDFunTy ty

tcIdDetails _ (IfRecSelId tc naughty)
  = do { tc' <- either (fmap RecSelData . tcIfaceTyCon)
                       (fmap (RecSelPatSyn . tyThingPatSyn) . tcIfaceDecl False)
                       tc
       ; return (RecSelId { sel_tycon = tc', sel_naughty = naughty }) }
  where
    tyThingPatSyn (AConLike (PatSynCon ps)) = ps
    tyThingPatSyn _ = panic "tcIdDetails: expecting patsyn"

tcIdInfo :: Bool -> Name -> Type -> IfaceIdInfo -> IfL IdInfo
tcIdInfo ignore_prags name ty info = do
    lcl_env <- getLclEnv
    -- Set the CgInfo to something sensible but uninformative before
    -- we start; default assumption is that it has CAFs
    let init_info | if_boot lcl_env = vanillaIdInfo `setUnfoldingInfo` BootUnfolding
                  | otherwise       = vanillaIdInfo
    if ignore_prags
        then return init_info
        else case info of
                NoInfo -> return init_info
                HasInfo info -> foldlM tcPrag init_info info
  where
    tcPrag :: IdInfo -> IfaceInfoItem -> IfL IdInfo
    tcPrag info HsNoCafRefs        = return (info `setCafInfo`   NoCafRefs)
    tcPrag info (HsArity arity)    = return (info `setArityInfo` arity)
    tcPrag info (HsStrictness str) = return (info `setStrictnessInfo` str)
    tcPrag info (HsInline prag)    = return (info `setInlinePragInfo` prag)
    tcPrag info HsLevity           = return (info `setNeverLevPoly` ty)

        -- The next two are lazy, so they don't transitively suck stuff in
    tcPrag info (HsUnfold lb if_unf)
      = do { unf <- tcUnfolding name ty info if_unf
           ; let info1 | lb        = info `setOccInfo` strongLoopBreaker
                       | otherwise = info
           ; return (info1 `setUnfoldingInfo` unf) }

tcJoinInfo :: IfaceJoinInfo -> Maybe JoinArity
tcJoinInfo (IfaceJoinPoint ar) = Just ar
tcJoinInfo IfaceNotJoinPoint   = Nothing

tcUnfolding :: Name -> Type -> IdInfo -> IfaceUnfolding -> IfL Unfolding
tcUnfolding name _ info (IfCoreUnfold stable if_expr)
  = do  { dflags <- getDynFlags
        ; mb_expr <- tcPragExpr name if_expr
        ; let unf_src | stable    = InlineStable
                      | otherwise = InlineRhs
        ; return $ case mb_expr of
            Nothing -> NoUnfolding
            Just expr -> mkUnfolding dflags unf_src
                           True {- Top level -}
                           (isBottomingSig strict_sig)
                           expr
        }
  where
     -- Strictness should occur before unfolding!
    strict_sig = strictnessInfo info
tcUnfolding name _ _ (IfCompulsory if_expr)
  = do  { mb_expr <- tcPragExpr name if_expr
        ; return (case mb_expr of
                    Nothing   -> NoUnfolding
                    Just expr -> mkCompulsoryUnfolding expr) }

tcUnfolding name _ _ (IfInlineRule arity unsat_ok boring_ok if_expr)
  = do  { mb_expr <- tcPragExpr name if_expr
        ; return (case mb_expr of
                    Nothing   -> NoUnfolding
                    Just expr -> mkCoreUnfolding InlineStable True expr guidance )}
  where
    guidance = UnfWhen { ug_arity = arity, ug_unsat_ok = unsat_ok, ug_boring_ok = boring_ok }

tcUnfolding name dfun_ty _ (IfDFunUnfold bs ops)
  = bindIfaceBndrs bs $ \ bs' ->
    do { mb_ops1 <- forkM_maybe doc $ mapM tcIfaceExpr ops
       ; return (case mb_ops1 of
                    Nothing   -> noUnfolding
                    Just ops1 -> mkDFunUnfolding bs' (classDataCon cls) ops1) }
  where
    doc = text "Class ops for dfun" <+> ppr name
    (_, _, cls, _) = tcSplitDFunTy dfun_ty

{-
For unfoldings we try to do the job lazily, so that we never type check
an unfolding that isn't going to be looked at.
-}

tcPragExpr :: Name -> IfaceExpr -> IfL (Maybe CoreExpr)
tcPragExpr name expr
  = forkM_maybe doc $ do
    core_expr' <- tcIfaceExpr expr

                -- Check for type consistency in the unfolding
    whenGOptM Opt_DoCoreLinting $ do
        in_scope <- get_in_scope
        dflags   <- getDynFlags
        case lintUnfolding dflags noSrcLoc in_scope core_expr' of
          Nothing       -> return ()
          Just fail_msg -> do { mod <- getIfModule
                              ; pprPanic "Iface Lint failure"
                                  (vcat [ text "In interface for" <+> ppr mod
                                        , hang doc 2 fail_msg
                                        , ppr name <+> equals <+> ppr core_expr'
                                        , text "Iface expr =" <+> ppr expr ]) }
    return core_expr'
  where
    doc = text "Unfolding of" <+> ppr name

    get_in_scope :: IfL VarSet -- Totally disgusting; but just for linting
    get_in_scope
        = do { (gbl_env, lcl_env) <- getEnvs
             ; rec_ids <- case if_rec_types gbl_env of
                            Nothing -> return []
                            Just (_, get_env) -> do
                               { type_env <- setLclEnv () get_env
                               ; return (typeEnvIds type_env) }
             ; return (bindingsVars (if_tv_env lcl_env) `unionVarSet`
                       bindingsVars (if_id_env lcl_env) `unionVarSet`
                       mkVarSet rec_ids) }

    bindingsVars :: FastStringEnv Var -> VarSet
    bindingsVars ufm = mkVarSet $ nonDetEltsUFM ufm
      -- It's OK to use nonDetEltsUFM here because we immediately forget
      -- the ordering by creating a set

{-
************************************************************************
*                                                                      *
                Getting from Names to TyThings
*                                                                      *
************************************************************************
-}

tcIfaceGlobal :: Name -> IfL TyThing
tcIfaceGlobal name
  | Just thing <- wiredInNameTyThing_maybe name
        -- Wired-in things include TyCons, DataCons, and Ids
        -- Even though we are in an interface file, we want to make
        -- sure the instances and RULES of this thing (particularly TyCon) are loaded
        -- Imagine: f :: Double -> Double
  = do { ifCheckWiredInThing thing; return thing }

  | otherwise
  = do  { env <- getGblEnv
        ; case if_rec_types env of {    -- Note [Tying the knot]
            Just (mod, get_type_env)
                | nameIsLocalOrFrom mod name
                -> do           -- It's defined in the module being compiled
                { type_env <- setLclEnv () get_type_env         -- yuk
                ; case lookupNameEnv type_env name of
                    Just thing -> return thing
                    Nothing   ->
                      pprPanic "tcIfaceGlobal (local): not found"
                               (ifKnotErr name (if_doc env) type_env)
                }

          ; _ -> do

        { hsc_env <- getTopEnv
        ; mb_thing <- liftIO (lookupTypeHscEnv hsc_env name)
        ; case mb_thing of {
            Just thing -> return thing ;
            Nothing    -> do

        { mb_thing <- importDecl name   -- It's imported; go get it
        ; case mb_thing of
            Failed err      -> failIfM err
            Succeeded thing -> return thing
    }}}}}

ifKnotErr :: Name -> SDoc -> TypeEnv -> SDoc
ifKnotErr name env_doc type_env = vcat
  [ text "You are in a maze of twisty little passages, all alike."
  , text "While forcing the thunk for TyThing" <+> ppr name
  , text "which was lazily initialized by" <+> env_doc <> text ","
  , text "I tried to tie the knot, but I couldn't find" <+> ppr name
  , text "in the current type environment."
  , text "If you are developing GHC, please read Note [Tying the knot]"
  , text "and Note [Type-checking inside the knot]."
  , text "Consider rebuilding GHC with profiling for a better stack trace."
  , hang (text "Contents of current type environment:")
       2 (ppr type_env)
  ]

-- Note [Tying the knot]
-- ~~~~~~~~~~~~~~~~~~~~~
-- The if_rec_types field is used when we are compiling M.hs, which indirectly
-- imports Foo.hi, which mentions M.T Then we look up M.T in M's type
-- environment, which is splatted into if_rec_types after we've built M's type
-- envt.
--
-- This is a dark and complicated part of GHC type checking, with a lot
-- of moving parts.  Interested readers should also look at:
--
--      * Note [Knot-tying typecheckIface]
--      * Note [DFun knot-tying]
--      * Note [hsc_type_env_var hack]
--
-- There is also a wiki page on the subject, see:
--
--      https://ghc.haskell.org/trac/ghc/wiki/Commentary/Compiler/TyingTheKnot

tcIfaceTyConByName :: IfExtName -> IfL TyCon
tcIfaceTyConByName name
  = do { thing <- tcIfaceGlobal name
       ; return (tyThingTyCon thing) }

tcIfaceTyCon :: IfaceTyCon -> IfL TyCon
tcIfaceTyCon (IfaceTyCon name info)
  = do { thing <- tcIfaceGlobal name
       ; return $ case ifaceTyConIsPromoted info of
           IsNotPromoted -> tyThingTyCon thing
           IsPromoted    -> promoteDataCon $ tyThingDataCon thing }

tcIfaceCoAxiom :: Name -> IfL (CoAxiom Branched)
tcIfaceCoAxiom name = do { thing <- tcIfaceImplicit name
                         ; return (tyThingCoAxiom thing) }

tcIfaceDataCon :: Name -> IfL DataCon
tcIfaceDataCon name = do { thing <- tcIfaceGlobal name
                         ; case thing of
                                AConLike (RealDataCon dc) -> return dc
                                _       -> pprPanic "tcIfaceExtDC" (ppr name$$ ppr thing) }

tcIfaceExtId :: Name -> IfL Id
tcIfaceExtId name = do { thing <- tcIfaceGlobal name
                       ; case thing of
                          AnId id -> return id
                          _       -> pprPanic "tcIfaceExtId" (ppr name$$ ppr thing) }

-- See Note [Resolving never-exported Names in TcIface]
tcIfaceImplicit :: Name -> IfL TyThing
tcIfaceImplicit n = do
    lcl_env <- getLclEnv
    case if_implicits_env lcl_env of
        Nothing -> tcIfaceGlobal n
        Just tenv ->
            case lookupTypeEnv tenv n of
                Nothing -> pprPanic "tcIfaceInst" (ppr n $$ ppr tenv)
                Just tything -> return tything

{-
************************************************************************
*                                                                      *
                Bindings
*                                                                      *
************************************************************************
-}

bindIfaceId :: IfaceIdBndr -> (Id -> IfL a) -> IfL a
bindIfaceId (fs, ty) thing_inside
  = do  { name <- newIfaceName (mkVarOccFS fs)
        ; ty' <- tcIfaceType ty
        ; let id = mkLocalIdOrCoVar name ty'
        ; extendIfaceIdEnv [id] (thing_inside id) }

bindIfaceIds :: [IfaceIdBndr] -> ([Id] -> IfL a) -> IfL a
bindIfaceIds [] thing_inside = thing_inside []
bindIfaceIds (b:bs) thing_inside
  = bindIfaceId b   $ \b'  ->
    bindIfaceIds bs $ \bs' ->
    thing_inside (b':bs')

bindIfaceBndr :: IfaceBndr -> (CoreBndr -> IfL a) -> IfL a
bindIfaceBndr (IfaceIdBndr bndr) thing_inside
  = bindIfaceId bndr thing_inside
bindIfaceBndr (IfaceTvBndr bndr) thing_inside
  = bindIfaceTyVar bndr thing_inside

bindIfaceBndrs :: [IfaceBndr] -> ([CoreBndr] -> IfL a) -> IfL a
bindIfaceBndrs []     thing_inside = thing_inside []
bindIfaceBndrs (b:bs) thing_inside
  = bindIfaceBndr b     $ \ b' ->
    bindIfaceBndrs bs   $ \ bs' ->
    thing_inside (b':bs')

-----------------------
bindIfaceForAllBndrs :: [IfaceForAllBndr] -> ([TyVarBinder] -> IfL a) -> IfL a
bindIfaceForAllBndrs [] thing_inside = thing_inside []
bindIfaceForAllBndrs (bndr:bndrs) thing_inside
  = bindIfaceForAllBndr bndr $ \tv vis ->
    bindIfaceForAllBndrs bndrs $ \bndrs' ->
    thing_inside (mkTyVarBinder vis tv : bndrs')

bindIfaceForAllBndr :: IfaceForAllBndr -> (TyVar -> ArgFlag -> IfL a) -> IfL a
bindIfaceForAllBndr (TvBndr tv vis) thing_inside
  = bindIfaceTyVar tv $ \tv' -> thing_inside tv' vis

bindIfaceTyVar :: IfaceTvBndr -> (TyVar -> IfL a) -> IfL a
bindIfaceTyVar (occ,kind) thing_inside
  = do  { name <- newIfaceName (mkTyVarOccFS occ)
        ; tyvar <- mk_iface_tyvar name kind
        ; extendIfaceTyVarEnv [tyvar] (thing_inside tyvar) }

mk_iface_tyvar :: Name -> IfaceKind -> IfL TyVar
mk_iface_tyvar name ifKind
   = do { kind <- tcIfaceType ifKind
        ; return (Var.mkTyVar name kind) }

bindIfaceTyConBinders :: [IfaceTyConBinder]
                      -> ([TyConBinder] -> IfL a) -> IfL a
bindIfaceTyConBinders [] thing_inside = thing_inside []
bindIfaceTyConBinders (b:bs) thing_inside
  = bindIfaceTyConBinderX bindIfaceTyVar b $ \ b'  ->
    bindIfaceTyConBinders bs               $ \ bs' ->
    thing_inside (b':bs')

bindIfaceTyConBinders_AT :: [IfaceTyConBinder]
                         -> ([TyConBinder] -> IfL a) -> IfL a
-- Used for type variable in nested associated data/type declarations
-- where some of the type variables are already in scope
--    class C a where { data T a b }
-- Here 'a' is in scope when we look at the 'data T'
bindIfaceTyConBinders_AT [] thing_inside
  = thing_inside []
bindIfaceTyConBinders_AT (b : bs) thing_inside
  = bindIfaceTyConBinderX bind_tv b  $ \b'  ->
    bindIfaceTyConBinders_AT      bs $ \bs' ->
    thing_inside (b':bs')
  where
    bind_tv tv thing
      = do { mb_tv <- lookupIfaceTyVar tv
           ; case mb_tv of
               Just b' -> thing b'
               Nothing -> bindIfaceTyVar tv thing }

bindIfaceTyConBinderX :: (IfaceTvBndr -> (TyVar -> IfL a) -> IfL a)
                      -> IfaceTyConBinder
                      -> (TyConBinder -> IfL a) -> IfL a
bindIfaceTyConBinderX bind_tv (TvBndr tv vis) thing_inside
  = bind_tv tv $ \tv' ->
    thing_inside (TvBndr tv' vis)