TcBinds.hs 74.7 KB
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{-
(c) The University of Glasgow 2006
(c) The GRASP/AQUA Project, Glasgow University, 1992-1998

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\section[TcBinds]{TcBinds}
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-}
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{-# LANGUAGE CPP, RankNTypes, ScopedTypeVariables #-}

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module TcBinds ( tcLocalBinds, tcTopBinds, tcRecSelBinds,
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                 tcHsBootSigs, tcPolyCheck,
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                 PragFun, tcSpecPrags, tcSpecWrapper,
                 tcVectDecls, 
                 TcSigInfo(..), TcSigFun, mkPragFun,
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                 instTcTySig, instTcTySigFromId, findScopedTyVars,
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                 badBootDeclErr, mkExport ) where
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import {-# SOURCE #-} TcMatches ( tcGRHSsPat, tcMatchesFun )
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import {-# SOURCE #-} TcExpr  ( tcMonoExpr )
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import {-# SOURCE #-} TcPatSyn ( tcInferPatSynDecl, tcCheckPatSynDecl, tcPatSynBuilderBind )
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import DynFlags
import HsSyn
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import HscTypes( isHsBootOrSig )
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import TcRnMonad
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import TcEnv
import TcUnify
import TcSimplify
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import TcEvidence
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import TcHsType
import TcPat
import TcMType
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import ConLike
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import Inst( deeplyInstantiate )
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import FamInstEnv( normaliseType )
import FamInst( tcGetFamInstEnvs )
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import Type( pprSigmaTypeExtraCts )
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import TyCon
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import TcType
import TysPrim
import Id
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import Var
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import VarSet
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import VarEnv( TidyEnv )
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import Module
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import Name
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import NameSet
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import NameEnv
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import SrcLoc
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import Bag
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import ListSetOps
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import ErrUtils
import Digraph
import Maybes
import Util
import BasicTypes
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import Outputable
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import FastString
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import Type(mkStrLitTy)
import Class(classTyCon)
import PrelNames(ipClassName)
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import TcValidity (checkValidType)
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import Control.Monad
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import Data.List (partition)
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#include "HsVersions.h"
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{-
************************************************************************
*                                                                      *
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\subsection{Type-checking bindings}
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*                                                                      *
************************************************************************
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@tcBindsAndThen@ typechecks a @HsBinds@.  The "and then" part is because
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it needs to know something about the {\em usage} of the things bound,
so that it can create specialisations of them.  So @tcBindsAndThen@
takes a function which, given an extended environment, E, typechecks
the scope of the bindings returning a typechecked thing and (most
important) an LIE.  It is this LIE which is then used as the basis for
specialising the things bound.

@tcBindsAndThen@ also takes a "combiner" which glues together the
bindings and the "thing" to make a new "thing".

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The real work is done by @tcBindWithSigsAndThen@.
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Recursive and non-recursive binds are handled in essentially the same
way: because of uniques there are no scoping issues left.  The only
difference is that non-recursive bindings can bind primitive values.

Even for non-recursive binding groups we add typings for each binder
to the LVE for the following reason.  When each individual binding is
checked the type of its LHS is unified with that of its RHS; and
type-checking the LHS of course requires that the binder is in scope.

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At the top-level the LIE is sure to contain nothing but constant
dictionaries, which we resolve at the module level.

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Note [Polymorphic recursion]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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The game plan for polymorphic recursion in the code above is
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        * Bind any variable for which we have a type signature
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          to an Id with a polymorphic type.  Then when type-checking
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          the RHSs we'll make a full polymorphic call.

This fine, but if you aren't a bit careful you end up with a horrendous
amount of partial application and (worse) a huge space leak. For example:

        f :: Eq a => [a] -> [a]
        f xs = ...f...

If we don't take care, after typechecking we get

        f = /\a -> \d::Eq a -> let f' = f a d
                               in
                               \ys:[a] -> ...f'...

Notice the the stupid construction of (f a d), which is of course
identical to the function we're executing.  In this case, the
polymorphic recursion isn't being used (but that's a very common case).
This can lead to a massive space leak, from the following top-level defn
(post-typechecking)

        ff :: [Int] -> [Int]
        ff = f Int dEqInt

Now (f dEqInt) evaluates to a lambda that has f' as a free variable; but
f' is another thunk which evaluates to the same thing... and you end
up with a chain of identical values all hung onto by the CAF ff.

        ff = f Int dEqInt

           = let f' = f Int dEqInt in \ys. ...f'...

           = let f' = let f' = f Int dEqInt in \ys. ...f'...
                      in \ys. ...f'...

Etc.

NOTE: a bit of arity anaysis would push the (f a d) inside the (\ys...),
which would make the space leak go away in this case

Solution: when typechecking the RHSs we always have in hand the
*monomorphic* Ids for each binding.  So we just need to make sure that
if (Method f a d) shows up in the constraints emerging from (...f...)
we just use the monomorphic Id.  We achieve this by adding monomorphic Ids
to the "givens" when simplifying constraints.  That's what the "lies_avail"
is doing.

Then we get

        f = /\a -> \d::Eq a -> letrec
                                 fm = \ys:[a] -> ...fm...
                               in
                               fm
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-}
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tcTopBinds :: HsValBinds Name -> TcM (TcGblEnv, TcLclEnv)
-- The TcGblEnv contains the new tcg_binds and tcg_spects
-- The TcLclEnv has an extended type envt for the new bindings
tcTopBinds (ValBindsOut binds sigs)
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  = do  { -- Pattern synonym bindings populate the global environment
          (binds', (tcg_env, tcl_env)) <- tcValBinds TopLevel binds sigs $
            do { gbl <- getGblEnv
               ; lcl <- getLclEnv
               ; return (gbl, lcl) }
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        ; specs <- tcImpPrags sigs   -- SPECIALISE prags for imported Ids

        ; let { tcg_env' = tcg_env { tcg_binds = foldr (unionBags . snd)
                                                       (tcg_binds tcg_env)
                                                       binds'
                                   , tcg_imp_specs = specs ++ tcg_imp_specs tcg_env } }

        ; return (tcg_env', tcl_env) }
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        -- The top level bindings are flattened into a giant
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        -- implicitly-mutually-recursive LHsBinds
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tcTopBinds (ValBindsIn {}) = panic "tcTopBinds"

tcRecSelBinds :: HsValBinds Name -> TcM TcGblEnv
tcRecSelBinds (ValBindsOut binds sigs)
  = tcExtendGlobalValEnv [sel_id | L _ (IdSig sel_id) <- sigs] $
    do { (rec_sel_binds, tcg_env) <- discardWarnings (tcValBinds TopLevel binds sigs getGblEnv)
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       ; let tcg_env'
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              | isHsBootOrSig (tcg_src tcg_env) = tcg_env
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              | otherwise = tcg_env { tcg_binds = foldr (unionBags . snd)
                                                        (tcg_binds tcg_env)
                                                        rec_sel_binds }
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              -- Do not add the code for record-selector bindings when
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              -- compiling hs-boot files
       ; return tcg_env' }
tcRecSelBinds (ValBindsIn {}) = panic "tcRecSelBinds"
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tcHsBootSigs :: HsValBinds Name -> TcM [Id]
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-- A hs-boot file has only one BindGroup, and it only has type
-- signatures in it.  The renamer checked all this
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tcHsBootSigs (ValBindsOut binds sigs)
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  = do  { checkTc (null binds) badBootDeclErr
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        ; concat <$> mapM (addLocM tc_boot_sig) (filter isTypeLSig sigs) }
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  where
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    tc_boot_sig (TypeSig lnames ty _) = mapM f lnames
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      where
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        f (L _ name) = do  { sigma_ty <- tcHsSigType (FunSigCtxt name True) ty
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                           ; return (mkVanillaGlobal name sigma_ty) }
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        -- Notice that we make GlobalIds, not LocalIds
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    tc_boot_sig s = pprPanic "tcHsBootSigs/tc_boot_sig" (ppr s)
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tcHsBootSigs groups = pprPanic "tcHsBootSigs" (ppr groups)
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badBootDeclErr :: MsgDoc
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badBootDeclErr = ptext (sLit "Illegal declarations in an hs-boot file")
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------------------------
tcLocalBinds :: HsLocalBinds Name -> TcM thing
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             -> TcM (HsLocalBinds TcId, thing)
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tcLocalBinds EmptyLocalBinds thing_inside
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  = do  { thing <- thing_inside
        ; return (EmptyLocalBinds, thing) }
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tcLocalBinds (HsValBinds (ValBindsOut binds sigs)) thing_inside
  = do  { (binds', thing) <- tcValBinds NotTopLevel binds sigs thing_inside
        ; return (HsValBinds (ValBindsOut binds' sigs), thing) }
tcLocalBinds (HsValBinds (ValBindsIn {})) _ = panic "tcLocalBinds"
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tcLocalBinds (HsIPBinds (IPBinds ip_binds _)) thing_inside
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  = do  { ipClass <- tcLookupClass ipClassName
        ; (given_ips, ip_binds') <-
            mapAndUnzipM (wrapLocSndM (tc_ip_bind ipClass)) ip_binds
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        -- If the binding binds ?x = E, we  must now
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        -- discharge any ?x constraints in expr_lie
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        -- See Note [Implicit parameter untouchables]
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        ; (ev_binds, result) <- checkConstraints (IPSkol ips)
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                                  [] given_ips thing_inside
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        ; return (HsIPBinds (IPBinds ip_binds' ev_binds), result) }
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  where
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    ips = [ip | L _ (IPBind (Left ip) _) <- ip_binds]
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        -- I wonder if we should do these one at at time
        -- Consider     ?x = 4
        --              ?y = ?x + 1
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    tc_ip_bind ipClass (IPBind (Left ip) expr)
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       = do { ty <- newFlexiTyVarTy openTypeKind
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            ; let p = mkStrLitTy $ hsIPNameFS ip
            ; ip_id <- newDict ipClass [ p, ty ]
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            ; expr' <- tcMonoExpr expr ty
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            ; let d = toDict ipClass p ty `fmap` expr'
            ; return (ip_id, (IPBind (Right ip_id) d)) }
    tc_ip_bind _ (IPBind (Right {}) _) = panic "tc_ip_bind"

    -- Coerces a `t` into a dictionry for `IP "x" t`.
    -- co : t -> IP "x" t
    toDict ipClass x ty =
      case unwrapNewTyCon_maybe (classTyCon ipClass) of
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        Just (_,_,ax) -> HsWrap $ mkWpCast $ mkTcSymCo $ mkTcUnbranchedAxInstCo Representational ax [x,ty]
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        Nothing       -> panic "The dictionary for `IP` is not a newtype?"

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{-
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Note [Implicit parameter untouchables]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We add the type variables in the types of the implicit parameters
as untouchables, not so much because we really must not unify them,
but rather because we otherwise end up with constraints like this
    Num alpha, Implic { wanted = alpha ~ Int }
The constraint solver solves alpha~Int by unification, but then
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doesn't float that solved constraint out (it's not an unsolved
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wanted).  Result disaster: the (Num alpha) is again solved, this
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time by defaulting.  No no no.

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However [Oct 10] this is all handled automatically by the
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untouchable-range idea.

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Note [Placeholder PatSyn kinds]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider this (Trac #9161)

  {-# LANGUAGE PatternSynonyms, DataKinds #-}
  pattern A = ()
  b :: A
  b = undefined

Here, the type signature for b mentions A.  But A is a pattern
synonym, which is typechecked (for very good reasons; a view pattern
in the RHS may mention a value binding) as part of a group of
bindings.  It is entirely resonable to reject this, but to do so
we need A to be in the kind environment when kind-checking the signature for B.

Hence the tcExtendKindEnv2 patsyn_placeholder_kinds, which adds a binding
    A -> AGlobal (AConLike (PatSynCon _|_))
to the environment. Then TcHsType.tcTyVar will find A in the kind environment,
and will give a 'wrongThingErr' as a result.  But the lookup of A won't fail.

The _|_ (= panic "fakePatSynCon") works because the wrongThingErr call, in
tcTyVar, doesn't look inside the TcTyThing.
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-}
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tcValBinds :: TopLevelFlag
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           -> [(RecFlag, LHsBinds Name)] -> [LSig Name]
           -> TcM thing
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           -> TcM ([(RecFlag, LHsBinds TcId)], thing)
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tcValBinds top_lvl binds sigs thing_inside
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  = do  {  -- Typecheck the signature
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        ; (poly_ids, sig_fn, nwc_tvs) <- tcExtendKindEnv2 patsyn_placeholder_kinds $
                                         -- See Note [Placeholder PatSyn kinds]
                                         tcTySigs sigs
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        ; let prag_fn = mkPragFun sigs (foldr (unionBags . snd) emptyBag binds)
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                -- Extend the envt right away with all
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                -- the Ids declared with type signatures
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                -- Use tcExtendIdEnv3 to avoid extending the TcIdBinder stack
        ; tcExtendIdEnv3 [(idName id, id) | id <- poly_ids] (mkVarSet nwc_tvs) $ do
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            { (binds', (extra_binds', thing)) <- tcBindGroups top_lvl sig_fn prag_fn binds $ do
                   { thing <- thing_inside
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                     -- See Note [Pattern synonym wrappers don't yield dependencies]
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                   ; patsyn_workers <- mapM tcPatSynBuilderBind patsyns
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                   ; let extra_binds = [ (NonRecursive, worker) | worker <- patsyn_workers ]
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                   ; return (extra_binds, thing) }
             ; return (binds' ++ extra_binds', thing) }}
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  where
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    patsyns
      = [psb | (_, lbinds) <- binds, L _ (PatSynBind psb) <- bagToList lbinds]
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    patsyn_placeholder_kinds -- See Note [Placeholder PatSyn kinds]
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      = [(name, placeholder_patsyn_tything)| PSB{ psb_id = L _ name } <- patsyns ]
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    placeholder_patsyn_tything
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      = AGlobal $ AConLike $ PatSynCon $ panic "fakePatSynCon"
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------------------------
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tcBindGroups :: TopLevelFlag -> TcSigFun -> PragFun
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             -> [(RecFlag, LHsBinds Name)] -> TcM thing
             -> TcM ([(RecFlag, LHsBinds TcId)], thing)
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-- Typecheck a whole lot of value bindings,
-- one strongly-connected component at a time
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-- Here a "strongly connected component" has the strightforward
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-- meaning of a group of bindings that mention each other,
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-- ignoring type signatures (that part comes later)
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tcBindGroups _ _ _ [] thing_inside
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  = do  { thing <- thing_inside
        ; return ([], thing) }
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tcBindGroups top_lvl sig_fn prag_fn (group : groups) thing_inside
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  = do  { (group', (groups', thing))
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                <- tc_group top_lvl sig_fn prag_fn group $
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                   tcBindGroups top_lvl sig_fn prag_fn groups thing_inside
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        ; return (group' ++ groups', thing) }
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------------------------
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tc_group :: forall thing.
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            TopLevelFlag -> TcSigFun -> PragFun
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         -> (RecFlag, LHsBinds Name) -> TcM thing
         -> TcM ([(RecFlag, LHsBinds TcId)], thing)
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-- Typecheck one strongly-connected component of the original program.
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-- We get a list of groups back, because there may
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-- be specialisations etc as well

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tc_group top_lvl sig_fn prag_fn (NonRecursive, binds) thing_inside
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        -- A single non-recursive binding
        -- We want to keep non-recursive things non-recursive
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        -- so that we desugar unlifted bindings correctly
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  = do { let bind = case bagToList binds of
                 [bind] -> bind
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                 []     -> panic "tc_group: empty list of binds"
                 _      -> panic "tc_group: NonRecursive binds is not a singleton bag"
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       ; (bind', thing) <- tc_single top_lvl sig_fn prag_fn bind thing_inside
       ; return ( [(NonRecursive, bind')], thing) }
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tc_group top_lvl sig_fn prag_fn (Recursive, binds) thing_inside
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  =     -- To maximise polymorphism, we do a new
        -- strongly-connected-component analysis, this time omitting
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        -- any references to variables with type signatures.
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        -- (This used to be optional, but isn't now.)
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    do  { traceTc "tc_group rec" (pprLHsBinds binds)
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        ; when hasPatSyn $ recursivePatSynErr binds
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        ; (binds1, thing) <- go sccs
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        ; return ([(Recursive, binds1)], thing) }
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                -- Rec them all together
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  where
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    hasPatSyn = anyBag (isPatSyn . unLoc) binds
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    isPatSyn PatSynBind{} = True
    isPatSyn _ = False

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    sccs :: [SCC (LHsBind Name)]
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    sccs = stronglyConnCompFromEdgedVertices (mkEdges sig_fn binds)

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    go :: [SCC (LHsBind Name)] -> TcM (LHsBinds TcId, thing)
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    go (scc:sccs) = do  { (binds1, ids1, closed) <- tc_scc scc
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                        ; (binds2, thing) <- tcExtendLetEnv top_lvl closed ids1 $
                                             go sccs
                        ; return (binds1 `unionBags` binds2, thing) }
    go []         = do  { thing <- thing_inside; return (emptyBag, thing) }
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    tc_scc (AcyclicSCC bind) = tc_sub_group NonRecursive [bind]
    tc_scc (CyclicSCC binds) = tc_sub_group Recursive    binds
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    tc_sub_group = tcPolyBinds top_lvl sig_fn prag_fn Recursive
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recursivePatSynErr :: OutputableBndr name => LHsBinds name -> TcM a
recursivePatSynErr binds
  = failWithTc $
    hang (ptext (sLit "Recursive pattern synonym definition with following bindings:"))
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       2 (vcat $ map pprLBind . bagToList $ binds)
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  where
    pprLoc loc  = parens (ptext (sLit "defined at") <+> ppr loc)
    pprLBind (L loc bind) = pprWithCommas ppr (collectHsBindBinders bind) <+>
                            pprLoc loc

tc_single :: forall thing.
            TopLevelFlag -> TcSigFun -> PragFun
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          -> LHsBind Name -> TcM thing
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          -> TcM (LHsBinds TcId, thing)
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tc_single _top_lvl sig_fn _prag_fn (L _ (PatSynBind psb@PSB{ psb_id = L _ name })) thing_inside
  = do { (pat_syn, aux_binds) <- tc_pat_syn_decl
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       ; let tything = AConLike (PatSynCon pat_syn)
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       ; thing <- tcExtendGlobalEnv [tything] thing_inside
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       ; return (aux_binds, thing)
       }
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  where
    tc_pat_syn_decl = case sig_fn name of
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        Nothing                  -> tcInferPatSynDecl psb
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        Just (TcPatSynInfo tpsi) -> tcCheckPatSynDecl psb tpsi
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        Just                  _  -> panic "tc_single"
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tc_single top_lvl sig_fn prag_fn lbind thing_inside
  = do { (binds1, ids, closed) <- tcPolyBinds top_lvl sig_fn prag_fn
                                    NonRecursive NonRecursive
                                    [lbind]
       ; thing <- tcExtendLetEnv top_lvl closed ids thing_inside
       ; return (binds1, thing) }
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-- | No signature or a partial signature
noCompleteSig :: Maybe TcSigInfo -> Bool
noCompleteSig Nothing    = True
noCompleteSig (Just sig) = isPartialSig sig

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------------------------
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mkEdges :: TcSigFun -> LHsBinds Name -> [Node BKey (LHsBind Name)]
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type BKey = Int -- Just number off the bindings
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mkEdges sig_fn binds
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  = [ (bind, key, [key | n <- nameSetElems (bind_fvs (unLoc bind)),
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                         Just key <- [lookupNameEnv key_map n], no_sig n ])
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    | (bind, key) <- keyd_binds
    ]
  where
    no_sig :: Name -> Bool
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    no_sig n = noCompleteSig (sig_fn n)
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    keyd_binds = bagToList binds `zip` [0::BKey ..]

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    key_map :: NameEnv BKey     -- Which binding it comes from
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    key_map = mkNameEnv [(bndr, key) | (L _ bind, key) <- keyd_binds
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                                     , bndr <- collectHsBindBinders bind ]
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------------------------
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tcPolyBinds :: TopLevelFlag -> TcSigFun -> PragFun
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            -> RecFlag         -- Whether the group is really recursive
            -> RecFlag         -- Whether it's recursive after breaking
                               -- dependencies based on type signatures
            -> [LHsBind Name]  -- None are PatSynBind
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            -> TcM (LHsBinds TcId, [TcId], TopLevelFlag)
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-- Typechecks a single bunch of values bindings all together,
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-- and generalises them.  The bunch may be only part of a recursive
-- group, because we use type signatures to maximise polymorphism
--
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-- Returns a list because the input may be a single non-recursive binding,
-- in which case the dependency order of the resulting bindings is
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-- important.
--
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-- Knows nothing about the scope of the bindings
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-- None of the bindings are pattern synonyms
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tcPolyBinds top_lvl sig_fn prag_fn rec_group rec_tc bind_list
  = setSrcSpan loc                              $
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    recoverM (recoveryCode binder_names sig_fn) $ do
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        -- Set up main recover; take advantage of any type sigs
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    { traceTc "------------------------------------------------" Outputable.empty
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    ; traceTc "Bindings for {" (ppr binder_names)
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    ; dflags   <- getDynFlags
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    ; type_env <- getLclTypeEnv
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    ; let plan = decideGeneralisationPlan dflags type_env
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                         binder_names bind_list sig_fn
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    ; traceTc "Generalisation plan" (ppr plan)
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    ; result@(tc_binds, poly_ids, _) <- case plan of
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         NoGen               -> tcPolyNoGen rec_tc prag_fn sig_fn bind_list
         InferGen mn cl      -> tcPolyInfer rec_tc prag_fn sig_fn mn cl bind_list
         CheckGen lbind sig  -> tcPolyCheck rec_tc prag_fn sig lbind
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        -- Check whether strict bindings are ok
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        -- These must be non-recursive etc, and are not generalised
        -- They desugar to a case expression in the end
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    ; checkStrictBinds top_lvl rec_group bind_list tc_binds poly_ids
    ; traceTc "} End of bindings for" (vcat [ ppr binder_names, ppr rec_group
                                            , vcat [ppr id <+> ppr (idType id) | id <- poly_ids]
                                          ])
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    ; return result }
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  where
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    binder_names = collectHsBindListBinders bind_list
    loc = foldr1 combineSrcSpans (map getLoc bind_list)
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         -- The mbinds have been dependency analysed and
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         -- may no longer be adjacent; so find the narrowest
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         -- span that includes them all
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------------------
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tcPolyNoGen     -- No generalisation whatsoever
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  :: RecFlag       -- Whether it's recursive after breaking
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                   -- dependencies based on type signatures
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  -> PragFun -> TcSigFun
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  -> [LHsBind Name]
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  -> TcM (LHsBinds TcId, [TcId], TopLevelFlag)
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tcPolyNoGen rec_tc prag_fn tc_sig_fn bind_list
  = do { (binds', mono_infos) <- tcMonoBinds rec_tc tc_sig_fn
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                                             (LetGblBndr prag_fn)
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                                             bind_list
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       ; mono_ids' <- mapM tc_mono_info mono_infos
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       ; return (binds', mono_ids', NotTopLevel) }
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  where
    tc_mono_info (name, _, mono_id)
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      = do { mono_ty' <- zonkTcType (idType mono_id)
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             -- Zonk, mainly to expose unboxed types to checkStrictBinds
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           ; let mono_id' = setIdType mono_id mono_ty'
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           ; _specs <- tcSpecPrags mono_id' (prag_fn name)
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           ; return mono_id' }
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           -- NB: tcPrags generates error messages for
           --     specialisation pragmas for non-overloaded sigs
           -- Indeed that is why we call it here!
           -- So we can safely ignore _specs
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------------------
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tcPolyCheck :: RecFlag       -- Whether it's recursive after breaking
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                             -- dependencies based on type signatures
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            -> PragFun
            -> TcSigInfo
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            -> LHsBind Name
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            -> TcM (LHsBinds TcId, [TcId], TopLevelFlag)
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-- There is just one binding,
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--   it binds a single variable,
--   it has a signature,
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tcPolyCheck rec_tc prag_fn
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            sig@(TcSigInfo { sig_id = poly_id, sig_tvs = tvs_w_scoped
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                           , sig_nwcs = sig_nwcs, sig_theta = theta
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                           , sig_tau = tau, sig_loc = loc
                           , sig_warn_redundant = warn_redundant })
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            bind
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  = ASSERT( null sig_nwcs ) -- We should be in tcPolyInfer if there are wildcards
    do { ev_vars <- newEvVars theta
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       ; let ctxt      = FunSigCtxt (idName poly_id) warn_redundant
             skol_info = SigSkol ctxt (mkPhiTy theta tau)
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             prag_sigs = prag_fn (idName poly_id)
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             tvs = map snd tvs_w_scoped
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       ; (ev_binds, (binds', [mono_info]))
            <- setSrcSpan loc $
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               checkConstraints skol_info tvs ev_vars $
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               tcMonoBinds rec_tc (\_ -> Just sig) LetLclBndr [bind]
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       ; spec_prags <- tcSpecPrags poly_id prag_sigs
       ; poly_id    <- addInlinePrags poly_id prag_sigs
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       ; let (_, _, mono_id) = mono_info
             export = ABE { abe_wrap = idHsWrapper
                          , abe_poly = poly_id
                          , abe_mono = mono_id
                          , abe_prags = SpecPrags spec_prags }
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             abs_bind = L loc $ AbsBinds
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                        { abs_tvs = tvs
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                        , abs_ev_vars = ev_vars, abs_ev_binds = [ev_binds]
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                        , abs_exports = [export], abs_binds = binds' }
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             closed | isEmptyVarSet (tyVarsOfType (idType poly_id)) = TopLevel
                    | otherwise                                     = NotTopLevel
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       ; return (unitBag abs_bind, [poly_id], closed) }
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tcPolyCheck _rec_tc _prag_fn sig _bind
  = pprPanic "tcPolyCheck" (ppr sig)

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------------------
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tcPolyInfer
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  :: RecFlag       -- Whether it's recursive after breaking
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                   -- dependencies based on type signatures
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  -> PragFun -> TcSigFun
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  -> Bool         -- True <=> apply the monomorphism restriction
  -> Bool         -- True <=> free vars have closed types
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  -> [LHsBind Name]
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  -> TcM (LHsBinds TcId, [TcId], TopLevelFlag)
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tcPolyInfer rec_tc prag_fn tc_sig_fn mono closed bind_list
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  = do { ((binds', mono_infos), tclvl, wanted)
             <- pushLevelAndCaptureConstraints  $
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                tcMonoBinds rec_tc tc_sig_fn LetLclBndr bind_list
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       ; let name_taus = [(name, idType mono_id) | (name, _, mono_id) <- mono_infos]
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       ; traceTc "simplifyInfer call" (ppr name_taus $$ ppr wanted)
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       ; (qtvs, givens, mr_bites, ev_binds)
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                 <- simplifyInfer tclvl mono name_taus wanted
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       ; inferred_theta  <- zonkTcThetaType (map evVarPred givens)
       ; exports <- checkNoErrs $ mapM (mkExport prag_fn qtvs inferred_theta)
                                       mono_infos

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       ; loc <- getSrcSpanM
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       ; let poly_ids = map abe_poly exports
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             final_closed | closed && not mr_bites = TopLevel
                          | otherwise              = NotTopLevel
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             abs_bind = L loc $
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                        AbsBinds { abs_tvs = qtvs
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                                 , abs_ev_vars = givens, abs_ev_binds = [ev_binds]
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                                 , abs_exports = exports, abs_binds = binds' }
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       ; traceTc "Binding:" (ppr final_closed $$
                             ppr (poly_ids `zip` map idType poly_ids))
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       ; return (unitBag abs_bind, poly_ids, final_closed) }
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         -- poly_ids are guaranteed zonked by mkExport
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--------------
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mkExport :: PragFun
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         -> [TyVar] -> TcThetaType      -- Both already zonked
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         -> MonoBindInfo
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         -> TcM (ABExport Id)
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-- Only called for generalisation plan IferGen, not by CheckGen or NoGen
--
-- mkExport generates exports with
--      zonked type variables,
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--      zonked poly_ids
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-- The former is just because no further unifications will change
-- the quantified type variables, so we can fix their final form
-- right now.
-- The latter is needed because the poly_ids are used to extend the
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-- type environment; see the invariant on TcEnv.tcExtendIdEnv
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-- Pre-condition: the qtvs and theta are already zonked
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mkExport prag_fn qtvs inferred_theta (poly_name, mb_sig, mono_id)
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  = do  { mono_ty <- zonkTcType (idType mono_id)
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        ; poly_id <- case mb_sig of
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              Nothing -> mkInferredPolyId poly_name qtvs inferred_theta mono_ty
              Just (TcPatSynInfo _) -> panic "mkExport"
              Just sig | isPartialSig sig
                       -> do { final_theta <- completeTheta inferred_theta sig
                             ; mkInferredPolyId poly_name qtvs final_theta mono_ty }
                       | otherwise
                       -> return (sig_id sig)
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        -- NB: poly_id has a zonked type
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        ; poly_id <- addInlinePrags poly_id prag_sigs
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        ; spec_prags <- tcSpecPrags poly_id prag_sigs
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                -- tcPrags requires a zonked poly_id
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        ; let sel_poly_ty = mkSigmaTy qtvs inferred_theta mono_ty
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        ; traceTc "mkExport: check sig"
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                  (vcat [ ppr poly_name, ppr sel_poly_ty, ppr (idType poly_id) ])
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        -- Perform the impedence-matching and ambiguity check
        -- right away.  If it fails, we want to fail now (and recover
        -- in tcPolyBinds).  If we delay checking, we get an error cascade.
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        -- Remember we are in the tcPolyInfer case, so the type envt is
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        -- closed (unless we are doing NoMonoLocalBinds in which case all bets
        -- are off)
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        -- See Note [Impedence matching]
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        ; (wrap, wanted) <- addErrCtxtM (mk_bind_msg inferred True poly_name (idType poly_id)) $
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                            captureConstraints $
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                            tcSubType_NC sig_ctxt sel_poly_ty (idType poly_id)
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        ; ev_binds <- simplifyTop wanted
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        ; return (ABE { abe_wrap = mkWpLet (EvBinds ev_binds) <.> wrap
                      , abe_poly = poly_id
                      , abe_mono = mono_id
                      , abe_prags = SpecPrags spec_prags }) }
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  where
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    inferred = isNothing mb_sig
    prag_sigs = prag_fn poly_name
    sig_ctxt  = InfSigCtxt poly_name
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mkInferredPolyId :: Name -> [TyVar] -> TcThetaType -> TcType -> TcM Id
-- In the inference case (no signature) this stuff figures out
-- the right type variables and theta to quantify over
-- See Note [Validity of inferred types]
mkInferredPolyId poly_name qtvs theta mono_ty
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  = do { fam_envs <- tcGetFamInstEnvs

       ; let (_co, norm_mono_ty) = normaliseType fam_envs Nominal mono_ty
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               -- Unification may not have normalised the type,
               -- (see Note [Lazy flattening] in TcFlatten) so do it
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               -- here to make it as uncomplicated as possible.
               -- Example: f :: [F Int] -> Bool
               -- should be rewritten to f :: [Char] -> Bool, if possible
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             my_tvs2 = closeOverKinds (growThetaTyVars theta (tyVarsOfType norm_mono_ty))
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                  -- Include kind variables!  Trac #7916
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             my_tvs   = filter (`elemVarSet` my_tvs2) qtvs   -- Maintain original order
             my_theta = filter (quantifyPred my_tvs2) theta
             inferred_poly_ty = mkSigmaTy my_tvs my_theta norm_mono_ty

       ; addErrCtxtM (mk_bind_msg True False poly_name inferred_poly_ty) $
         checkValidType (InfSigCtxt poly_name) inferred_poly_ty
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       ; return (mkLocalId poly_name inferred_poly_ty) }
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mk_bind_msg :: Bool -> Bool -> Name -> TcType -> TidyEnv -> TcM (TidyEnv, SDoc)
mk_bind_msg inferred want_ambig poly_name poly_ty tidy_env
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 = do { (tidy_env', tidy_ty) <- zonkTidyTcType tidy_env poly_ty
      ; return (tidy_env', mk_msg tidy_ty) }
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 where
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   mk_msg ty = vcat [ ptext (sLit "When checking that") <+> quotes (ppr poly_name)
                      <+> ptext (sLit "has the") <+> what <+> ptext (sLit "type")
                    , nest 2 (ppr poly_name <+> dcolon <+> ppr ty)
                    , ppWhen want_ambig $
                      ptext (sLit "Probable cause: the inferred type is ambiguous") ]
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   what | inferred  = ptext (sLit "inferred")
        | otherwise = ptext (sLit "specified")
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-- | Report the inferred constraints for an extra-constraints wildcard/hole as
-- an error message, unless the PartialTypeSignatures flag is enabled. In this
-- case, the extra inferred constraints are accepted without complaining.
-- Returns the annotated constraints combined with the inferred constraints.
completeTheta :: TcThetaType -> TcSigInfo -> TcM TcThetaType
completeTheta _ (TcPatSynInfo _)
  = panic "Extra-constraints wildcard not supported in a pattern signature"
completeTheta inferred_theta
              sig@(TcSigInfo { sig_id = poly_id
                             , sig_extra_cts = mb_extra_cts
                             , sig_theta = annotated_theta })
  | Just loc <- mb_extra_cts
  = do { annotated_theta <- zonkTcThetaType annotated_theta
       ; let inferred_diff = minusList inferred_theta annotated_theta
             final_theta   = annotated_theta ++ inferred_diff
       ; partial_sigs      <- xoptM Opt_PartialTypeSignatures
       ; warn_partial_sigs <- woptM Opt_WarnPartialTypeSignatures
       ; msg <- mkLongErrAt loc (mk_msg inferred_diff partial_sigs) empty
       ; case partial_sigs of
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           True | warn_partial_sigs -> reportWarning msg
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                | otherwise         -> return ()
           False                    -> reportError msg
       ; return final_theta }

  | otherwise
  = zonkTcThetaType annotated_theta
    -- No extra-constraints wildcard means no extra constraints will be added
    -- to the context, so just return the possibly empty (zonked)
    -- annotated_theta.
  where
    pts_hint = text "To use the inferred type, enable PartialTypeSignatures"
    mk_msg inferred_diff suppress_hint
       = vcat [ hang ((text "Found hole") <+> quotes (char '_'))
                   2 (text "with inferred constraints:")
                      <+> pprTheta inferred_diff
              , if suppress_hint then empty else pts_hint
              , typeSigCtxt (idName poly_id) sig ]
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{-
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Note [Partial type signatures and generalisation]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When we have a partial type signature, like
   f :: _ -> Int
then we *always* use the InferGen plan, and hence tcPolyInfer.
We do this even for a local binding with -XMonoLocalBinds.
Reasons:
  * The TcSigInfo for 'f' has a unification variable for the '_',
    whose TcLevel is one level deeper than the current level.
    (See pushTcLevelM in tcTySig.)  But NoGen doesn't increase
    the TcLevel like InferGen, so we lose the level invariant.

  * The signature might be   f :: forall a. _ -> a
    so it really is polymorphic.  It's not clear what it would
    mean to use NoGen on this, and indeed the ASSERT in tcLhs,
    in the (Just sig) case, checks that if there is a signature
    then we are using LetLclBndr, and hence a nested AbsBinds with
    increased TcLevel

It might be possible to fix these difficulties somehow, but there
doesn't seem much point.  Indeed, adding a partial type signature is a
way to get per-binding inferred generalisation.

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Note [Validity of inferred types]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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We need to check inferred type for validity, in case it uses language
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extensions that are not turned on.  The principle is that if the user
simply adds the inferred type to the program source, it'll compile fine.
See #8883.

Examples that might fail:
 - an inferred theta that requires type equalities e.g. (F a ~ G b)
                                or multi-parameter type classes
 - an inferred type that includes unboxed tuples

However we don't do the ambiguity check (checkValidType omits it for
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InfSigCtxt) because the impedence-matching stage, which follows
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immediately, will do it and we don't want two error messages.
Moreover, because of the impedence matching stage, the ambiguity-check
suggestion of -XAllowAmbiguiousTypes will not work.


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Note [Impedence matching]
~~~~~~~~~~~~~~~~~~~~~~~~~
Consider
   f 0 x = x
   f n x = g [] (not x)

   g [] y = f 10 y
   g _  y = f 9  y

After typechecking we'll get
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  f_mono_ty :: a -> Bool -> Bool
  g_mono_ty :: [b] -> Bool -> Bool
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with constraints
  (Eq a, Num a)

Note that f is polymorphic in 'a' and g in 'b'; and these are not linked.
The types we really want for f and g are
   f :: forall a. (Eq a, Num a) => a -> Bool -> Bool
   g :: forall b. [b] -> Bool -> Bool

We can get these by "impedence matching":
   tuple :: forall a b. (Eq a, Num a) => (a -> Bool -> Bool, [b] -> Bool -> Bool)
   tuple a b d1 d1 = let ...bind f_mono, g_mono in (f_mono, g_mono)

   f a d1 d2 = case tuple a Any d1 d2 of (f, g) -> f
   g b = case tuple Integer b dEqInteger dNumInteger of (f,g) -> g

Suppose the shared quantified tyvars are qtvs and constraints theta.
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Then we want to check that
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   f's polytype  is more polymorphic than   forall qtvs. theta => f_mono_ty
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and the proof is the impedence matcher.
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Notice that the impedence matcher may do defaulting.  See Trac #7173.

It also cleverly does an ambiguity check; for example, rejecting
   f :: F a -> a
where F is a non-injective type function.
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-}
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