TcBinds.hs

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

\section[TcBinds]{TcBinds}
-}

{-# LANGUAGE CPP, RankNTypes, ScopedTypeVariables #-}

module TcBinds ( tcLocalBinds, tcTopBinds, tcRecSelBinds,
                 tcValBinds, tcHsBootSigs, tcPolyCheck,
                 tcSpecPrags, tcSpecWrapper,
                 tcVectDecls, addTypecheckedBinds,
                 TcSigInfo(..), TcSigFun,
                 TcPragEnv, mkPragEnv,
                 tcUserTypeSig, instTcTySig, chooseInferredQuantifiers,
                 instTcTySigFromId, tcExtendTyVarEnvFromSig,
                 badBootDeclErr ) where

import {-# SOURCE #-} TcMatches ( tcGRHSsPat, tcMatchesFun )
import {-# SOURCE #-} TcExpr  ( tcMonoExpr )
import {-# SOURCE #-} TcPatSyn ( tcInferPatSynDecl, tcCheckPatSynDecl
                               , tcPatSynBuilderBind, tcPatSynSig )
import DynFlags
import HsSyn
import HscTypes( isHsBootOrSig )
import TcRnMonad
import TcEnv
import TcUnify
import TcSimplify
import TcEvidence
import TcHsType
import TcPat
import TcMType
import Inst( topInstantiate, deeplyInstantiate )
import FamInstEnv( normaliseType )
import FamInst( tcGetFamInstEnvs )
import TyCon
import TcType
import TysPrim
import Id
import Var
import VarSet
import VarEnv( TidyEnv )
import Module
import Name
import NameSet
import NameEnv
import SrcLoc
import Bag
import ListSetOps
import ErrUtils
import Digraph
import Maybes
import Util
import BasicTypes
import Outputable
import FastString
import Type(mkStrLitTy, tidyOpenType)
import PrelNames( mkUnboundName, gHC_PRIM, ipClassName )
import TcValidity (checkValidType)
import qualified GHC.LanguageExtensions as LangExt

import Control.Monad

#include "HsVersions.h"

{- *********************************************************************
*                                                                      *
               A useful helper function
*                                                                      *
********************************************************************* -}

addTypecheckedBinds :: TcGblEnv -> [LHsBinds Id] -> TcGblEnv
addTypecheckedBinds tcg_env binds
  | isHsBootOrSig (tcg_src tcg_env) = tcg_env
    -- Do not add the code for record-selector bindings
    -- when compiling hs-boot files
  | otherwise = tcg_env { tcg_binds = foldr unionBags
                                            (tcg_binds tcg_env)
                                            binds }

{-
************************************************************************
*                                                                      *
\subsection{Type-checking bindings}
*                                                                      *
************************************************************************

@tcBindsAndThen@ typechecks a @HsBinds@.  The "and then" part is because
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".

The real work is done by @tcBindWithSigsAndThen@.

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.

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

Note [Polymorphic recursion]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The game plan for polymorphic recursion in the code above is

        * Bind any variable for which we have a type signature
          to an Id with a polymorphic type.  Then when type-checking
          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
-}

tcTopBinds :: [(RecFlag, LHsBinds Name)] -> [LSig 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 binds sigs
  = 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) }
        ; specs <- tcImpPrags sigs   -- SPECIALISE prags for imported Ids

        ; let { tcg_env' = tcg_env { tcg_imp_specs = specs ++ tcg_imp_specs tcg_env }
                           `addTypecheckedBinds` map snd binds' }

        ; return (tcg_env', tcl_env) }
        -- The top level bindings are flattened into a giant
        -- implicitly-mutually-recursive LHsBinds

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
       ; let tcg_env' = tcg_env `addTypecheckedBinds` map snd rec_sel_binds
       ; return tcg_env' }
tcRecSelBinds (ValBindsIn {}) = panic "tcRecSelBinds"

tcHsBootSigs :: [(RecFlag, LHsBinds Name)] -> [LSig Name] -> TcM [Id]
-- A hs-boot file has only one BindGroup, and it only has type
-- signatures in it.  The renamer checked all this
tcHsBootSigs binds sigs
  = do  { checkTc (null binds) badBootDeclErr
        ; concat <$> mapM (addLocM tc_boot_sig) (filter isTypeLSig sigs) }
  where
    tc_boot_sig (TypeSig lnames hs_ty) = mapM f lnames
      where
        f (L _ name)
          = do { sigma_ty <- solveEqualities $
                             tcHsSigWcType (FunSigCtxt name False) hs_ty
               ; return (mkVanillaGlobal name sigma_ty) }
        -- Notice that we make GlobalIds, not LocalIds
    tc_boot_sig s = pprPanic "tcHsBootSigs/tc_boot_sig" (ppr s)

badBootDeclErr :: MsgDoc
badBootDeclErr = ptext (sLit "Illegal declarations in an hs-boot file")

------------------------
tcLocalBinds :: HsLocalBinds Name -> TcM thing
             -> TcM (HsLocalBinds TcId, thing)

tcLocalBinds EmptyLocalBinds thing_inside
  = do  { thing <- thing_inside
        ; return (EmptyLocalBinds, thing) }

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"

tcLocalBinds (HsIPBinds (IPBinds ip_binds _)) thing_inside
  = do  { ipClass <- tcLookupClass ipClassName
        ; (given_ips, ip_binds') <-
            mapAndUnzipM (wrapLocSndM (tc_ip_bind ipClass)) ip_binds

        -- If the binding binds ?x = E, we  must now
        -- discharge any ?x constraints in expr_lie
        -- See Note [Implicit parameter untouchables]
        ; (ev_binds, result) <- checkConstraints (IPSkol ips)
                                  [] given_ips thing_inside

        ; return (HsIPBinds (IPBinds ip_binds' ev_binds), result) }
  where
    ips = [ip | L _ (IPBind (Left (L _ ip)) _) <- ip_binds]

        -- I wonder if we should do these one at at time
        -- Consider     ?x = 4
        --              ?y = ?x + 1
    tc_ip_bind ipClass (IPBind (Left (L _ ip)) expr)
       = do { ty <- newOpenFlexiTyVarTy
            ; let p = mkStrLitTy $ hsIPNameFS ip
            ; ip_id <- newDict ipClass [ p, ty ]
            ; expr' <- tcMonoExpr expr ty
            ; 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 = HsWrap $ mkWpCastR $
                          wrapIP $ mkClassPred ipClass [x,ty]

{- 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
doesn't float that solved constraint out (it's not an unsolved
wanted).  Result disaster: the (Num alpha) is again solved, this
time by defaulting.  No no no.

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

Note [Inlining and hs-boot files]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider this example (Trac #10083):

    ---------- RSR.hs-boot ------------
    module RSR where
      data RSR
      eqRSR :: RSR -> RSR -> Bool

    ---------- SR.hs ------------
    module SR where
      import {-# SOURCE #-} RSR
      data SR = MkSR RSR
      eqSR (MkSR r1) (MkSR r2) = eqRSR r1 r2

    ---------- RSR.hs ------------
    module RSR where
      import SR
      data RSR = MkRSR SR -- deriving( Eq )
      eqRSR (MkRSR s1) (MkRSR s2) = (eqSR s1 s2)
      foo x y = not (eqRSR x y)

When compiling RSR we get this code

    RSR.eqRSR :: RSR -> RSR -> Bool
    RSR.eqRSR = \ (ds1 :: RSR.RSR) (ds2 :: RSR.RSR) ->
                case ds1 of _ { RSR.MkRSR s1 ->
                case ds2 of _ { RSR.MkRSR s2 ->
                SR.eqSR s1 s2 }}

    RSR.foo :: RSR -> RSR -> Bool
    RSR.foo = \ (x :: RSR) (y :: RSR) -> not (RSR.eqRSR x y)

Now, when optimising foo:
    Inline eqRSR (small, non-rec)
    Inline eqSR  (small, non-rec)
but the result of inlining eqSR from SR is another call to eqRSR, so
everything repeats.  Neither eqSR nor eqRSR are (apparently) loop
breakers.

Solution: when compiling RSR, add a NOINLINE pragma to every function
exported by the boot-file for RSR (if it exists).

ALAS: doing so makes the boostrappted GHC itself slower by 8% overall
      (on Trac #9872a-d, and T1969.  So I un-did this change, and
      parked it for now.  Sigh.
-}

tcValBinds :: TopLevelFlag
           -> [(RecFlag, LHsBinds Name)] -> [LSig Name]
           -> TcM thing
           -> TcM ([(RecFlag, LHsBinds TcId)], thing)

tcValBinds top_lvl binds sigs thing_inside
  = do  { let patsyns = getPatSynBinds binds

            -- Typecheck the signature
        ; (poly_ids, sig_fn) <- tcAddPatSynPlaceholders patsyns $
                                tcTySigs sigs

        ; _self_boot <- tcSelfBootInfo
        ; let prag_fn = mkPragEnv sigs (foldr (unionBags . snd) emptyBag binds)

-- -------  See Note [Inlining and hs-boot files] (change parked) --------
--              prag_fn | isTopLevel top_lvl   -- See Note [Inlining and hs-boot files]
--                      , SelfBoot { sb_ids = boot_id_names } <- self_boot
--                      = foldNameSet add_no_inl prag_fn1 boot_id_names
--                      | otherwise
--                      = prag_fn1
--              add_no_inl boot_id_name prag_fn
--                = extendPragEnv prag_fn (boot_id_name, no_inl_sig boot_id_name)
--              no_inl_sig name = L boot_loc (InlineSig (L boot_loc name) neverInlinePragma)
--              boot_loc = mkGeneralSrcSpan (fsLit "The hs-boot file for this module")

                -- Extend the envt right away with all the Ids
                -- declared with complete type signatures
                -- Do not extend the TcIdBinderStack; instead
                -- we extend it on a per-rhs basis in tcExtendForRhs
        ; tcExtendLetEnvIds top_lvl [(idName id, id) | id <- poly_ids] $ do
            { (binds', (extra_binds', thing)) <- tcBindGroups top_lvl sig_fn prag_fn binds $ do
                   { thing <- thing_inside
                     -- See Note [Pattern synonym builders don't yield dependencies]
                   ; patsyn_builders <- mapM tcPatSynBuilderBind patsyns
                   ; let extra_binds = [ (NonRecursive, builder) | builder <- patsyn_builders ]
                   ; return (extra_binds, thing) }
            ; return (binds' ++ extra_binds', thing) }}

------------------------
tcBindGroups :: TopLevelFlag -> TcSigFun -> TcPragEnv
             -> [(RecFlag, LHsBinds Name)] -> TcM thing
             -> TcM ([(RecFlag, LHsBinds TcId)], thing)
-- Typecheck a whole lot of value bindings,
-- one strongly-connected component at a time
-- Here a "strongly connected component" has the strightforward
-- meaning of a group of bindings that mention each other,
-- ignoring type signatures (that part comes later)

tcBindGroups _ _ _ [] thing_inside
  = do  { thing <- thing_inside
        ; return ([], thing) }

tcBindGroups top_lvl sig_fn prag_fn (group : groups) thing_inside
  = do  { (group', (groups', thing))
                <- tc_group top_lvl sig_fn prag_fn group $
                   tcBindGroups top_lvl sig_fn prag_fn groups thing_inside
        ; return (group' ++ groups', thing) }

------------------------
tc_group :: forall thing.
            TopLevelFlag -> TcSigFun -> TcPragEnv
         -> (RecFlag, LHsBinds Name) -> TcM thing
         -> TcM ([(RecFlag, LHsBinds TcId)], thing)

-- Typecheck one strongly-connected component of the original program.
-- We get a list of groups back, because there may
-- be specialisations etc as well

tc_group top_lvl sig_fn prag_fn (NonRecursive, binds) thing_inside
        -- A single non-recursive binding
        -- We want to keep non-recursive things non-recursive
        -- so that we desugar unlifted bindings correctly
  = do { let bind = case bagToList binds of
                 [bind] -> bind
                 []     -> panic "tc_group: empty list of binds"
                 _      -> panic "tc_group: NonRecursive binds is not a singleton bag"
       ; (bind', thing) <- tc_single top_lvl sig_fn prag_fn bind thing_inside
       ; return ( [(NonRecursive, bind')], thing) }

tc_group top_lvl sig_fn prag_fn (Recursive, binds) thing_inside
  =     -- To maximise polymorphism, we do a new
        -- strongly-connected-component analysis, this time omitting
        -- any references to variables with type signatures.
        -- (This used to be optional, but isn't now.)
        -- See Note [Polymorphic recursion] in HsBinds.
    do  { traceTc "tc_group rec" (pprLHsBinds binds)
        ; when hasPatSyn $ recursivePatSynErr binds
        ; (binds1, thing) <- go sccs
        ; return ([(Recursive, binds1)], thing) }
                -- Rec them all together
  where
    hasPatSyn = anyBag (isPatSyn . unLoc) binds
    isPatSyn PatSynBind{} = True
    isPatSyn _ = False

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

    go :: [SCC (LHsBind Name)] -> TcM (LHsBinds TcId, thing)
    go (scc:sccs) = do  { (binds1, ids1) <- tc_scc scc
                        ; (binds2, thing) <- tcExtendLetEnv top_lvl ids1 $
                                             go sccs
                        ; return (binds1 `unionBags` binds2, thing) }
    go []         = do  { thing <- thing_inside; return (emptyBag, thing) }

    tc_scc (AcyclicSCC bind) = tc_sub_group NonRecursive [bind]
    tc_scc (CyclicSCC binds) = tc_sub_group Recursive    binds

    tc_sub_group = tcPolyBinds top_lvl sig_fn prag_fn Recursive

recursivePatSynErr :: OutputableBndr name => LHsBinds name -> TcM a
recursivePatSynErr binds
  = failWithTc $
    hang (ptext (sLit "Recursive pattern synonym definition with following bindings:"))
       2 (vcat $ map pprLBind . bagToList $ binds)
  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 -> TcPragEnv
          -> LHsBind Name -> TcM thing
          -> TcM (LHsBinds TcId, thing)
tc_single _top_lvl sig_fn _prag_fn (L _ (PatSynBind psb@PSB{ psb_id = L _ name })) thing_inside
  = do { (aux_binds, tcg_env) <- tc_pat_syn_decl
       ; thing <- setGblEnv tcg_env thing_inside
       ; return (aux_binds, thing)
       }
  where
    tc_pat_syn_decl :: TcM (LHsBinds TcId, TcGblEnv)
    tc_pat_syn_decl = case sig_fn name of
        Nothing                 -> tcInferPatSynDecl psb
        Just (TcPatSynSig tpsi) -> tcCheckPatSynDecl psb tpsi
        Just                 _  -> panic "tc_single"

tc_single top_lvl sig_fn prag_fn lbind thing_inside
  = do { (binds1, ids) <- tcPolyBinds top_lvl sig_fn prag_fn
                                      NonRecursive NonRecursive
                                      [lbind]
       ; thing <- tcExtendLetEnv top_lvl ids thing_inside
       ; return (binds1, thing) }

------------------------
type BKey = Int -- Just number off the bindings

mkEdges :: TcSigFun -> LHsBinds Name -> [Node BKey (LHsBind Name)]
-- See Note [Polymorphic recursion] in HsBinds.
mkEdges sig_fn binds
  = [ (bind, key, [key | n <- nameSetElems (bind_fvs (unLoc bind)),
                         Just key <- [lookupNameEnv key_map n], no_sig n ])
    | (bind, key) <- keyd_binds
    ]
  where
    no_sig :: Name -> Bool
    no_sig n = noCompleteSig (sig_fn n)

    keyd_binds = bagToList binds `zip` [0::BKey ..]

    key_map :: NameEnv BKey     -- Which binding it comes from
    key_map = mkNameEnv [(bndr, key) | (L _ bind, key) <- keyd_binds
                                     , bndr <- collectHsBindBinders bind ]

------------------------
tcPolyBinds :: TopLevelFlag -> TcSigFun -> TcPragEnv
            -> RecFlag         -- Whether the group is really recursive
            -> RecFlag         -- Whether it's recursive after breaking
                               -- dependencies based on type signatures
            -> [LHsBind Name]  -- None are PatSynBind
            -> TcM (LHsBinds TcId, [TcId])

-- Typechecks a single bunch of values bindings all together,
-- and generalises them.  The bunch may be only part of a recursive
-- group, because we use type signatures to maximise polymorphism
--
-- Returns a list because the input may be a single non-recursive binding,
-- in which case the dependency order of the resulting bindings is
-- important.
--
-- Knows nothing about the scope of the bindings
-- None of the bindings are pattern synonyms

tcPolyBinds top_lvl sig_fn prag_fn rec_group rec_tc bind_list
  = setSrcSpan loc                              $
    recoverM (recoveryCode binder_names sig_fn) $ do
        -- Set up main recover; take advantage of any type sigs

    { traceTc "------------------------------------------------" Outputable.empty
    ; traceTc "Bindings for {" (ppr binder_names)
    ; dflags   <- getDynFlags
    ; type_env <- getLclTypeEnv
    ; let plan = decideGeneralisationPlan dflags type_env
                         binder_names bind_list sig_fn
    ; traceTc "Generalisation plan" (ppr plan)
    ; result@(tc_binds, poly_ids) <- case plan of
         NoGen              -> tcPolyNoGen rec_tc prag_fn sig_fn bind_list
         InferGen mn        -> tcPolyInfer rec_tc prag_fn sig_fn mn bind_list
         CheckGen lbind sig -> tcPolyCheck rec_tc prag_fn sig lbind

        -- Check whether strict bindings are ok
        -- These must be non-recursive etc, and are not generalised
        -- They desugar to a case expression in the end
    ; 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]
                                          ])

    ; return result }
  where
    binder_names = collectHsBindListBinders bind_list
    loc = foldr1 combineSrcSpans (map getLoc bind_list)
         -- The mbinds have been dependency analysed and
         -- may no longer be adjacent; so find the narrowest
         -- span that includes them all

------------------
tcPolyNoGen     -- No generalisation whatsoever
  :: RecFlag       -- Whether it's recursive after breaking
                   -- dependencies based on type signatures
  -> TcPragEnv -> TcSigFun
  -> [LHsBind Name]
  -> TcM (LHsBinds TcId, [TcId])

tcPolyNoGen rec_tc prag_fn tc_sig_fn bind_list
  = do { (binds', mono_infos) <- tcMonoBinds rec_tc tc_sig_fn
                                             (LetGblBndr prag_fn)
                                             bind_list
       ; mono_ids' <- mapM tc_mono_info mono_infos
       ; return (binds', mono_ids') }
  where
    tc_mono_info (MBI { mbi_poly_name = name, mbi_mono_id = mono_id })
      = do { mono_ty' <- zonkTcType (idType mono_id)
             -- Zonk, mainly to expose unboxed types to checkStrictBinds
           ; let mono_id' = setIdType mono_id mono_ty'
           ; _specs <- tcSpecPrags mono_id' (lookupPragEnv prag_fn name)
           ; return mono_id' }
           -- 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

------------------
tcPolyCheck :: RecFlag       -- Whether it's recursive after breaking
                             -- dependencies based on type signatures
            -> TcPragEnv
            -> TcIdSigInfo
            -> LHsBind Name
            -> TcM (LHsBinds TcId, [TcId])
-- There is just one binding,
--   it binds a single variable,
--   it has a complete type signature,
tcPolyCheck rec_tc prag_fn
            sig@(TISI { sig_bndr  = CompleteSig poly_id
                      , sig_skols = skol_prs
                      , sig_theta = theta
                      , sig_tau   = tau
                      , sig_ctxt  = ctxt
                      , sig_loc   = loc })
            bind
  = do { ev_vars <- newEvVars theta
       ; let skol_info = SigSkol ctxt (mkPhiTy theta tau)
             prag_sigs = lookupPragEnv prag_fn name
             skol_tvs  = map snd skol_prs
                 -- Find the location of the original source type sig, if
                 -- there is was one.  This will appear in messages like
                 -- "type variable x is bound by .. at <loc>"
             name = idName poly_id
       ; (ev_binds, (binds', _))
            <- setSrcSpan loc $
               checkConstraints skol_info skol_tvs ev_vars $
               tcMonoBinds rec_tc (\_ -> Just (TcIdSig sig)) LetLclBndr [bind]

       ; spec_prags <- tcSpecPrags poly_id prag_sigs
       ; poly_id    <- addInlinePrags poly_id prag_sigs

       ; let bind' = case bagToList binds' of
                       [b] -> b
                       _   -> pprPanic "tcPolyCheck" (ppr binds')
             abs_bind = L loc $ AbsBindsSig
                        { abs_tvs = skol_tvs
                        , abs_ev_vars = ev_vars
                        , abs_sig_export = poly_id
                        , abs_sig_prags = SpecPrags spec_prags
                        , abs_sig_ev_bind = ev_binds
                        , abs_sig_bind    = bind' }

       ; return (unitBag abs_bind, [poly_id]) }

tcPolyCheck _rec_tc _prag_fn sig _bind
  = pprPanic "tcPolyCheck" (ppr sig)

------------------
{-
Note [Instantiate when inferring a type]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider
  f = (*)
As there is no incentive to instantiate the RHS, tcMonoBinds will
produce a type of forall a. Num a => a -> a -> a for `f`. This will then go
through simplifyInfer and such, remaining unchanged.

There are two problems with this:
 1) If the definition were `g _ = (*)`, we get a very unusual type of
    `forall {a}. a -> forall b. Num b => b -> b -> b` for `g`. This is
    surely confusing for users.

 2) The monomorphism restriction can't work. The MR is dealt with in
    simplifyInfer, and simplifyInfer has no way of instantiating. This
    could perhaps be worked around, but it may be hard to know even
    when instantiation should happen.

There is an easy solution to all three problems: instantiate (deeply) when
inferring a type. So that's what we do. Note that this decision is
user-facing.

Here are the details:
 * tcMonoBinds produces the "monomorphic" ids to be put in the AbsBinds.
   It is inconvenient to instantiate in this function or below. So the
   monomorphic ids will be uninstantiated (and hence actually polymorphic,
   but that doesn't ruin anyone's day).

 * In the same captureConstraints as the tcMonoBinds, we instantiate all
   the types of the monomorphic ids. Instantiating will produce constraints
   to solve and instantiated types. These constraints and the instantiated
   types go into simplifyInfer. HsWrappers are produced that go from
   the "mono" types to the instantiated ones.

 * simplifyInfer does its magic, figuring out how to regeneralize.

 * mkExport then does the impedence matching and needs to connect the
   monomorphic ids to the polymorphic types as decided by simplifyInfer.
   Because the instantiation happens before simplifyInfer, we also pass in
   the HsWrappers obtained via instantiating so that mkExport can connect
   all the pieces.

 * We produce an AbsBinds with the right (instantiated and then, perhaps,
   regeneralized) polytypes and the not-yet-instantiated "monomorphic" ids,
   using the built HsWrappers to connect. Done!
-}

tcPolyInfer
  :: RecFlag       -- Whether it's recursive after breaking
                   -- dependencies based on type signatures
  -> TcPragEnv -> TcSigFun
  -> Bool         -- True <=> apply the monomorphism restriction
  -> [LHsBind Name]
  -> TcM (LHsBinds TcId, [TcId])
tcPolyInfer rec_tc prag_fn tc_sig_fn mono bind_list
  = do { (tclvl, wanted, (binds', mono_infos, wrappers, insted_tys))
             <- pushLevelAndCaptureConstraints  $
             do { (binds', mono_infos)
                    <- tcMonoBinds rec_tc tc_sig_fn LetLclBndr bind_list
                  -- See Note [Instantiate when inferring a type]
                ; traceTc "Note [Instantiate when inferring a type]" $
                    vcat (map (pprBndr LetBind . mbi_mono_id) mono_infos)
                ; (wrappers, insted_tys)
                    <- tcExtendIdBndrs
                         [ TcIdBndr mono_id NotTopLevel
                         | MBI { mbi_mono_id = mono_id } <- mono_infos ] $
                       mapAndUnzipM deeply_instantiate mono_infos
                     -- during instantiation, we might encounter an error
                     -- whose message will want to list these binders as
                     -- relevant.

                ; return (binds', mono_infos, wrappers, insted_tys) }

       ; let name_taus = [ (mbi_poly_name info, tau)
                         | (info, tau) <- zip mono_infos insted_tys]
             sigs      = [ sig | MBI { mbi_sig = Just sig } <- mono_infos ]

       ; traceTc "simplifyInfer call" (ppr tclvl $$ ppr name_taus $$ ppr wanted)
       ; (qtvs, givens, ev_binds)
                 <- simplifyInfer tclvl mono sigs name_taus wanted

       ; let inferred_theta = map evVarPred givens
       ; exports <- checkNoErrs $
                    zipWith3M (mkExport prag_fn qtvs inferred_theta)
                              mono_infos wrappers insted_tys

       ; loc <- getSrcSpanM
       ; let poly_ids = map abe_poly exports
             abs_bind = L loc $
                        AbsBinds { abs_tvs = qtvs
                                 , abs_ev_vars = givens, abs_ev_binds = [ev_binds]
                                 , abs_exports = exports, abs_binds = binds' }

       ; traceTc "Binding:" (ppr (poly_ids `zip` map idType poly_ids))
       ; return (unitBag abs_bind, poly_ids) }
         -- poly_ids are guaranteed zonked by mkExport

  where
    deeply_instantiate :: MonoBindInfo -> TcM (HsWrapper, TcRhoType)
    deeply_instantiate (MBI { mbi_mono_id = mono_id, mbi_orig = orig })
      = do { mono_ty <- zonkTcType (idType mono_id)
              -- NB: zonk to uncover any foralls
           ; addErrCtxtM (instErrCtxt mono_id mono_ty) $
             deeplyInstantiate orig mono_ty }

--------------
mkExport :: TcPragEnv
         -> [TyVar] -> TcThetaType      -- Both already zonked
         -> MonoBindInfo
         -> HsWrapper -- the instantiation wrapper;
                      -- see Note [Instantiate when inferring a type]
         -> TcTauType -- the instantiated type
         -> TcM (ABExport Id)
-- Only called for generalisation plan InferGen, not by CheckGen or NoGen
--
-- mkExport generates exports with
--      zonked type variables,
--      zonked poly_ids
-- 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
-- type environment; see the invariant on TcEnv.tcExtendIdEnv

-- Pre-condition: the qtvs and theta are already zonked

mkExport prag_fn qtvs theta
         mono_info@(MBI { mbi_poly_name = poly_name
                        , mbi_sig       = mb_sig
                        , mbi_mono_id   = mono_id })
         inst_wrap inst_ty
  = do  { inst_ty <- zonkTcType inst_ty

        ; poly_id <- case mb_sig of
              Just sig | Just poly_id <- completeIdSigPolyId_maybe sig
                       -> return poly_id
              _other   -> checkNoErrs $
                          mkInferredPolyId qtvs theta
                                           poly_name mb_sig inst_ty
              -- The checkNoErrs ensures that if the type is ambiguous
              -- we don't carry on to the impedence matching, and generate
              -- a duplicate ambiguity error.  There is a similar
              -- checkNoErrs for complete type signatures too.

        -- NB: poly_id has a zonked type
        ; poly_id <- addInlinePrags poly_id prag_sigs
        ; spec_prags <- tcSpecPrags poly_id prag_sigs
                -- tcPrags requires a zonked poly_id

        -- See Note [Impedence matching]
        -- NB: we have already done checkValidType, including an ambiguity check,
        --     on the type; either when we checked the sig or in mkInferredPolyId
        ; let sel_poly_ty = mkInvSigmaTy qtvs theta inst_ty
                -- this type is just going into tcSubType, so Inv vs. Spec doesn't
                -- matter

              poly_ty     = idType poly_id
        ; wrap <- if sel_poly_ty `eqType` poly_ty  -- NB: eqType ignores visibility
                  then return idHsWrapper  -- Fast path; also avoids complaint when we infer
                                           -- an ambiguouse type and have AllowAmbiguousType
                                           -- e..g infer  x :: forall a. F a -> Int
                  else addErrCtxtM (mk_impedence_match_msg mono_info sel_poly_ty poly_ty) $
                       tcSubType_NC sig_ctxt sel_poly_ty poly_ty

        ; warn_missing_sigs <- woptM Opt_WarnMissingLocalSigs
        ; when warn_missing_sigs $ localSigWarn poly_id mb_sig

        ; return (ABE { abe_wrap = wrap
                        -- abe_wrap :: idType poly_id ~ (forall qtvs. theta => inst_ty)
                      , abe_inst_wrap = inst_wrap
                      , abe_poly = poly_id
                      , abe_mono = mono_id
                      , abe_prags = SpecPrags spec_prags}) }
  where
    prag_sigs = lookupPragEnv prag_fn poly_name
    sig_ctxt  = InfSigCtxt poly_name

mkInferredPolyId :: [TyVar] -> TcThetaType
                 -> Name -> Maybe TcIdSigInfo -> TcType
                 -> TcM TcId
mkInferredPolyId qtvs inferred_theta poly_name mb_sig mono_ty
  = do { fam_envs <- tcGetFamInstEnvs
       ; let (_co, mono_ty') = normaliseType fam_envs Nominal mono_ty
               -- Unification may not have normalised the type,
               -- (see Note [Lazy flattening] in TcFlatten) so do it