2 % (c) The University of Glasgow 2006
3 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
5 \section[TcBinds]{TcBinds}
8 module TcBinds ( tcLocalBinds, tcTopBinds,
9 tcHsBootSigs, tcPolyBinds,
10 PragFun, tcSpecPrags, mkPragFun,
11 TcSigInfo(..), SigFun, mkSigFun,
12 badBootDeclErr ) where
14 import {-# SOURCE #-} TcMatches ( tcGRHSsPat, tcMatchesFun )
15 import {-# SOURCE #-} TcExpr ( tcMonoExpr )
28 import RnBinds( misplacedSigErr )
47 import Data.List( partition )
50 #include "HsVersions.h"
54 %************************************************************************
56 \subsection{Type-checking bindings}
58 %************************************************************************
60 @tcBindsAndThen@ typechecks a @HsBinds@. The "and then" part is because
61 it needs to know something about the {\em usage} of the things bound,
62 so that it can create specialisations of them. So @tcBindsAndThen@
63 takes a function which, given an extended environment, E, typechecks
64 the scope of the bindings returning a typechecked thing and (most
65 important) an LIE. It is this LIE which is then used as the basis for
66 specialising the things bound.
68 @tcBindsAndThen@ also takes a "combiner" which glues together the
69 bindings and the "thing" to make a new "thing".
71 The real work is done by @tcBindWithSigsAndThen@.
73 Recursive and non-recursive binds are handled in essentially the same
74 way: because of uniques there are no scoping issues left. The only
75 difference is that non-recursive bindings can bind primitive values.
77 Even for non-recursive binding groups we add typings for each binder
78 to the LVE for the following reason. When each individual binding is
79 checked the type of its LHS is unified with that of its RHS; and
80 type-checking the LHS of course requires that the binder is in scope.
82 At the top-level the LIE is sure to contain nothing but constant
83 dictionaries, which we resolve at the module level.
86 tcTopBinds :: HsValBinds Name
87 -> TcM ( LHsBinds TcId -- Typechecked bindings
88 , [LTcSpecPrag] -- SPECIALISE prags for imported Ids
89 , TcLclEnv) -- Augmented environment
91 -- Note: returning the TcLclEnv is more than we really
92 -- want. The bit we care about is the local bindings
93 -- and the free type variables thereof
95 = do { (ValBindsOut prs sigs, env) <- tcValBinds TopLevel binds getLclEnv
96 ; let binds = foldr (unionBags . snd) emptyBag prs
97 ; specs <- tcImpPrags sigs
98 ; return (binds, specs, env) }
99 -- The top level bindings are flattened into a giant
100 -- implicitly-mutually-recursive LHsBinds
102 tcHsBootSigs :: HsValBinds Name -> TcM [Id]
103 -- A hs-boot file has only one BindGroup, and it only has type
104 -- signatures in it. The renamer checked all this
105 tcHsBootSigs (ValBindsOut binds sigs)
106 = do { checkTc (null binds) badBootDeclErr
107 ; mapM (addLocM tc_boot_sig) (filter isTypeLSig sigs) }
109 tc_boot_sig (TypeSig (L _ name) ty)
110 = do { sigma_ty <- tcHsSigType (FunSigCtxt name) ty
111 ; return (mkVanillaGlobal name sigma_ty) }
112 -- Notice that we make GlobalIds, not LocalIds
113 tc_boot_sig s = pprPanic "tcHsBootSigs/tc_boot_sig" (ppr s)
114 tcHsBootSigs groups = pprPanic "tcHsBootSigs" (ppr groups)
116 badBootDeclErr :: Message
117 badBootDeclErr = ptext (sLit "Illegal declarations in an hs-boot file")
119 ------------------------
120 tcLocalBinds :: HsLocalBinds Name -> TcM thing
121 -> TcM (HsLocalBinds TcId, thing)
123 tcLocalBinds EmptyLocalBinds thing_inside
124 = do { thing <- thing_inside
125 ; return (EmptyLocalBinds, thing) }
127 tcLocalBinds (HsValBinds binds) thing_inside
128 = do { (binds', thing) <- tcValBinds NotTopLevel binds thing_inside
129 ; return (HsValBinds binds', thing) }
131 tcLocalBinds (HsIPBinds (IPBinds ip_binds _)) thing_inside
132 = do { (given_ips, ip_binds') <- mapAndUnzipM (wrapLocSndM tc_ip_bind) ip_binds
134 -- If the binding binds ?x = E, we must now
135 -- discharge any ?x constraints in expr_lie
136 -- See Note [Implicit parameter untouchables]
137 ; (ev_binds, result) <- checkConstraints (IPSkol ips)
138 [] given_ips thing_inside
140 ; return (HsIPBinds (IPBinds ip_binds' ev_binds), result) }
142 ips = [ip | L _ (IPBind ip _) <- ip_binds]
144 -- I wonder if we should do these one at at time
147 tc_ip_bind (IPBind ip expr)
148 = do { ty <- newFlexiTyVarTy argTypeKind
149 ; ip_id <- newIP ip ty
150 ; expr' <- tcMonoExpr expr ty
151 ; return (ip_id, (IPBind (IPName ip_id) expr')) }
154 Note [Implicit parameter untouchables]
155 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
156 We add the type variables in the types of the implicit parameters
157 as untouchables, not so much because we really must not unify them,
158 but rather because we otherwise end up with constraints like this
159 Num alpha, Implic { wanted = alpha ~ Int }
160 The constraint solver solves alpha~Int by unification, but then
161 doesn't float that solved constraint out (it's not an unsolved
162 wanted. Result disaster: the (Num alpha) is again solved, this
163 time by defaulting. No no no.
165 However [Oct 10] this is all handled automatically by the
166 untouchable-range idea.
169 tcValBinds :: TopLevelFlag
170 -> HsValBinds Name -> TcM thing
171 -> TcM (HsValBinds TcId, thing)
173 tcValBinds _ (ValBindsIn binds _) _
174 = pprPanic "tcValBinds" (ppr binds)
176 tcValBinds top_lvl (ValBindsOut binds sigs) thing_inside
177 = do { -- Typecheck the signature
178 ; let { prag_fn = mkPragFun sigs (foldr (unionBags . snd) emptyBag binds)
179 ; ty_sigs = filter isTypeLSig sigs
180 ; sig_fn = mkSigFun ty_sigs }
182 ; poly_ids <- checkNoErrs (mapAndRecoverM tcTySig ty_sigs)
183 -- No recovery from bad signatures, because the type sigs
184 -- may bind type variables, so proceeding without them
185 -- can lead to a cascade of errors
186 -- ToDo: this means we fall over immediately if any type sig
187 -- is wrong, which is over-conservative, see Trac bug #745
189 -- Extend the envt right away with all
190 -- the Ids declared with type signatures
191 ; (binds', thing) <- tcExtendIdEnv poly_ids $
192 tcBindGroups top_lvl sig_fn prag_fn
195 ; return (ValBindsOut binds' sigs, thing) }
197 ------------------------
198 tcBindGroups :: TopLevelFlag -> SigFun -> PragFun
199 -> [(RecFlag, LHsBinds Name)] -> TcM thing
200 -> TcM ([(RecFlag, LHsBinds TcId)], thing)
201 -- Typecheck a whole lot of value bindings,
202 -- one strongly-connected component at a time
203 -- Here a "strongly connected component" has the strightforward
204 -- meaning of a group of bindings that mention each other,
205 -- ignoring type signatures (that part comes later)
207 tcBindGroups _ _ _ [] thing_inside
208 = do { thing <- thing_inside
209 ; return ([], thing) }
211 tcBindGroups top_lvl sig_fn prag_fn (group : groups) thing_inside
212 = do { (group', (groups', thing))
213 <- tc_group top_lvl sig_fn prag_fn group $
214 tcBindGroups top_lvl sig_fn prag_fn groups thing_inside
215 ; return (group' ++ groups', thing) }
217 ------------------------
218 tc_group :: forall thing.
219 TopLevelFlag -> SigFun -> PragFun
220 -> (RecFlag, LHsBinds Name) -> TcM thing
221 -> TcM ([(RecFlag, LHsBinds TcId)], thing)
223 -- Typecheck one strongly-connected component of the original program.
224 -- We get a list of groups back, because there may
225 -- be specialisations etc as well
227 tc_group top_lvl sig_fn prag_fn (NonRecursive, binds) thing_inside
228 -- A single non-recursive binding
229 -- We want to keep non-recursive things non-recursive
230 -- so that we desugar unlifted bindings correctly
231 = do { (binds1, ids) <- tcPolyBinds top_lvl sig_fn prag_fn NonRecursive NonRecursive
233 ; thing <- tcExtendIdEnv ids thing_inside
234 ; return ( [(NonRecursive, binds1)], thing) }
236 tc_group top_lvl sig_fn prag_fn (Recursive, binds) thing_inside
237 = -- To maximise polymorphism (assumes -XRelaxedPolyRec), we do a new
238 -- strongly-connected-component analysis, this time omitting
239 -- any references to variables with type signatures.
240 do { traceTc "tc_group rec" (pprLHsBinds binds)
241 ; (binds1, _ids, thing) <- go sccs
242 -- Here is where we should do bindInstsOfLocalFuns
243 -- if we start having Methods again
244 ; return ([(Recursive, binds1)], thing) }
245 -- Rec them all together
247 sccs :: [SCC (LHsBind Name)]
248 sccs = stronglyConnCompFromEdgedVertices (mkEdges sig_fn binds)
250 go :: [SCC (LHsBind Name)] -> TcM (LHsBinds TcId, [TcId], thing)
251 go (scc:sccs) = do { (binds1, ids1) <- tc_scc scc
252 ; (binds2, ids2, thing) <- tcExtendIdEnv ids1 $ go sccs
253 ; return (binds1 `unionBags` binds2, ids1 ++ ids2, thing) }
254 go [] = do { thing <- thing_inside; return (emptyBag, [], thing) }
256 tc_scc (AcyclicSCC bind) = tc_sub_group NonRecursive [bind]
257 tc_scc (CyclicSCC binds) = tc_sub_group Recursive binds
259 tc_sub_group = tcPolyBinds top_lvl sig_fn prag_fn Recursive
262 ------------------------
264 bindLocalInsts :: TopLevelFlag
265 -> TcM (LHsBinds TcId, [TcId], a)
266 -> TcM (LHsBinds TcId, TcEvBinds, a)
267 bindLocalInsts top_lvl thing_inside
269 = do { (binds, _, thing) <- thing_inside; return (binds, emptyBag, thing) }
270 -- For the top level don't bother with all this bindInstsOfLocalFuns stuff.
271 -- All the top level things are rec'd together anyway, so it's fine to
272 -- leave them to the tcSimplifyTop, and quite a bit faster too
274 | otherwise -- Nested case
275 = do { ((binds, ids, thing), lie) <- captureConstraints thing_inside
276 ; lie_binds <- bindLocalMethods lie ids
277 ; return (binds, lie_binds, thing) }
280 ------------------------
281 mkEdges :: SigFun -> LHsBinds Name
282 -> [(LHsBind Name, BKey, [BKey])]
284 type BKey = Int -- Just number off the bindings
287 = [ (bind, key, [key | n <- nameSetToList (bind_fvs (unLoc bind)),
288 Just key <- [lookupNameEnv key_map n], no_sig n ])
289 | (bind, key) <- keyd_binds
292 no_sig :: Name -> Bool
293 no_sig n = isNothing (sig_fn n)
295 keyd_binds = bagToList binds `zip` [0::BKey ..]
297 key_map :: NameEnv BKey -- Which binding it comes from
298 key_map = mkNameEnv [(bndr, key) | (L _ bind, key) <- keyd_binds
299 , bndr <- bindersOfHsBind bind ]
301 bindersOfHsBind :: HsBind Name -> [Name]
302 bindersOfHsBind (PatBind { pat_lhs = pat }) = collectPatBinders pat
303 bindersOfHsBind (FunBind { fun_id = L _ f }) = [f]
304 bindersOfHsBind (AbsBinds {}) = panic "bindersOfHsBind AbsBinds"
305 bindersOfHsBind (VarBind {}) = panic "bindersOfHsBind VarBind"
307 ------------------------
308 tcPolyBinds :: TopLevelFlag -> SigFun -> PragFun
309 -> RecFlag -- Whether the group is really recursive
310 -> RecFlag -- Whether it's recursive after breaking
311 -- dependencies based on type signatures
313 -> TcM (LHsBinds TcId, [TcId])
315 -- Typechecks a single bunch of bindings all together,
316 -- and generalises them. The bunch may be only part of a recursive
317 -- group, because we use type signatures to maximise polymorphism
319 -- Returns a list because the input may be a single non-recursive binding,
320 -- in which case the dependency order of the resulting bindings is
323 -- Knows nothing about the scope of the bindings
325 tcPolyBinds top_lvl sig_fn prag_fn rec_group rec_tc bind_list
327 recoverM (recoveryCode binder_names sig_fn) $ do
328 -- Set up main recoer; take advantage of any type sigs
330 { traceTc "------------------------------------------------" empty
331 ; traceTc "Bindings for" (ppr binder_names)
333 ; tc_sig_fn <- tcInstSigs sig_fn binder_names
336 ; let plan = decideGeneralisationPlan dflags top_lvl binder_names bind_list tc_sig_fn
337 ; traceTc "Generalisation plan" (ppr plan)
338 ; (binds, poly_ids) <- case plan of
339 NoGen -> tcPolyNoGen tc_sig_fn prag_fn rec_tc bind_list
340 InferGen mono -> tcPolyInfer top_lvl mono tc_sig_fn prag_fn rec_tc bind_list
341 CheckGen sig -> tcPolyCheck sig prag_fn rec_tc bind_list
343 -- Check whether strict bindings are ok
344 -- These must be non-recursive etc, and are not generalised
345 -- They desugar to a case expression in the end
346 ; checkStrictBinds top_lvl rec_group bind_list poly_ids
348 ; return (binds, poly_ids) }
350 binder_names = collectHsBindListBinders bind_list
351 loc = getLoc (head bind_list)
352 -- TODO: location a bit awkward, but the mbinds have been
353 -- dependency analysed and may no longer be adjacent
357 :: TcSigFun -> PragFun
358 -> RecFlag -- Whether it's recursive after breaking
359 -- dependencies based on type signatures
361 -> TcM (LHsBinds TcId, [TcId])
362 -- No generalisation whatsoever
364 tcPolyNoGen tc_sig_fn prag_fn rec_tc bind_list
365 = do { (binds', mono_infos) <- tcMonoBinds tc_sig_fn (LetGblBndr prag_fn)
367 ; mono_ids' <- mapM tc_mono_info mono_infos
368 ; return (binds', mono_ids') }
370 tc_mono_info (name, _, mono_id)
371 = do { mono_ty' <- zonkTcTypeCarefully (idType mono_id)
372 -- Zonk, mainly to expose unboxed types to checkStrictBinds
373 ; let mono_id' = setIdType mono_id mono_ty'
374 ; _specs <- tcSpecPrags mono_id' (prag_fn name)
376 -- NB: tcPrags generates error messages for
377 -- specialisation pragmas for non-overloaded sigs
378 -- Indeed that is why we call it here!
379 -- So we can safely ignore _specs
382 tcPolyCheck :: TcSigInfo -> PragFun
383 -> RecFlag -- Whether it's recursive after breaking
384 -- dependencies based on type signatures
386 -> TcM (LHsBinds TcId, [TcId])
387 -- There is just one binding,
388 -- it binds a single variable,
389 -- it has a signature,
390 tcPolyCheck sig@(TcSigInfo { sig_id = id, sig_tvs = tvs, sig_scoped = scoped
391 , sig_theta = theta, sig_loc = loc })
392 prag_fn rec_tc bind_list
393 = do { ev_vars <- newEvVars theta
395 ; let skol_info = SigSkol (FunSigCtxt (idName id))
396 ; (ev_binds, (binds', [mono_info]))
397 <- checkConstraints skol_info tvs ev_vars $
398 tcExtendTyVarEnv2 (scoped `zip` mkTyVarTys tvs) $
399 tcMonoBinds (\_ -> Just sig) LetLclBndr rec_tc bind_list
401 ; export <- mkExport prag_fn tvs theta mono_info
403 ; let (_, poly_id, _, _) = export
404 abs_bind = L loc $ AbsBinds
406 , abs_ev_vars = ev_vars, abs_ev_binds = ev_binds
407 , abs_exports = [export], abs_binds = binds' }
408 ; return (unitBag abs_bind, [poly_id]) }
413 -> Bool -- True <=> apply the monomorphism restriction
414 -> TcSigFun -> PragFun
415 -> RecFlag -- Whether it's recursive after breaking
416 -- dependencies based on type signatures
418 -> TcM (LHsBinds TcId, [TcId])
419 tcPolyInfer top_lvl mono sig_fn prag_fn rec_tc bind_list
420 = do { ((binds', mono_infos), wanted)
421 <- captureConstraints $
422 tcMonoBinds sig_fn LetLclBndr rec_tc bind_list
424 ; unifyCtxts [sig | (_, Just sig, _) <- mono_infos]
426 ; let get_tvs | isTopLevel top_lvl = tyVarsOfType
427 | otherwise = exactTyVarsOfType
428 -- See Note [Silly type synonym] in TcType
429 tau_tvs = foldr (unionVarSet . get_tvs . getMonoType) emptyVarSet mono_infos
431 ; (qtvs, givens, ev_binds) <- simplifyInfer mono tau_tvs wanted
433 ; exports <- mapM (mkExport prag_fn qtvs (map evVarPred givens))
436 ; let poly_ids = [poly_id | (_, poly_id, _, _) <- exports]
437 ; traceTc "Binding:" (ppr (poly_ids `zip` map idType poly_ids))
440 ; let abs_bind = L loc $ AbsBinds { abs_tvs = qtvs
441 , abs_ev_vars = givens, abs_ev_binds = ev_binds
442 , abs_exports = exports, abs_binds = binds' }
444 ; return (unitBag abs_bind, poly_ids) -- poly_ids are guaranteed zonked by mkExport
449 mkExport :: PragFun -> [TyVar] -> TcThetaType
451 -> TcM ([TyVar], Id, Id, TcSpecPrags)
452 -- mkExport generates exports with
453 -- zonked type variables,
455 -- The former is just because no further unifications will change
456 -- the quantified type variables, so we can fix their final form
458 -- The latter is needed because the poly_ids are used to extend the
459 -- type environment; see the invariant on TcEnv.tcExtendIdEnv
461 -- Pre-condition: the inferred_tvs are already zonked
463 mkExport prag_fn inferred_tvs theta
464 (poly_name, mb_sig, mono_id)
465 = do { (tvs, poly_id) <- mk_poly_id mb_sig
466 -- poly_id has a zonked type
468 ; poly_id' <- addInlinePrags poly_id prag_sigs
470 ; spec_prags <- tcSpecPrags poly_id prag_sigs
471 -- tcPrags requires a zonked poly_id
473 ; return (tvs, poly_id', mono_id, SpecPrags spec_prags) }
475 prag_sigs = prag_fn poly_name
476 poly_ty = mkSigmaTy inferred_tvs theta (idType mono_id)
478 mk_poly_id Nothing = do { poly_ty' <- zonkTcTypeCarefully poly_ty
479 ; return (inferred_tvs, mkLocalId poly_name poly_ty') }
480 mk_poly_id (Just sig) = do { tvs <- mapM zonk_tv (sig_tvs sig)
481 ; return (tvs, sig_id sig) }
483 zonk_tv tv = do { ty <- zonkTcTyVar tv; return (tcGetTyVar "mkExport" ty) }
485 ------------------------
486 type PragFun = Name -> [LSig Name]
488 mkPragFun :: [LSig Name] -> LHsBinds Name -> PragFun
489 mkPragFun sigs binds = \n -> lookupNameEnv prag_env n `orElse` []
491 prs = mapCatMaybes get_sig sigs
493 get_sig :: LSig Name -> Maybe (Located Name, LSig Name)
494 get_sig (L l (SpecSig nm ty inl)) = Just (nm, L l $ SpecSig nm ty (add_arity nm inl))
495 get_sig (L l (InlineSig nm inl)) = Just (nm, L l $ InlineSig nm (add_arity nm inl))
498 add_arity (L _ n) inl_prag -- Adjust inl_sat field to match visible arity of function
499 | Just ar <- lookupNameEnv ar_env n,
500 Inline <- inl_inline inl_prag = inl_prag { inl_sat = Just ar }
501 -- add arity only for real INLINE pragmas, not INLINABLE
502 | otherwise = inl_prag
504 prag_env :: NameEnv [LSig Name]
505 prag_env = foldl add emptyNameEnv prs
506 add env (L _ n,p) = extendNameEnv_Acc (:) singleton env n p
508 -- ar_env maps a local to the arity of its definition
509 ar_env :: NameEnv Arity
510 ar_env = foldrBag lhsBindArity emptyNameEnv binds
512 lhsBindArity :: LHsBind Name -> NameEnv Arity -> NameEnv Arity
513 lhsBindArity (L _ (FunBind { fun_id = id, fun_matches = ms })) env
514 = extendNameEnv env (unLoc id) (matchGroupArity ms)
515 lhsBindArity _ env = env -- PatBind/VarBind
518 tcSpecPrags :: Id -> [LSig Name]
520 -- Add INLINE and SPECIALSE pragmas
521 -- INLINE prags are added to the (polymorphic) Id directly
522 -- SPECIALISE prags are passed to the desugarer via TcSpecPrags
523 -- Pre-condition: the poly_id is zonked
524 -- Reason: required by tcSubExp
525 tcSpecPrags poly_id prag_sigs
526 = do { unless (null bad_sigs) warn_discarded_sigs
527 ; mapAndRecoverM (wrapLocM (tcSpec poly_id)) spec_sigs }
529 spec_sigs = filter isSpecLSig prag_sigs
530 bad_sigs = filter is_bad_sig prag_sigs
531 is_bad_sig s = not (isSpecLSig s || isInlineLSig s)
533 warn_discarded_sigs = warnPrags poly_id bad_sigs $
534 ptext (sLit "Discarding unexpected pragmas for")
538 tcSpec :: TcId -> Sig Name -> TcM TcSpecPrag
539 tcSpec poly_id prag@(SpecSig _ hs_ty inl)
540 -- The Name in the SpecSig may not be the same as that of the poly_id
541 -- Example: SPECIALISE for a class method: the Name in the SpecSig is
542 -- for the selector Id, but the poly_id is something like $cop
543 = addErrCtxt (spec_ctxt prag) $
544 do { spec_ty <- tcHsSigType sig_ctxt hs_ty
545 ; warnIf (not (isOverloadedTy poly_ty || isInlinePragma inl))
546 (ptext (sLit "SPECIALISE pragma for non-overloaded function") <+> quotes (ppr poly_id))
547 -- Note [SPECIALISE pragmas]
548 ; wrap <- tcSubType origin skol_info (idType poly_id) spec_ty
549 ; return (SpecPrag poly_id wrap inl) }
551 name = idName poly_id
552 poly_ty = idType poly_id
553 origin = SpecPragOrigin name
554 sig_ctxt = FunSigCtxt name
555 skol_info = SigSkol sig_ctxt
556 spec_ctxt prag = hang (ptext (sLit "In the SPECIALISE pragma")) 2 (ppr prag)
558 tcSpec _ prag = pprPanic "tcSpec" (ppr prag)
561 tcImpPrags :: [LSig Name] -> TcM [LTcSpecPrag]
563 = do { this_mod <- getModule
565 = case sigName prag of
567 Just name -> not (nameIsLocalOrFrom this_mod name)
568 (spec_prags, others) = partition isSpecLSig $
570 ; mapM_ misplacedSigErr others
571 -- Messy that this misplaced-sig error comes here
572 -- but the others come from the renamer
573 ; mapAndRecoverM (wrapLocM tcImpSpec) spec_prags }
575 tcImpSpec :: Sig Name -> TcM TcSpecPrag
576 tcImpSpec prag@(SpecSig (L _ name) _ _)
577 = do { id <- tcLookupId name
578 ; checkTc (isAnyInlinePragma (idInlinePragma id))
581 tcImpSpec p = pprPanic "tcImpSpec" (ppr p)
583 impSpecErr :: Name -> SDoc
585 = hang (ptext (sLit "You cannot SPECIALISE") <+> quotes (ppr name))
586 2 (vcat [ ptext (sLit "because its definition has no INLINE/INLINABLE pragma")
587 , ptext (sLit "(or you compiled its definining module without -O)")])
589 -- If typechecking the binds fails, then return with each
590 -- signature-less binder given type (forall a.a), to minimise
591 -- subsequent error messages
592 recoveryCode :: [Name] -> SigFun -> TcM (LHsBinds TcId, [Id])
593 recoveryCode binder_names sig_fn
594 = do { traceTc "tcBindsWithSigs: error recovery" (ppr binder_names)
595 ; poly_ids <- mapM mk_dummy binder_names
596 ; return (emptyBag, poly_ids) }
599 | isJust (sig_fn name) = tcLookupId name -- Had signature; look it up
600 | otherwise = return (mkLocalId name forall_a_a) -- No signature
603 forall_a_a = mkForAllTy openAlphaTyVar (mkTyVarTy openAlphaTyVar)
606 Note [SPECIALISE pragmas]
607 ~~~~~~~~~~~~~~~~~~~~~~~~~
608 There is no point in a SPECIALISE pragma for a non-overloaded function:
609 reverse :: [a] -> [a]
610 {-# SPECIALISE reverse :: [Int] -> [Int] #-}
612 But SPECIALISE INLINE *can* make sense for GADTS:
614 ArrInt :: !Int -> ByteArray# -> Arr Int
615 ArrPair :: !Int -> Arr e1 -> Arr e2 -> Arr (e1, e2)
617 (!:) :: Arr e -> Int -> e
618 {-# SPECIALISE INLINE (!:) :: Arr Int -> Int -> Int #-}
619 {-# SPECIALISE INLINE (!:) :: Arr (a, b) -> Int -> (a, b) #-}
620 (ArrInt _ ba) !: (I# i) = I# (indexIntArray# ba i)
621 (ArrPair _ a1 a2) !: i = (a1 !: i, a2 !: i)
623 When (!:) is specialised it becomes non-recursive, and can usefully
624 be inlined. Scary! So we only warn for SPECIALISE *without* INLINE
625 for a non-overloaded function.
627 %************************************************************************
629 \subsection{tcMonoBind}
631 %************************************************************************
633 @tcMonoBinds@ deals with a perhaps-recursive group of HsBinds.
634 The signatures have been dealt with already.
637 tcMonoBinds :: TcSigFun -> LetBndrSpec
638 -> RecFlag -- Whether the binding is recursive for typechecking purposes
639 -- i.e. the binders are mentioned in their RHSs, and
640 -- we are not resuced by a type signature
642 -> TcM (LHsBinds TcId, [MonoBindInfo])
644 tcMonoBinds sig_fn no_gen is_rec
645 [ L b_loc (FunBind { fun_id = L nm_loc name, fun_infix = inf,
646 fun_matches = matches, bind_fvs = fvs })]
647 -- Single function binding,
648 | NonRecursive <- is_rec -- ...binder isn't mentioned in RHS
649 , Nothing <- sig_fn name -- ...with no type signature
650 = -- In this very special case we infer the type of the
651 -- right hand side first (it may have a higher-rank type)
652 -- and *then* make the monomorphic Id for the LHS
653 -- e.g. f = \(x::forall a. a->a) -> <body>
654 -- We want to infer a higher-rank type for f
656 do { ((co_fn, matches'), rhs_ty) <- tcInfer (tcMatchesFun name inf matches)
658 ; mono_id <- newNoSigLetBndr no_gen name rhs_ty
659 ; return (unitBag (L b_loc (FunBind { fun_id = L nm_loc mono_id, fun_infix = inf,
660 fun_matches = matches', bind_fvs = fvs,
661 fun_co_fn = co_fn, fun_tick = Nothing })),
662 [(name, Nothing, mono_id)]) }
664 tcMonoBinds sig_fn no_gen _ binds
665 = do { tc_binds <- mapM (wrapLocM (tcLhs sig_fn no_gen)) binds
667 -- Bring the monomorphic Ids, into scope for the RHSs
668 ; let mono_info = getMonoBindInfo tc_binds
669 rhs_id_env = [(name,mono_id) | (name, Nothing, mono_id) <- mono_info]
670 -- A monomorphic binding for each term variable that lacks
671 -- a type sig. (Ones with a sig are already in scope.)
673 ; binds' <- tcExtendIdEnv2 rhs_id_env $ do
674 traceTc "tcMonoBinds" $ vcat [ ppr n <+> ppr id <+> ppr (idType id)
675 | (n,id) <- rhs_id_env]
676 mapM (wrapLocM tcRhs) tc_binds
677 ; return (listToBag binds', mono_info) }
679 ------------------------
680 -- tcLhs typechecks the LHS of the bindings, to construct the environment in which
681 -- we typecheck the RHSs. Basically what we are doing is this: for each binder:
682 -- if there's a signature for it, use the instantiated signature type
683 -- otherwise invent a type variable
684 -- You see that quite directly in the FunBind case.
686 -- But there's a complication for pattern bindings:
687 -- data T = MkT (forall a. a->a)
689 -- Here we can guess a type variable for the entire LHS (which will be refined to T)
690 -- but we want to get (f::forall a. a->a) as the RHS environment.
691 -- The simplest way to do this is to typecheck the pattern, and then look up the
692 -- bound mono-ids. Then we want to retain the typechecked pattern to avoid re-doing
693 -- it; hence the TcMonoBind data type in which the LHS is done but the RHS isn't
695 data TcMonoBind -- Half completed; LHS done, RHS not done
696 = TcFunBind MonoBindInfo SrcSpan Bool (MatchGroup Name)
697 | TcPatBind [MonoBindInfo] (LPat TcId) (GRHSs Name) TcSigmaType
699 type MonoBindInfo = (Name, Maybe TcSigInfo, TcId)
700 -- Type signature (if any), and
701 -- the monomorphic bound things
703 getMonoType :: MonoBindInfo -> TcTauType
704 getMonoType (_,_,mono_id) = idType mono_id
706 tcLhs :: TcSigFun -> LetBndrSpec -> HsBind Name -> TcM TcMonoBind
707 tcLhs sig_fn no_gen (FunBind { fun_id = L nm_loc name, fun_infix = inf, fun_matches = matches })
708 | Just sig <- sig_fn name
709 = do { mono_id <- newSigLetBndr no_gen name sig
710 ; return (TcFunBind (name, Just sig, mono_id) nm_loc inf matches) }
712 = do { mono_ty <- newFlexiTyVarTy argTypeKind
713 ; mono_id <- newNoSigLetBndr no_gen name mono_ty
714 ; return (TcFunBind (name, Nothing, mono_id) nm_loc inf matches) }
716 tcLhs sig_fn no_gen (PatBind { pat_lhs = pat, pat_rhs = grhss })
717 = do { let tc_pat exp_ty = tcLetPat sig_fn no_gen pat exp_ty $
718 mapM lookup_info (collectPatBinders pat)
720 -- After typechecking the pattern, look up the binder
721 -- names, which the pattern has brought into scope.
722 lookup_info :: Name -> TcM MonoBindInfo
723 lookup_info name = do { mono_id <- tcLookupId name
724 ; return (name, sig_fn name, mono_id) }
726 ; ((pat', infos), pat_ty) <- addErrCtxt (patMonoBindsCtxt pat grhss) $
729 ; return (TcPatBind infos pat' grhss pat_ty) }
731 tcLhs _ _ other_bind = pprPanic "tcLhs" (ppr other_bind)
732 -- AbsBind, VarBind impossible
735 tcRhs :: TcMonoBind -> TcM (HsBind TcId)
736 -- When we are doing pattern bindings, or multiple function bindings at a time
737 -- we *don't* bring any scoped type variables into scope
738 -- Wny not? They are not completely rigid.
739 -- That's why we have the special case for a single FunBind in tcMonoBinds
740 tcRhs (TcFunBind (_,_,mono_id) loc inf matches)
741 = do { (co_fn, matches') <- tcMatchesFun (idName mono_id) inf
742 matches (idType mono_id)
743 ; return (FunBind { fun_id = L loc mono_id, fun_infix = inf
744 , fun_matches = matches'
746 , bind_fvs = placeHolderNames, fun_tick = Nothing }) }
748 tcRhs (TcPatBind _ pat' grhss pat_ty)
749 = do { grhss' <- addErrCtxt (patMonoBindsCtxt pat' grhss) $
750 tcGRHSsPat grhss pat_ty
751 ; return (PatBind { pat_lhs = pat', pat_rhs = grhss', pat_rhs_ty = pat_ty
752 , bind_fvs = placeHolderNames }) }
755 ---------------------
756 getMonoBindInfo :: [Located TcMonoBind] -> [MonoBindInfo]
757 getMonoBindInfo tc_binds
758 = foldr (get_info . unLoc) [] tc_binds
760 get_info (TcFunBind info _ _ _) rest = info : rest
761 get_info (TcPatBind infos _ _ _) rest = infos ++ rest
765 %************************************************************************
769 %************************************************************************
771 unifyCtxts checks that all the signature contexts are the same
772 The type signatures on a mutually-recursive group of definitions
773 must all have the same context (or none).
775 The trick here is that all the signatures should have the same
776 context, and we want to share type variables for that context, so that
777 all the right hand sides agree a common vocabulary for their type
780 We unify them because, with polymorphic recursion, their types
781 might not otherwise be related. This is a rather subtle issue.
784 unifyCtxts :: [TcSigInfo] -> TcM ()
785 -- Post-condition: the returned Insts are full zonked
786 unifyCtxts [] = return ()
787 unifyCtxts (sig1 : sigs)
788 = do { traceTc "unifyCtxts" (ppr (sig1 : sigs))
789 ; mapM_ unify_ctxt sigs }
791 theta1 = sig_theta sig1
792 unify_ctxt :: TcSigInfo -> TcM ()
793 unify_ctxt sig@(TcSigInfo { sig_theta = theta })
794 = setSrcSpan (sig_loc sig) $
795 addErrCtxt (sigContextsCtxt sig1 sig) $
796 do { cois <- unifyTheta theta1 theta
797 ; -- Check whether all coercions are identity coercions
798 -- That can happen if we have, say
800 -- g :: C (F a) => ...
801 -- where F is a type function and (F a ~ [a])
802 -- Then unification might succeed with a coercion. But it's much
803 -- much simpler to require that such signatures have identical contexts
804 checkTc (all isIdentityCoI cois)
805 (ptext (sLit "Mutually dependent functions have syntactically distinct contexts"))
810 @getTyVarsToGen@ decides what type variables to generalise over.
812 For a "restricted group" -- see the monomorphism restriction
813 for a definition -- we bind no dictionaries, and
814 remove from tyvars_to_gen any constrained type variables
816 *Don't* simplify dicts at this point, because we aren't going
817 to generalise over these dicts. By the time we do simplify them
818 we may well know more. For example (this actually came up)
820 f x = array ... xs where xs = [1,2,3,4,5]
821 We don't want to generate lots of (fromInt Int 1), (fromInt Int 2)
822 stuff. If we simplify only at the f-binding (not the xs-binding)
823 we'll know that the literals are all Ints, and we can just produce
826 Find all the type variables involved in overloading, the
827 "constrained_tyvars". These are the ones we *aren't* going to
828 generalise. We must be careful about doing this:
830 (a) If we fail to generalise a tyvar which is not actually
831 constrained, then it will never, ever get bound, and lands
832 up printed out in interface files! Notorious example:
833 instance Eq a => Eq (Foo a b) where ..
834 Here, b is not constrained, even though it looks as if it is.
835 Another, more common, example is when there's a Method inst in
836 the LIE, whose type might very well involve non-overloaded
838 [NOTE: Jan 2001: I don't understand the problem here so I'm doing
839 the simple thing instead]
841 (b) On the other hand, we mustn't generalise tyvars which are constrained,
842 because we are going to pass on out the unmodified LIE, with those
843 tyvars in it. They won't be in scope if we've generalised them.
845 So we are careful, and do a complete simplification just to find the
846 constrained tyvars. We don't use any of the results, except to
847 find which tyvars are constrained.
849 Note [Polymorphic recursion]
850 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
851 The game plan for polymorphic recursion in the code above is
853 * Bind any variable for which we have a type signature
854 to an Id with a polymorphic type. Then when type-checking
855 the RHSs we'll make a full polymorphic call.
857 This fine, but if you aren't a bit careful you end up with a horrendous
858 amount of partial application and (worse) a huge space leak. For example:
860 f :: Eq a => [a] -> [a]
863 If we don't take care, after typechecking we get
865 f = /\a -> \d::Eq a -> let f' = f a d
869 Notice the the stupid construction of (f a d), which is of course
870 identical to the function we're executing. In this case, the
871 polymorphic recursion isn't being used (but that's a very common case).
872 This can lead to a massive space leak, from the following top-level defn
878 Now (f dEqInt) evaluates to a lambda that has f' as a free variable; but
879 f' is another thunk which evaluates to the same thing... and you end
880 up with a chain of identical values all hung onto by the CAF ff.
884 = let f' = f Int dEqInt in \ys. ...f'...
886 = let f' = let f' = f Int dEqInt in \ys. ...f'...
891 NOTE: a bit of arity anaysis would push the (f a d) inside the (\ys...),
892 which would make the space leak go away in this case
894 Solution: when typechecking the RHSs we always have in hand the
895 *monomorphic* Ids for each binding. So we just need to make sure that
896 if (Method f a d) shows up in the constraints emerging from (...f...)
897 we just use the monomorphic Id. We achieve this by adding monomorphic Ids
898 to the "givens" when simplifying constraints. That's what the "lies_avail"
903 f = /\a -> \d::Eq a -> letrec
904 fm = \ys:[a] -> ...fm...
908 %************************************************************************
912 %************************************************************************
914 Type signatures are tricky. See Note [Signature skolems] in TcType
916 @tcSigs@ checks the signatures for validity, and returns a list of
917 {\em freshly-instantiated} signatures. That is, the types are already
918 split up, and have fresh type variables installed. All non-type-signature
919 "RenamedSigs" are ignored.
921 The @TcSigInfo@ contains @TcTypes@ because they are unified with
922 the variable's type, and after that checked to see whether they've
927 The -XScopedTypeVariables flag brings lexically-scoped type variables
928 into scope for any explicitly forall-quantified type variables:
929 f :: forall a. a -> a
931 Then 'a' is in scope inside 'e'.
933 However, we do *not* support this
934 - For pattern bindings e.g
938 - For multiple function bindings, unless Opt_RelaxedPolyRec is on
939 f :: forall a. a -> a
941 g :: forall b. b -> b
943 Reason: we use mutable variables for 'a' and 'b', since they may
944 unify to each other, and that means the scoped type variable would
945 not stand for a completely rigid variable.
947 Currently, we simply make Opt_ScopedTypeVariables imply Opt_RelaxedPolyRec
950 Note [More instantiated than scoped]
951 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
952 There may be more instantiated type variables than lexically-scoped
954 type T a = forall b. b -> (a,b)
956 Here, the signature for f will have one scoped type variable, c,
957 but two instantiated type variables, c' and b'.
959 We assume that the scoped ones are at the *front* of sig_tvs,
960 and remember the names from the original HsForAllTy in the TcSigFun.
962 Note [Signature skolems]
963 ~~~~~~~~~~~~~~~~~~~~~~~~
964 When instantiating a type signature, we do so with either skolems or
965 SigTv meta-type variables depending on the use_skols boolean. This
966 variable is set True when we are typechecking a single function
967 binding; and False for pattern bindings and a group of several
970 Reason: in the latter cases, the "skolems" can be unified together,
971 so they aren't properly rigid in the type-refinement sense.
972 NB: unless we are doing H98, each function with a sig will be done
973 separately, even if it's mutually recursive, so use_skols will be True
976 Note [Only scoped tyvars are in the TyVarEnv]
977 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
978 We are careful to keep only the *lexically scoped* type variables in
979 the type environment. Why? After all, the renamer has ensured
980 that only legal occurrences occur, so we could put all type variables
983 But we want to check that two distinct lexically scoped type variables
984 do not map to the same internal type variable. So we need to know which
985 the lexically-scoped ones are... and at the moment we do that by putting
986 only the lexically scoped ones into the environment.
988 Note [Instantiate sig with fresh variables]
989 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
990 It's vital to instantiate a type signature with fresh variables.
992 type T = forall a. [a] -> [a]
994 f = g where { g :: T; g = <rhs> }
996 We must not use the same 'a' from the defn of T at both places!!
997 (Instantiation is only necessary because of type synonyms. Otherwise,
998 it's all cool; each signature has distinct type variables from the renamer.)
1001 type SigFun = Name -> Maybe ([Name], SrcSpan)
1002 -- Maps a let-binder to the list of
1003 -- type variables brought into scope
1004 -- by its type signature, plus location
1005 -- Nothing => no type signature
1007 mkSigFun :: [LSig Name] -> SigFun
1008 -- Search for a particular type signature
1009 -- Precondition: the sigs are all type sigs
1010 -- Precondition: no duplicates
1011 mkSigFun sigs = lookupNameEnv env
1013 env = mkNameEnv (mapCatMaybes mk_pair sigs)
1014 mk_pair (L loc (TypeSig (L _ name) lhs_ty)) = Just (name, (hsExplicitTvs lhs_ty, loc))
1015 mk_pair (L loc (IdSig id)) = Just (idName id, ([], loc))
1017 -- The scoped names are the ones explicitly mentioned
1018 -- in the HsForAll. (There may be more in sigma_ty, because
1019 -- of nested type synonyms. See Note [More instantiated than scoped].)
1020 -- See Note [Only scoped tyvars are in the TyVarEnv]
1024 tcTySig :: LSig Name -> TcM TcId
1025 tcTySig (L span (TypeSig (L _ name) ty))
1027 do { sigma_ty <- tcHsSigType (FunSigCtxt name) ty
1028 ; return (mkLocalId name sigma_ty) }
1029 tcTySig (L _ (IdSig id))
1031 tcTySig s = pprPanic "tcTySig" (ppr s)
1034 tcInstSigs :: SigFun -> [Name] -> TcM TcSigFun
1035 tcInstSigs sig_fn bndrs
1036 = do { prs <- mapMaybeM (tcInstSig sig_fn use_skols) bndrs
1037 ; return (lookupNameEnv (mkNameEnv prs)) }
1039 use_skols = isSingleton bndrs -- See Note [Signature skolems]
1041 tcInstSig :: SigFun -> Bool -> Name -> TcM (Maybe (Name, TcSigInfo))
1042 -- For use_skols :: Bool see Note [Signature skolems]
1044 -- We must instantiate with fresh uniques,
1045 -- (see Note [Instantiate sig with fresh variables])
1046 -- although we keep the same print-name.
1048 tcInstSig sig_fn use_skols name
1049 | Just (scoped_tvs, loc) <- sig_fn name
1050 = do { poly_id <- tcLookupId name -- Cannot fail; the poly ids are put into
1051 -- scope when starting the binding group
1052 ; (tvs, theta, tau) <- tcInstSigType use_skols name (idType poly_id)
1053 ; let sig = TcSigInfo { sig_id = poly_id
1054 , sig_scoped = scoped_tvs
1055 , sig_tvs = tvs, sig_theta = theta, sig_tau = tau
1057 ; return (Just (name, sig)) }
1061 -------------------------------
1062 data GeneralisationPlan
1063 = NoGen -- No generalisation, no AbsBinds
1064 | InferGen Bool -- Implicit generalisation; there is an AbsBinds
1065 -- True <=> apply the MR; generalise only unconstrained type vars
1066 | CheckGen TcSigInfo -- Explicit generalisation; there is an AbsBinds
1068 -- A consequence of the no-AbsBinds choice (NoGen) is that there is
1069 -- no "polymorphic Id" and "monmomorphic Id"; there is just the one
1071 instance Outputable GeneralisationPlan where
1072 ppr NoGen = ptext (sLit "NoGen")
1073 ppr (InferGen b) = ptext (sLit "InferGen") <+> ppr b
1074 ppr (CheckGen s) = ptext (sLit "CheckGen") <+> ppr s
1076 decideGeneralisationPlan
1077 :: DynFlags -> TopLevelFlag -> [Name] -> [LHsBind Name] -> TcSigFun -> GeneralisationPlan
1078 decideGeneralisationPlan dflags top_lvl _bndrs binds sig_fn
1079 | bang_pat_binds = NoGen
1080 | mono_pat_binds = NoGen
1081 | Just sig <- one_funbind_with_sig binds = if null (sig_tvs sig) && null (sig_theta sig)
1082 then NoGen -- Optimise common case
1084 | (xopt Opt_MonoLocalBinds dflags
1085 && isNotTopLevel top_lvl) = NoGen
1086 | otherwise = InferGen mono_restriction
1089 bang_pat_binds = any (isBangHsBind . unLoc) binds
1090 -- Bang patterns must not be polymorphic,
1091 -- because we are going to force them
1094 mono_pat_binds = xopt Opt_MonoPatBinds dflags
1095 && any (is_pat_bind . unLoc) binds
1097 mono_restriction = xopt Opt_MonomorphismRestriction dflags
1098 && any (restricted . unLoc) binds
1100 no_sig n = isNothing (sig_fn n)
1102 -- With OutsideIn, all nested bindings are monomorphic
1103 -- except a single function binding with a signature
1104 one_funbind_with_sig [L _ FunBind { fun_id = v }] = sig_fn (unLoc v)
1105 one_funbind_with_sig _ = Nothing
1107 -- The Haskell 98 monomorphism resetriction
1108 restricted (PatBind {}) = True
1109 restricted (VarBind { var_id = v }) = no_sig v
1110 restricted (FunBind { fun_id = v, fun_matches = m }) = restricted_match m
1112 restricted (AbsBinds {}) = panic "isRestrictedGroup/unrestricted AbsBinds"
1114 restricted_match (MatchGroup (L _ (Match [] _ _) : _) _) = True
1115 restricted_match _ = False
1116 -- No args => like a pattern binding
1117 -- Some args => a function binding
1119 is_pat_bind (PatBind {}) = True
1120 is_pat_bind _ = False
1123 checkStrictBinds :: TopLevelFlag -> RecFlag
1124 -> [LHsBind Name] -> [Id]
1126 -- Check that non-overloaded unlifted bindings are
1127 -- a) non-recursive,
1128 -- b) not top level,
1129 -- c) not a multiple-binding group (more or less implied by (a))
1131 checkStrictBinds top_lvl rec_group binds poly_ids
1132 | unlifted || bang_pat
1133 = do { checkTc (isNotTopLevel top_lvl)
1134 (strictBindErr "Top-level" unlifted binds)
1135 ; checkTc (isNonRec rec_group)
1136 (strictBindErr "Recursive" unlifted binds)
1137 ; checkTc (isSingleton binds)
1138 (strictBindErr "Multiple" unlifted binds)
1139 -- This should be a checkTc, not a warnTc, but as of GHC 6.11
1140 -- the versions of alex and happy available have non-conforming
1141 -- templates, so the GHC build fails if it's an error:
1142 ; warnUnlifted <- doptM Opt_WarnLazyUnliftedBindings
1143 ; warnTc (warnUnlifted && not bang_pat && lifted_pat)
1144 -- No outer bang, but it's a compound pattern
1145 -- E.g (I# x#) = blah
1146 -- Warn about this, but not about
1149 (unliftedMustBeBang binds) }
1153 unlifted = any is_unlifted poly_ids
1154 bang_pat = any (isBangHsBind . unLoc) binds
1155 lifted_pat = any (isLiftedPatBind . unLoc) binds
1156 is_unlifted id = case tcSplitForAllTys (idType id) of
1157 (_, rho) -> isUnLiftedType rho
1159 unliftedMustBeBang :: [LHsBind Name] -> SDoc
1160 unliftedMustBeBang binds
1161 = hang (text "Pattern bindings containing unlifted types should use an outermost bang pattern:")
1162 2 (pprBindList binds)
1164 strictBindErr :: String -> Bool -> [LHsBind Name] -> SDoc
1165 strictBindErr flavour unlifted binds
1166 = hang (text flavour <+> msg <+> ptext (sLit "aren't allowed:"))
1167 2 (pprBindList binds)
1169 msg | unlifted = ptext (sLit "bindings for unlifted types")
1170 | otherwise = ptext (sLit "bang-pattern bindings")
1172 pprBindList :: [LHsBind Name] -> SDoc
1173 pprBindList binds = vcat (map ppr binds)
1177 %************************************************************************
1179 \subsection[TcBinds-errors]{Error contexts and messages}
1181 %************************************************************************
1185 -- This one is called on LHS, when pat and grhss are both Name
1186 -- and on RHS, when pat is TcId and grhss is still Name
1187 patMonoBindsCtxt :: OutputableBndr id => LPat id -> GRHSs Name -> SDoc
1188 patMonoBindsCtxt pat grhss
1189 = hang (ptext (sLit "In a pattern binding:")) 2 (pprPatBind pat grhss)
1191 -----------------------------------------------
1192 sigContextsCtxt :: TcSigInfo -> TcSigInfo -> SDoc
1193 sigContextsCtxt sig1 sig2
1194 = vcat [ptext (sLit "When matching the contexts of the signatures for"),
1195 nest 2 (vcat [ppr id1 <+> dcolon <+> ppr (idType id1),
1196 ppr id2 <+> dcolon <+> ppr (idType id2)]),
1197 ptext (sLit "The signature contexts in a mutually recursive group should all be identical")]