%
% (c) The University of Glasgow 2006
% (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
%
\section[TcBinds]{TcBinds}
\begin{code}
module TcBinds ( tcLocalBinds, tcTopBinds, tcRecSelBinds,
tcHsBootSigs, tcPolyCheck,
PragFun, tcSpecPrags, tcVectDecls, mkPragFun,
TcSigInfo(..), TcSigFun,
instTcTySig, instTcTySigFromId, findScopedTyVars,
badBootDeclErr ) where
import TcMatches ( tcGRHSsPat, tcMatchesFun )
import TcExpr ( tcMonoExpr )
import TcPatSyn ( tcPatSynDecl )
import DynFlags
import HsSyn
import HscTypes( isHsBoot )
import TcRnMonad
import TcEnv
import TcUnify
import TcSimplify
import TcEvidence
import TcHsType
import TcPat
import TcMType
import PatSyn
import ConLike
import Type( tidyOpenType )
import FunDeps( growThetaTyVars )
import TyCon
import TcType
import TysPrim
import Id
import Var
import VarSet
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)
import Class(classTyCon)
import PrelNames(ipClassName)
import Control.Monad
#include "HsVersions.h"
\end{code}
%************************************************************************
%* *
\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
\begin{code}
tcTopBinds :: HsValBinds Name -> TcM (TcGblEnv, TcLclEnv)
tcTopBinds (ValBindsOut binds sigs)
= do {
(binds', (tcg_env, tcl_env)) <- tcValBinds TopLevel binds sigs $
do { gbl <- getGblEnv
; lcl <- getLclEnv
; return (gbl, lcl) }
; specs <- tcImpPrags sigs
; 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) }
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)
; let tcg_env'
| isHsBoot (tcg_src tcg_env) = tcg_env
| otherwise = tcg_env { tcg_binds = foldr (unionBags . snd)
(tcg_binds tcg_env)
rec_sel_binds }
; return tcg_env' }
tcRecSelBinds (ValBindsIn {}) = panic "tcRecSelBinds"
tcHsBootSigs :: HsValBinds Name -> TcM [Id]
tcHsBootSigs (ValBindsOut binds sigs)
= do { checkTc (null binds) badBootDeclErr
; concat <$> mapM (addLocM tc_boot_sig) (filter isTypeLSig sigs) }
where
tc_boot_sig (TypeSig lnames ty) = mapM f lnames
where
f (L _ name) = do { sigma_ty <- tcHsSigType (FunSigCtxt name) ty
; return (mkVanillaGlobal name sigma_ty) }
tc_boot_sig s = pprPanic "tcHsBootSigs/tc_boot_sig" (ppr s)
tcHsBootSigs groups = pprPanic "tcHsBootSigs" (ppr groups)
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
; (ev_binds, result) <- checkConstraints (IPSkol ips)
[] given_ips thing_inside
; return (HsIPBinds (IPBinds ip_binds' ev_binds), result) }
where
ips = [ip | L _ (IPBind (Left ip) _) <- ip_binds]
tc_ip_bind ipClass (IPBind (Left ip) expr)
= do { ty <- newFlexiTyVarTy openTypeKind
; 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"
toDict ipClass x ty =
case unwrapNewTyCon_maybe (classTyCon ipClass) of
Just (_,_,ax) -> HsWrap $ mkWpCast $ mkTcSymCo $ mkTcUnbranchedAxInstCo Representational ax [x,ty]
Nothing -> panic "The dictionary for `IP` is not a newtype?"
\end{code}
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 [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.
\begin{code}
tcValBinds :: TopLevelFlag
-> [(RecFlag, LHsBinds Name)] -> [LSig Name]
-> TcM thing
-> TcM ([(RecFlag, LHsBinds TcId)], thing)
tcValBinds top_lvl binds sigs thing_inside
= do {
; (poly_ids, sig_fn) <- tcExtendKindEnv2 patsyn_placeholder_kinds $
tcTySigs sigs
; let prag_fn = mkPragFun sigs (foldr (unionBags . snd) emptyBag binds)
; tcExtendIdEnv2 [(idName id, id) | id <- poly_ids] $
tcBindGroups top_lvl sig_fn prag_fn
binds thing_inside }
where
patsyn_placeholder_kinds
= [ (name, placeholder_patsyn_tything)
| (_, lbinds) <- binds
, L _ (PatSynBind{ patsyn_id = L _ name }) <- bagToList lbinds ]
placeholder_patsyn_tything
= AGlobal $ AConLike $ PatSynCon $ panic "fakePatSynCon"
tcBindGroups :: TopLevelFlag -> TcSigFun -> PragFun
-> [(RecFlag, LHsBinds Name)] -> TcM thing
-> TcM ([(RecFlag, LHsBinds TcId)], thing)
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 -> PragFun
-> (RecFlag, LHsBinds Name) -> TcM thing
-> TcM ([(RecFlag, LHsBinds TcId)], thing)
tc_group top_lvl sig_fn prag_fn (NonRecursive, binds) thing_inside
= do { let bind = case bagToList binds of
[] -> panic "tc_group: empty list of binds"
[bind] -> bind
_ -> 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
=
do { traceTc "tc_group rec" (pprLHsBinds binds)
; when hasPatSyn $ recursivePatSynErr binds
; (binds1, _ids, thing) <- go sccs
; return ([(Recursive, binds1)], thing) }
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, [TcId], thing)
go (scc:sccs) = do { (binds1, ids1, closed) <- tc_scc scc
; (binds2, ids2, thing) <- tcExtendLetEnv top_lvl closed ids1 $
go sccs
; return (binds1 `unionBags` binds2, ids1 ++ ids2, 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 -> PragFun
-> LHsBind Name -> TcM thing
-> TcM (LHsBinds TcId, thing)
tc_single _top_lvl _sig_fn _prag_fn (L _ ps@PatSynBind{}) thing_inside
= do { (pat_syn, aux_binds) <-
tcPatSynDecl (patsyn_id ps) (patsyn_args ps) (patsyn_def ps) (patsyn_dir ps)
; let tything = AConLike (PatSynCon pat_syn)
implicit_ids = (patSynMatcher pat_syn) :
(maybeToList (patSynWrapper pat_syn))
; thing <- tcExtendGlobalEnv [tything] $
tcExtendGlobalEnvImplicit (map AnId implicit_ids) $
thing_inside
; return (aux_binds, thing)
}
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) }
mkEdges :: TcSigFun -> LHsBinds Name
-> [(LHsBind Name, BKey, [BKey])]
type BKey = Int
mkEdges sig_fn binds
= [ (bind, key, [key | n <- nameSetToList (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 = isNothing (sig_fn n)
keyd_binds = bagToList binds `zip` [0::BKey ..]
key_map :: NameEnv BKey
key_map = mkNameEnv [(bndr, key) | (L _ bind, key) <- keyd_binds
, bndr <- bindersOfHsBind bind ]
bindersOfHsBind :: HsBind Name -> [Name]
bindersOfHsBind (PatBind { pat_lhs = pat }) = collectPatBinders pat
bindersOfHsBind (FunBind { fun_id = L _ f }) = [f]
bindersOfHsBind (PatSynBind { patsyn_id = L _ psyn }) = [psyn]
bindersOfHsBind (AbsBinds {}) = panic "bindersOfHsBind AbsBinds"
bindersOfHsBind (VarBind {}) = panic "bindersOfHsBind VarBind"
tcPolyBinds :: TopLevelFlag -> TcSigFun -> PragFun
-> RecFlag
-> RecFlag
-> [LHsBind Name]
-> TcM (LHsBinds TcId, [TcId], TopLevelFlag)
tcPolyBinds top_lvl sig_fn prag_fn rec_group rec_tc bind_list
= setSrcSpan loc $
recoverM (recoveryCode binder_names sig_fn) $ do
{ traceTc "------------------------------------------------" 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 cl -> tcPolyInfer rec_tc prag_fn sig_fn mn cl bind_list
CheckGen lbind sig -> tcPolyCheck rec_tc prag_fn sig lbind
; 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)
tcPolyNoGen
:: RecFlag
-> PragFun -> TcSigFun
-> [LHsBind Name]
-> TcM (LHsBinds TcId, [TcId], TopLevelFlag)
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', NotTopLevel) }
where
tc_mono_info (name, _, mono_id)
= do { mono_ty' <- zonkTcType (idType mono_id)
; let mono_id' = setIdType mono_id mono_ty'
; _specs <- tcSpecPrags mono_id' (prag_fn name)
; return mono_id' }
tcPolyCheck :: RecFlag
-> PragFun -> TcSigInfo
-> LHsBind Name
-> TcM (LHsBinds TcId, [TcId], TopLevelFlag)
tcPolyCheck rec_tc prag_fn
sig@(TcSigInfo { sig_id = poly_id, sig_tvs = tvs_w_scoped
, sig_theta = theta, sig_tau = tau, sig_loc = loc })
bind
= do { ev_vars <- newEvVars theta
; let skol_info = SigSkol (FunSigCtxt (idName poly_id)) (mkPhiTy theta tau)
prag_sigs = prag_fn (idName poly_id)
tvs = map snd tvs_w_scoped
; (ev_binds, (binds', [mono_info]))
<- setSrcSpan loc $
checkConstraints skol_info tvs ev_vars $
tcExtendTyVarEnv2 [(n,tv) | (Just n, tv) <- tvs_w_scoped] $
tcMonoBinds rec_tc (\_ -> Just sig) LetLclBndr [bind]
; spec_prags <- tcSpecPrags poly_id prag_sigs
; poly_id <- addInlinePrags poly_id prag_sigs
; let (_, _, mono_id) = mono_info
export = ABE { abe_wrap = idHsWrapper
, abe_poly = poly_id
, abe_mono = mono_id
, abe_prags = SpecPrags spec_prags }
abs_bind = L loc $ AbsBinds
{ abs_tvs = tvs
, abs_ev_vars = ev_vars, abs_ev_binds = ev_binds
, abs_exports = [export], abs_binds = binds' }
closed | isEmptyVarSet (tyVarsOfType (idType poly_id)) = TopLevel
| otherwise = NotTopLevel
; return (unitBag abs_bind, [poly_id], closed) }
tcPolyInfer
:: RecFlag
-> PragFun -> TcSigFun
-> Bool
-> Bool
-> [LHsBind Name]
-> TcM (LHsBinds TcId, [TcId], TopLevelFlag)
tcPolyInfer rec_tc prag_fn tc_sig_fn mono closed bind_list
= do { ((binds', mono_infos), wanted)
<- captureConstraints $
tcMonoBinds rec_tc tc_sig_fn LetLclBndr bind_list
; let name_taus = [(name, idType mono_id) | (name, _, mono_id) <- mono_infos]
; traceTc "simplifyInfer call" (ppr name_taus $$ ppr wanted)
; (qtvs, givens, mr_bites, ev_binds) <-
simplifyInfer closed mono name_taus wanted
; theta <- zonkTcThetaType (map evVarPred givens)
; exports <- checkNoErrs $ mapM (mkExport prag_fn qtvs theta) mono_infos
; loc <- getSrcSpanM
; let poly_ids = map abe_poly exports
final_closed | closed && not mr_bites = TopLevel
| otherwise = NotTopLevel
abs_bind = L loc $
AbsBinds { abs_tvs = qtvs
, abs_ev_vars = givens, abs_ev_binds = ev_binds
, abs_exports = exports, abs_binds = binds' }
; traceTc "Binding:" (ppr final_closed $$
ppr (poly_ids `zip` map idType poly_ids))
; return (unitBag abs_bind, poly_ids, final_closed) }
mkExport :: PragFun
-> [TyVar] -> TcThetaType
-> MonoBindInfo
-> TcM (ABExport Id)
mkExport prag_fn qtvs theta (poly_name, mb_sig, mono_id)
= do { mono_ty <- zonkTcType (idType mono_id)
; let poly_id = case mb_sig of
Nothing -> mkLocalId poly_name inferred_poly_ty
Just sig -> sig_id sig
my_tvs2 = closeOverKinds (growThetaTyVars theta (tyVarsOfType mono_ty))
my_tvs = filter (`elemVarSet` my_tvs2) qtvs
my_theta = filter (quantifyPred my_tvs2) theta
inferred_poly_ty = mkSigmaTy my_tvs my_theta mono_ty
; poly_id <- addInlinePrags poly_id prag_sigs
; spec_prags <- tcSpecPrags poly_id prag_sigs
; let sel_poly_ty = mkSigmaTy qtvs theta mono_ty
; traceTc "mkExport: check sig"
(ppr poly_name $$ ppr sel_poly_ty $$ ppr (idType poly_id))
; (wrap, wanted) <- addErrCtxtM (mk_msg poly_id) $
captureConstraints $
tcSubType origin sig_ctxt sel_poly_ty (idType poly_id)
; ev_binds <- simplifyTop wanted
; return (ABE { abe_wrap = mkWpLet (EvBinds ev_binds) <.> wrap
, abe_poly = poly_id
, abe_mono = mono_id
, abe_prags = SpecPrags spec_prags }) }
where
inferred = isNothing mb_sig
mk_msg poly_id tidy_env
= return (tidy_env', msg)
where
msg | inferred = hang (ptext (sLit "When checking that") <+> pp_name)
2 (ptext (sLit "has the inferred type") <+> pp_ty)
$$ ptext (sLit "Probable cause: the inferred type is ambiguous")
| otherwise = hang (ptext (sLit "When checking that") <+> pp_name)
2 (ptext (sLit "has the specified type") <+> pp_ty)
pp_name = quotes (ppr poly_name)
pp_ty = quotes (ppr tidy_ty)
(tidy_env', tidy_ty) = tidyOpenType tidy_env (idType poly_id)
prag_sigs = prag_fn poly_name
origin = AmbigOrigin sig_ctxt
sig_ctxt = InfSigCtxt poly_name
\end{code}
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
f_mono_ty :: a -> Bool -> Bool
g_mono_ty :: [b] -> Bool -> Bool
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.
Then we want to check that
f's polytype is more polymorphic than forall qtvs. theta => f_mono_ty
and the proof is the impedence matcher.
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.
\begin{code}
type PragFun = Name -> [LSig Name]
mkPragFun :: [LSig Name] -> LHsBinds Name -> PragFun
mkPragFun sigs binds = \n -> lookupNameEnv prag_env n `orElse` []
where
prs = mapCatMaybes get_sig sigs
get_sig :: LSig Name -> Maybe (Located Name, LSig Name)
get_sig (L l (SpecSig nm ty inl)) = Just (nm, L l $ SpecSig nm ty (add_arity nm inl))
get_sig (L l (InlineSig nm inl)) = Just (nm, L l $ InlineSig nm (add_arity nm inl))
get_sig _ = Nothing
add_arity (L _ n) inl_prag
| Just ar <- lookupNameEnv ar_env n,
Inline <- inl_inline inl_prag = inl_prag { inl_sat = Just ar }
| otherwise = inl_prag
prag_env :: NameEnv [LSig Name]
prag_env = foldl add emptyNameEnv prs
add env (L _ n,p) = extendNameEnv_Acc (:) singleton env n p
ar_env :: NameEnv Arity
ar_env = foldrBag lhsBindArity emptyNameEnv binds
lhsBindArity :: LHsBind Name -> NameEnv Arity -> NameEnv Arity
lhsBindArity (L _ (FunBind { fun_id = id, fun_matches = ms })) env
= extendNameEnv env (unLoc id) (matchGroupArity ms)
lhsBindArity _ env = env
tcSpecPrags :: Id -> [LSig Name]
-> TcM [LTcSpecPrag]
tcSpecPrags poly_id prag_sigs
= do { unless (null bad_sigs) warn_discarded_sigs
; mapAndRecoverM (wrapLocM (tcSpec poly_id)) spec_sigs }
where
spec_sigs = filter isSpecLSig prag_sigs
bad_sigs = filter is_bad_sig prag_sigs
is_bad_sig s = not (isSpecLSig s || isInlineLSig s)
warn_discarded_sigs = warnPrags poly_id bad_sigs $
ptext (sLit "Discarding unexpected pragmas for")
tcSpec :: TcId -> Sig Name -> TcM TcSpecPrag
tcSpec poly_id prag@(SpecSig fun_name hs_ty inl)
= addErrCtxt (spec_ctxt prag) $
do { spec_ty <- tcHsSigType sig_ctxt hs_ty
; warnIf (not (isOverloadedTy poly_ty || isInlinePragma inl))
(ptext (sLit "SPECIALISE pragma for non-overloaded function")
<+> quotes (ppr fun_name))
; wrap <- tcSubType origin sig_ctxt (idType poly_id) spec_ty
; return (SpecPrag poly_id wrap inl) }
where
name = idName poly_id
poly_ty = idType poly_id
origin = SpecPragOrigin name
sig_ctxt = FunSigCtxt name
spec_ctxt prag = hang (ptext (sLit "In the SPECIALISE pragma")) 2 (ppr prag)
tcSpec _ prag = pprPanic "tcSpec" (ppr prag)
tcImpPrags :: [LSig Name] -> TcM [LTcSpecPrag]
tcImpPrags prags
= do { this_mod <- getModule
; dflags <- getDynFlags
; if (not_specialising dflags) then
return []
else
mapAndRecoverM (wrapLocM tcImpSpec)
[L loc (name,prag) | (L loc prag@(SpecSig (L _ name) _ _)) <- prags
, not (nameIsLocalOrFrom this_mod name) ] }
where
not_specialising dflags
| not (gopt Opt_Specialise dflags) = True
| otherwise = case hscTarget dflags of
HscNothing -> True
HscInterpreted -> True
_other -> False
tcImpSpec :: (Name, Sig Name) -> TcM TcSpecPrag
tcImpSpec (name, prag)
= do { id <- tcLookupId name
; unless (isAnyInlinePragma (idInlinePragma id))
(addWarnTc (impSpecErr name))
; tcSpec id prag }
impSpecErr :: Name -> SDoc
impSpecErr name
= hang (ptext (sLit "You cannot SPECIALISE") <+> quotes (ppr name))
2 (vcat [ ptext (sLit "because its definition has no INLINE/INLINABLE pragma")
, parens $ sep
[ ptext (sLit "or its defining module") <+> quotes (ppr mod)
, ptext (sLit "was compiled without -O")]])
where
mod = nameModule name
tcVectDecls :: [LVectDecl Name] -> TcM ([LVectDecl TcId])
tcVectDecls decls
= do { decls' <- mapM (wrapLocM tcVect) decls
; let ids = [lvectDeclName decl | decl <- decls', not $ lvectInstDecl decl]
dups = findDupsEq (==) ids
; mapM_ reportVectDups dups
; traceTcConstraints "End of tcVectDecls"
; return decls'
}
where
reportVectDups (first:_second:_more)
= addErrAt (getSrcSpan first) $
ptext (sLit "Duplicate vectorisation declarations for") <+> ppr first
reportVectDups _ = return ()
tcVect :: VectDecl Name -> TcM (VectDecl TcId)
tcVect (HsVect name rhs)
= addErrCtxt (vectCtxt name) $
do { var <- wrapLocM tcLookupId name
; let L rhs_loc (HsVar rhs_var_name) = rhs
; rhs_id <- tcLookupId rhs_var_name
; return $ HsVect var (L rhs_loc (HsVar rhs_id))
}
tcVect (HsNoVect name)
= addErrCtxt (vectCtxt name) $
do { var <- wrapLocM tcLookupId name
; return $ HsNoVect var
}
tcVect (HsVectTypeIn isScalar lname rhs_name)
= addErrCtxt (vectCtxt lname) $
do { tycon <- tcLookupLocatedTyCon lname
; checkTc ( not isScalar
|| isJust rhs_name
|| tyConArity tycon == 0
)
scalarTyConMustBeNullary
; rhs_tycon <- fmapMaybeM (tcLookupTyCon . unLoc) rhs_name
; return $ HsVectTypeOut isScalar tycon rhs_tycon
}
tcVect (HsVectTypeOut _ _ _)
= panic "TcBinds.tcVect: Unexpected 'HsVectTypeOut'"
tcVect (HsVectClassIn lname)
= addErrCtxt (vectCtxt lname) $
do { cls <- tcLookupLocatedClass lname
; return $ HsVectClassOut cls
}
tcVect (HsVectClassOut _)
= panic "TcBinds.tcVect: Unexpected 'HsVectClassOut'"
tcVect (HsVectInstIn linstTy)
= addErrCtxt (vectCtxt linstTy) $
do { (cls, tys) <- tcHsVectInst linstTy
; inst <- tcLookupInstance cls tys
; return $ HsVectInstOut inst
}
tcVect (HsVectInstOut _)
= panic "TcBinds.tcVect: Unexpected 'HsVectInstOut'"
vectCtxt :: Outputable thing => thing -> SDoc
vectCtxt thing = ptext (sLit "When checking the vectorisation declaration for") <+> ppr thing
scalarTyConMustBeNullary :: MsgDoc
scalarTyConMustBeNullary = ptext (sLit "VECTORISE SCALAR type constructor must be nullary")
recoveryCode :: [Name] -> TcSigFun -> TcM (LHsBinds TcId, [Id], TopLevelFlag)
recoveryCode binder_names sig_fn
= do { traceTc "tcBindsWithSigs: error recovery" (ppr binder_names)
; poly_ids <- mapM mk_dummy binder_names
; return (emptyBag, poly_ids, if all is_closed poly_ids
then TopLevel else NotTopLevel) }
where
mk_dummy name
| isJust (sig_fn name) = tcLookupId name
| otherwise = return (mkLocalId name forall_a_a)
is_closed poly_id = isEmptyVarSet (tyVarsOfType (idType poly_id))
forall_a_a :: TcType
forall_a_a = mkForAllTy openAlphaTyVar (mkTyVarTy openAlphaTyVar)
\end{code}
Note [SPECIALISE pragmas]
~~~~~~~~~~~~~~~~~~~~~~~~~
There is no point in a SPECIALISE pragma for a non-overloaded function:
reverse :: [a] -> [a]
{-# SPECIALISE reverse :: [Int] -> [Int] #-}
But SPECIALISE INLINE *can* make sense for GADTS:
data Arr e where
ArrInt :: !Int -> ByteArray# -> Arr Int
ArrPair :: !Int -> Arr e1 -> Arr e2 -> Arr (e1, e2)
(!:) :: Arr e -> Int -> e
{-# SPECIALISE INLINE (!:) :: Arr Int -> Int -> Int #-}
{-# SPECIALISE INLINE (!:) :: Arr (a, b) -> Int -> (a, b) #-}
(ArrInt _ ba) !: (I# i) = I# (indexIntArray# ba i)
(ArrPair _ a1 a2) !: i = (a1 !: i, a2 !: i)
When (!:) is specialised it becomes non-recursive, and can usefully
be inlined. Scary! So we only warn for SPECIALISE *without* INLINE
for a non-overloaded function.
%************************************************************************
%* *
\subsection{tcMonoBind}
%* *
%************************************************************************
@tcMonoBinds@ deals with a perhaps-recursive group of HsBinds.
The signatures have been dealt with already.
Note [Pattern bindings]
~~~~~~~~~~~~~~~~~~~~~~~
The rule for typing pattern bindings is this:
..sigs..
p = e
where 'p' binds v1..vn, and 'e' may mention v1..vn,
typechecks exactly like
..sigs..
x = e -- Inferred type
v1 = case x of p -> v1
..
vn = case x of p -> vn
Note that
(f :: forall a. a -> a) = id
should not typecheck because
case id of { (f :: forall a. a->a) -> f }
will not typecheck.
\begin{code}
tcMonoBinds :: RecFlag
-> TcSigFun -> LetBndrSpec
-> [LHsBind Name]
-> TcM (LHsBinds TcId, [MonoBindInfo])
tcMonoBinds is_rec sig_fn no_gen
[ L b_loc (FunBind { fun_id = L nm_loc name, fun_infix = inf,
fun_matches = matches, bind_fvs = fvs })]
| NonRecursive <- is_rec
, Nothing <- sig_fn name
=
setSrcSpan b_loc $
do { rhs_ty <- newFlexiTyVarTy openTypeKind
; mono_id <- newNoSigLetBndr no_gen name rhs_ty
; (co_fn, matches') <- tcExtendIdBndrs [TcIdBndr mono_id NotTopLevel] $
tcMatchesFun name inf matches rhs_ty
; return (unitBag $ L b_loc (FunBind { fun_id = L nm_loc mono_id, fun_infix = inf,
fun_matches = matches', bind_fvs = fvs,
fun_co_fn = co_fn, fun_tick = Nothing }),
[(name, Nothing, mono_id)]) }
tcMonoBinds _ sig_fn no_gen binds
= do { tc_binds <- mapM (wrapLocM (tcLhs sig_fn no_gen)) binds
; let mono_info = getMonoBindInfo tc_binds
rhs_id_env = [(name,mono_id) | (name, Nothing, mono_id) <- mono_info]
; traceTc "tcMonoBinds" $ vcat [ ppr n <+> ppr id <+> ppr (idType id)
| (n,id) <- rhs_id_env]
; binds' <- tcExtendIdEnv2 rhs_id_env $
mapM (wrapLocM tcRhs) tc_binds
; return (listToBag binds', mono_info) }
data TcMonoBind
= TcFunBind MonoBindInfo SrcSpan Bool (MatchGroup Name (LHsExpr Name))
| TcPatBind [MonoBindInfo] (LPat TcId) (GRHSs Name (LHsExpr Name)) TcSigmaType
type MonoBindInfo = (Name, Maybe TcSigInfo, TcId)
tcLhs :: TcSigFun -> LetBndrSpec -> HsBind Name -> TcM TcMonoBind
tcLhs sig_fn no_gen (FunBind { fun_id = L nm_loc name, fun_infix = inf, fun_matches = matches })
| Just sig <- sig_fn name
= ASSERT2( case no_gen of { LetLclBndr -> True; LetGblBndr {} -> False }
, ppr name )
do { mono_name <- newLocalName name
; let mono_id = mkLocalId mono_name (sig_tau sig)
; return (TcFunBind (name, Just sig, mono_id) nm_loc inf matches) }
| otherwise
= do { mono_ty <- newFlexiTyVarTy openTypeKind
; mono_id <- newNoSigLetBndr no_gen name mono_ty
; return (TcFunBind (name, Nothing, mono_id) nm_loc inf matches) }
tcLhs sig_fn no_gen (PatBind { pat_lhs = pat, pat_rhs = grhss })
= do { let tc_pat exp_ty = tcLetPat sig_fn no_gen pat exp_ty $
mapM lookup_info (collectPatBinders pat)
lookup_info :: Name -> TcM MonoBindInfo
lookup_info name = do { mono_id <- tcLookupId name
; return (name, sig_fn name, mono_id) }
; ((pat', infos), pat_ty) <- addErrCtxt (patMonoBindsCtxt pat grhss) $
tcInfer tc_pat
; return (TcPatBind infos pat' grhss pat_ty) }
tcLhs _ _ other_bind = pprPanic "tcLhs" (ppr other_bind)
tcRhs :: TcMonoBind -> TcM (HsBind TcId)
tcRhs (TcFunBind (_,_,mono_id) loc inf matches)
= tcExtendIdBndrs [TcIdBndr mono_id NotTopLevel] $
do { traceTc "tcRhs: fun bind" (ppr mono_id $$ ppr (idType mono_id))
; (co_fn, matches') <- tcMatchesFun (idName mono_id) inf
matches (idType mono_id)
; return (FunBind { fun_id = L loc mono_id, fun_infix = inf
, fun_matches = matches'
, fun_co_fn = co_fn
, bind_fvs = placeHolderNames, fun_tick = Nothing }) }
tcRhs (TcPatBind infos pat' grhss pat_ty)
= tcExtendIdBndrs [ TcIdBndr mono_id NotTopLevel | (_,_,mono_id) <- infos ] $
do { traceTc "tcRhs: pat bind" (ppr pat' $$ ppr pat_ty)
; grhss' <- addErrCtxt (patMonoBindsCtxt pat' grhss) $
tcGRHSsPat grhss pat_ty
; return (PatBind { pat_lhs = pat', pat_rhs = grhss', pat_rhs_ty = pat_ty
, bind_fvs = placeHolderNames
, pat_ticks = (Nothing,[]) }) }
getMonoBindInfo :: [Located TcMonoBind] -> [MonoBindInfo]
getMonoBindInfo tc_binds
= foldr (get_info . unLoc) [] tc_binds
where
get_info (TcFunBind info _ _ _) rest = info : rest
get_info (TcPatBind infos _ _ _) rest = infos ++ rest
\end{code}
%************************************************************************
%* *
Signatures
%* *
%************************************************************************
Type signatures are tricky. See Note [Signature skolems] in TcType
@tcSigs@ checks the signatures for validity, and returns a list of
{\em freshly-instantiated} signatures. That is, the types are already
split up, and have fresh type variables installed. All non-type-signature
"RenamedSigs" are ignored.
The @TcSigInfo@ contains @TcTypes@ because they are unified with
the variable's type, and after that checked to see whether they've
been instantiated.
Note [Scoped tyvars]
~~~~~~~~~~~~~~~~~~~~
The -XScopedTypeVariables flag brings lexically-scoped type variables
into scope for any explicitly forall-quantified type variables:
f :: forall a. a -> a
f x = e
Then 'a' is in scope inside 'e'.
However, we do *not* support this
- For pattern bindings e.g
f :: forall a. a->a
(f,g) = e
Note [Signature skolems]
~~~~~~~~~~~~~~~~~~~~~~~~
When instantiating a type signature, we do so with either skolems or
SigTv meta-type variables depending on the use_skols boolean. This
variable is set True when we are typechecking a single function
binding; and False for pattern bindings and a group of several
function bindings.
Reason: in the latter cases, the "skolems" can be unified together,
so they aren't properly rigid in the type-refinement sense.
NB: unless we are doing H98, each function with a sig will be done
separately, even if it's mutually recursive, so use_skols will be True
Note [Only scoped tyvars are in the TyVarEnv]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We are careful to keep only the *lexically scoped* type variables in
the type environment. Why? After all, the renamer has ensured
that only legal occurrences occur, so we could put all type variables
into the type env.
But we want to check that two distinct lexically scoped type variables
do not map to the same internal type variable. So we need to know which
the lexically-scoped ones are... and at the moment we do that by putting
only the lexically scoped ones into the environment.
Note [Instantiate sig with fresh variables]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
It's vital to instantiate a type signature with fresh variables.
For example:
type T = forall a. [a] -> [a]
f :: T;
f = g where { g :: T; g = }
We must not use the same 'a' from the defn of T at both places!!
(Instantiation is only necessary because of type synonyms. Otherwise,
it's all cool; each signature has distinct type variables from the renamer.)
Note [Fail eagerly on bad signatures]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
If a type signaure is wrong, fail immediately:
* the type sigs may bind type variables, so proceeding without them
can lead to a cascade of errors
* the type signature might be ambiguous, in which case checking
the code against the signature will give a very similar error
to the ambiguity error.
ToDo: this means we fall over if any type sig
is wrong (eg at the top level of the module),
which is over-conservative
\begin{code}
tcTySigs :: [LSig Name] -> TcM ([TcId], TcSigFun)
tcTySigs hs_sigs
= checkNoErrs $
do { ty_sigs_s<- mapAndRecoverM tcTySig hs_sigs
; let ty_sigs = concat ty_sigs_s
env = mkNameEnv [(idName (sig_id sig), sig) | sig <- ty_sigs]
; return (map sig_id ty_sigs, lookupNameEnv env) }
tcTySig :: LSig Name -> TcM [TcSigInfo]
tcTySig (L loc (IdSig id))
= do { sig <- instTcTySigFromId loc id
; return [sig] }
tcTySig (L loc (TypeSig names@(L _ name1 : _) hs_ty))
= setSrcSpan loc $
do { sigma_ty <- tcHsSigType (FunSigCtxt name1) hs_ty
; mapM (instTcTySig hs_ty sigma_ty) (map unLoc names) }
tcTySig _ = return []
instTcTySigFromId :: SrcSpan -> Id -> TcM TcSigInfo
instTcTySigFromId loc id
= do { (tvs, theta, tau) <- tcInstType (tcInstSigTyVarsLoc loc)
(idType id)
; return (TcSigInfo { sig_id = id, sig_loc = loc
, sig_tvs = [(Nothing, tv) | tv <- tvs]
, sig_theta = theta, sig_tau = tau }) }
where
instTcTySig :: LHsType Name -> TcType
-> Name -> TcM TcSigInfo
instTcTySig hs_ty@(L loc _) sigma_ty name
= do { (inst_tvs, theta, tau) <- tcInstType tcInstSigTyVars sigma_ty
; return (TcSigInfo { sig_id = mkLocalId name sigma_ty
, sig_loc = loc
, sig_tvs = findScopedTyVars hs_ty sigma_ty inst_tvs
, sig_theta = theta, sig_tau = tau }) }
data GeneralisationPlan
= NoGen
| InferGen
Bool
Bool
| CheckGen (LHsBind Name) TcSigInfo
instance Outputable GeneralisationPlan where
ppr NoGen = ptext (sLit "NoGen")
ppr (InferGen b c) = ptext (sLit "InferGen") <+> ppr b <+> ppr c
ppr (CheckGen _ s) = ptext (sLit "CheckGen") <+> ppr s
decideGeneralisationPlan
:: DynFlags -> TcTypeEnv -> [Name]
-> [LHsBind Name] -> TcSigFun -> GeneralisationPlan
decideGeneralisationPlan dflags type_env bndr_names lbinds sig_fn
| strict_pat_binds = NoGen
| Just (lbind, sig) <- one_funbind_with_sig lbinds = CheckGen lbind sig
| mono_local_binds = NoGen
| otherwise = InferGen mono_restriction closed_flag
where
bndr_set = mkNameSet bndr_names
binds = map unLoc lbinds
strict_pat_binds = any isStrictHsBind binds
mono_restriction = xopt Opt_MonomorphismRestriction dflags
&& any restricted binds
is_closed_ns :: NameSet -> Bool -> Bool
is_closed_ns ns b = foldNameSet ((&&) . is_closed_id) b ns
is_closed_id :: Name -> Bool
is_closed_id name
| name `elemNameSet` bndr_set
= True
| Just thing <- lookupNameEnv type_env name
= case thing of
ATcId { tct_closed = cl } -> isTopLevel cl
ATyVar {} -> False
AGlobal {} -> True
_ -> pprPanic "is_closed_id" (ppr name)
| otherwise
= WARN( isInternalName name, ppr name ) True
closed_flag = foldr (is_closed_ns . bind_fvs) True binds
mono_local_binds = xopt Opt_MonoLocalBinds dflags
&& not closed_flag
no_sig n = isNothing (sig_fn n)
one_funbind_with_sig [lbind@(L _ (FunBind { fun_id = v }))]
= case sig_fn (unLoc v) of
Nothing -> Nothing
Just sig -> Just (lbind, sig)
one_funbind_with_sig _
= Nothing
restricted (PatBind {}) = True
restricted (VarBind { var_id = v }) = no_sig v
restricted (FunBind { fun_id = v, fun_matches = m }) = restricted_match m
&& no_sig (unLoc v)
restricted (PatSynBind {}) = panic "isRestrictedGroup/unrestricted PatSynBind"
restricted (AbsBinds {}) = panic "isRestrictedGroup/unrestricted AbsBinds"
restricted_match (MG { mg_alts = L _ (Match [] _ _) : _ }) = True
restricted_match _ = False
checkStrictBinds :: TopLevelFlag -> RecFlag
-> [LHsBind Name]
-> LHsBinds TcId -> [Id]
-> TcM ()
checkStrictBinds top_lvl rec_group orig_binds tc_binds poly_ids
| unlifted_bndrs || any_strict_pat
= do { checkTc (isNotTopLevel top_lvl)
(strictBindErr "Top-level" unlifted_bndrs orig_binds)
; checkTc (isNonRec rec_group)
(strictBindErr "Recursive" unlifted_bndrs orig_binds)
; checkTc (all is_monomorphic (bagToList tc_binds))
(polyBindErr orig_binds)
; checkTc (isSingleton orig_binds)
(strictBindErr "Multiple" unlifted_bndrs orig_binds)
; checkTc (not any_pat_looks_lazy)
(unliftedMustBeBang orig_binds) }
| otherwise
= traceTc "csb2" (ppr poly_ids) >>
return ()
where
unlifted_bndrs = any is_unlifted poly_ids
any_strict_pat = any (isStrictHsBind . unLoc) orig_binds
any_pat_looks_lazy = any (looksLazyPatBind . unLoc) orig_binds
is_unlifted id = case tcSplitForAllTys (idType id) of
(_, rho) -> isUnLiftedType rho
is_monomorphic (L _ (AbsBinds { abs_tvs = tvs, abs_ev_vars = evs }))
= null tvs && null evs
is_monomorphic _ = True
unliftedMustBeBang :: [LHsBind Name] -> SDoc
unliftedMustBeBang binds
= hang (text "Pattern bindings containing unlifted types should use an outermost bang pattern:")
2 (vcat (map ppr binds))
polyBindErr :: [LHsBind Name] -> SDoc
polyBindErr binds
= hang (ptext (sLit "You can't mix polymorphic and unlifted bindings"))
2 (vcat [vcat (map ppr binds),
ptext (sLit "Probable fix: use a bang pattern")])
strictBindErr :: String -> Bool -> [LHsBind Name] -> SDoc
strictBindErr flavour unlifted_bndrs binds
= hang (text flavour <+> msg <+> ptext (sLit "aren't allowed:"))
2 (vcat (map ppr binds))
where
msg | unlifted_bndrs = ptext (sLit "bindings for unlifted types")
| otherwise = ptext (sLit "bang-pattern or unboxed-tuple bindings")
\end{code}
Note [Binding scoped type variables]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
%************************************************************************
%* *
\subsection[TcBinds-errors]{Error contexts and messages}
%* *
%************************************************************************
\begin{code}
patMonoBindsCtxt :: (OutputableBndr id, Outputable body) => LPat id -> GRHSs Name body -> SDoc
patMonoBindsCtxt pat grhss
= hang (ptext (sLit "In a pattern binding:")) 2 (pprPatBind pat grhss)
\end{code}