Commit 0452021e authored by Simon Peyton Jones's avatar Simon Peyton Jones

Fix typechecking of pattern bindings that have type signatures (Trac #7268)

Pattern bindings are jolly tricky to typecheck, especially if there are
also type signatures involved.  Trac #7268 pointed out that I'd got it
wrong; this fixes it.  See Note [Typing patterns in pattern bindings] in TcPat.
parent 1d4704d4
......@@ -325,9 +325,10 @@ tc_group top_lvl sig_fn prag_fn (NonRecursive, binds) thing_inside
; return ( [(NonRecursive, binds1)], thing) }
tc_group top_lvl sig_fn prag_fn (Recursive, binds) thing_inside
= -- To maximise polymorphism (assumes -XRelaxedPolyRec), we do a new
= -- 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.)
do { traceTc "tc_group rec" (pprLHsBinds binds)
; (binds1, _ids, thing) <- go sccs
-- Here is where we should do bindInstsOfLocalFuns
......@@ -1006,7 +1007,12 @@ 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
= do { mono_id <- newSigLetBndr no_gen name sig
= ASSERT2( case no_gen of { LetLclBndr -> True; LetGblBndr {} -> False }
, ppr name ) -- { f :: ty; f x = e } is always done via CheckGen
-- which gives rise to LetLclBndr. It wouldn't make
-- sense to have a *polymorphic* function Id at this point
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
......@@ -1098,17 +1104,6 @@ However, we do *not* support this
f :: forall a. a->a
(f,g) = e
- For multiple function bindings, unless Opt_RelaxedPolyRec is on
f :: forall a. a -> a
f = g
g :: forall b. b -> b
g = ...f...
Reason: we use mutable variables for 'a' and 'b', since they may
unify to each other, and that means the scoped type variable would
not stand for a completely rigid variable.
Currently, we simply make Opt_ScopedTypeVariables imply Opt_RelaxedPolyRec
Note [More instantiated than scoped]
There may be more instantiated type variables than lexically-scoped
......@@ -15,7 +15,7 @@ TcPat: Typechecking patterns
module TcPat ( tcLetPat, TcSigFun, TcSigInfo(..), TcPragFun
, LetBndrSpec(..), addInlinePrags, warnPrags
, tcPat, tcPats, newNoSigLetBndr, newSigLetBndr
, tcPat, tcPats, newNoSigLetBndr
, addDataConStupidTheta, badFieldCon, polyPatSig ) where
#include "HsVersions.h"
......@@ -112,8 +112,8 @@ data PatCtxt
= LamPat -- Used for lambdas, case etc
(HsMatchContext Name)
| LetPat -- Used only for let(rec) bindings
-- See Note [Let binders]
| LetPat -- Used only for let(rec) pattern bindings
-- See Note [Typing patterns in pattern bindings]
TcSigFun -- Tells type sig if any
LetBndrSpec -- True <=> no generalisation of this let
......@@ -121,8 +121,10 @@ data LetBndrSpec
= LetLclBndr -- The binder is just a local one;
-- an AbsBinds will provide the global version
| LetGblBndr TcPragFun -- There isn't going to be an AbsBinds;
-- here is the inline-pragma information
| LetGblBndr TcPragFun -- Genrealisation plan is NoGen, so there isn't going
-- to be an AbsBinds; So we must bind the global version
-- of the binder right away.
-- Oh, and dhhere is the inline-pragma information
makeLazy :: PatEnv -> PatEnv
makeLazy penv = penv { pe_lazy = True }
......@@ -177,15 +179,6 @@ if the original function had a signature like
But that's ok: tcMatchesFun (called by tcRhs) can deal with that
It happens, too! See Note [Polymorphic methods] in TcClassDcl.
Note [Let binders]
eg x :: Int
y :: Bool
(x,y) = e
...more notes to add here..
Note [Existential check]
Lazy patterns can't bind existentials. They arise in two ways:
......@@ -215,34 +208,30 @@ tcPatBndr :: PatEnv -> Name -> TcSigmaType -> TcM (TcCoercion, TcId)
-- Then coi : pat_ty ~ typeof(xp)
tcPatBndr (PE { pe_ctxt = LetPat lookup_sig no_gen}) bndr_name pat_ty
| Just sig <- lookup_sig bndr_name
= do { bndr_id <- newSigLetBndr no_gen bndr_name sig
-- See Note [Typing patterns in pattern bindings]
| LetGblBndr prags <- no_gen
, Just sig <- lookup_sig bndr_name
= do { bndr_id <- addInlinePrags (sig_id sig) (prags bndr_name)
; traceTc "tcPatBndr(gbl,sig)" (ppr bndr_id $$ ppr (idType bndr_id))
; co <- unifyPatType (idType bndr_id) pat_ty
; return (co, bndr_id) }
| otherwise
| otherwise
= do { bndr_id <- newNoSigLetBndr no_gen bndr_name pat_ty
; traceTc "tcPatBndr(no-sig)" (ppr bndr_id $$ ppr (idType bndr_id))
; return (mkTcReflCo pat_ty, bndr_id) }
tcPatBndr (PE { pe_ctxt = _lam_or_proc }) bndr_name pat_ty
= do { bndr <- mkLocalBinder bndr_name pat_ty
; return (mkTcReflCo pat_ty, bndr) }
newSigLetBndr :: LetBndrSpec -> Name -> TcSigInfo -> TcM TcId
newSigLetBndr LetLclBndr name sig
= do { mono_name <- newLocalName name
; mkLocalBinder mono_name (sig_tau sig) }
newSigLetBndr (LetGblBndr prags) name sig
= addInlinePrags (sig_id sig) (prags name)
newNoSigLetBndr :: LetBndrSpec -> Name -> TcType -> TcM TcId
-- In the polymorphic case (no_gen = False), generate a "monomorphic version"
-- In the polymorphic case (no_gen = LetLclBndr), generate a "monomorphic version"
-- of the Id; the original name will be bound to the polymorphic version
-- by the AbsBinds
-- In the monomorphic case there is no AbsBinds, and we use the original
-- name directly
-- In the monomorphic case (no_gen = LetBglBndr) there is no AbsBinds, and we
-- use the original name directly
newNoSigLetBndr LetLclBndr name ty
=do { mono_name <- newLocalName name
; mkLocalBinder mono_name ty }
......@@ -280,16 +269,34 @@ mkLocalBinder name ty
= return (Id.mkLocalId name ty)
Note [Polymorphism and pattern bindings]
When is_mono holds we are not generalising
But the signature can still be polymorphic!
data T = MkT (forall a. a->a)
Note [Typing patterns in pattern bindings]
Suppose we are typing a pattern binding
pat = rhs
Then the PatCtxt will be (LetPat sig_fn let_bndr_spec).
There can still be signatures for the binders:
data T = MkT (forall a. a->a) Int
x :: forall a. a->a
MkT x = <rhs>
So the no_gen flag decides whether the pattern-bound variables should
have exactly the type in the type signature (when not generalising) or
the instantiated version (when generalising)
y :: Int
MkT x y = <rhs>
Two cases, dealt with by the LetPat case of tcPatBndr
* If we are generalising (generalisation plan is InferGen or
CheckGen), then the let_bndr_spec will be LetLclBndr. In that case
we want to bind a cloned, local version of the variable, with the
type given by the pattern context, *not* by the signature (even if
there is one; see Trac #7268). The mkExport part of the
generalisation step will do the checking and impedence matching
against the signature.
* If for some some reason we are not generalising (plan = NoGen), the
LetBndrSpec will be LetGblBndr. In that case we must bind the
global version of the Id, and do so with precisely the type given
in the signature. (Then we unify with the type from the pattern
context type.
%* *
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