Fix scoped type variables for expression type signatures

[ghc-hetmet.git] / compiler / typecheck / TcBinds.lhs

diff --git a/compiler/typecheck/TcBinds.lhs b/compiler/typecheck/TcBinds.lhs
index 24f98d8..a3b17a6 100644 (file)
--- a/compiler/typecheck/TcBinds.lhs
+++ b/compiler/typecheck/TcBinds.lhs
@@ -19,10 +19,9 @@ import DynFlags              ( dopt, DynFlags,
                          DynFlag(Opt_MonomorphismRestriction, Opt_MonoPatBinds, Opt_GlasgowExts) )
 import HsSyn           ( HsExpr(..), HsBind(..), LHsBinds, LHsBind, Sig(..),
                          HsLocalBinds(..), HsValBinds(..), HsIPBinds(..),
-                         LSig, Match(..), IPBind(..), Prag(..),
-                         HsType(..), LHsType, HsExplicitForAll(..), hsLTyVarNames, 
+                         LSig, Match(..), IPBind(..), Prag(..), LHsType,
                          isVanillaLSig, sigName, placeHolderNames, isPragLSig,
-                         LPat, GRHSs, MatchGroup(..), pprLHsBinds, mkHsWrap,
+                         LPat, GRHSs, MatchGroup(..), pprLHsBinds, mkHsWrap, hsExplicitTvs,
                          collectHsBindBinders, collectPatBinders, pprPatBind, isBangHsBind
                        )
 import TcHsSyn         ( zonkId )
@@ -973,13 +972,12 @@ mkTcSigFun :: [LSig Name] -> TcSigFun
 -- Precondition: no duplicates
 mkTcSigFun sigs = lookupNameEnv env
   where
-    env = mkNameEnv [(name, scoped_tyvars hs_ty)
-                   | L span (TypeSig (L _ name) (L _ hs_ty)) <- sigs]
-    scoped_tyvars (HsForAllTy Explicit tvs _ _) = hsLTyVarNames tvs
-    scoped_tyvars other                                = []
+    env = mkNameEnv [(name, hsExplicitTvs lhs_ty)
+                   | L span (TypeSig (L _ name) lhs_ty) <- sigs]
        -- The scoped names are the ones explicitly mentioned
        -- in the HsForAll.  (There may be more in sigma_ty, because
        -- of nested type synonyms.  See Note [Scoped] with TcSigInfo.)
+       -- See Note [Only scoped tyvars are in the TyVarEnv]
 
 ---------------
 data TcSigInfo
@@ -998,6 +996,19 @@ data TcSigInfo
        sig_loc    :: InstLoc           -- The location of the signature
     }
 
+
+--     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 [Scoped]
 -- There may be more instantiated type variables than scoped 
 -- ones.  For example:
@@ -1010,7 +1021,7 @@ data TcSigInfo
 -- and remember the names from the original HsForAllTy in sig_scoped
 
 --     Note [Instantiate sig]
--- It's vital to instantiate a type signature with fresh variable.
+-- It's vital to instantiate a type signature with fresh variables.
 -- For example:
 --     type S = forall a. a->a
 --     f,g :: S
@@ -1046,7 +1057,7 @@ tcInstSig :: Bool -> Name -> [Name] -> TcM TcSigInfo
 -- Instantiate the signature, with either skolems or 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 bindigs and a group of several function bindings.
+-- 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