X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=compiler%2Ftypes%2FType.lhs;h=4be386c1edbddd55d650a6b3b50798898100ff7b;hb=a385f0af5ea320a18d580f6a36c59c55b3516efd;hp=df9e3c7218730eec854b89575f2b778af27a0bcf;hpb=e314b86f6290e5440a46cd5cc29f7878cb78c6fb;p=ghc-hetmet.git

diff --git a/compiler/types/Type.lhs b/compiler/types/Type.lhs
index df9e3c7..4be386c 100644
--- a/compiler/types/Type.lhs
+++ b/compiler/types/Type.lhs
@@ -1258,6 +1258,19 @@ extendTvSubstList (TvSubst in_scope env) tvs tys
 -- the types given; but it's just a thunk so with a bit of luck
 -- it'll never be evaluated
 
+-- Note [Generating the in-scope set for a substitution]
+-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+-- If we want to substitute [a -> ty1, b -> ty2] I used to 
+-- think it was enough to generate an in-scope set that includes
+-- fv(ty1,ty2).  But that's not enough; we really should also take the
+-- free vars of the type we are substituting into!  Example:
+--	(forall b. (a,b,x)) [a -> List b]
+-- Then if we use the in-scope set {b}, there is a danger we will rename
+-- the forall'd variable to 'x' by mistake, getting this:
+--	(forall x. (List b, x, x)
+-- Urk!  This means looking at all the calls to mkOpenTvSubst....
+
+
 mkOpenTvSubst :: TvSubstEnv -> TvSubst
 mkOpenTvSubst env = TvSubst (mkInScopeSet (tyVarsOfTypes (varEnvElts env))) env
 
@@ -1310,9 +1323,9 @@ zip_ty_env tvs      tys      env   = pprTrace "Var/Type length mismatch: " (ppr
 
 instance Outputable TvSubst where
   ppr (TvSubst ins env) 
-    = brackets $ sep[ ptext SLIT("TvSubst"),
-		      nest 2 (ptext SLIT("In scope:") <+> ppr ins), 
-		      nest 2 (ptext SLIT("Env:") <+> ppr env) ]
+    = brackets $ sep[ ptext (sLit "TvSubst"),
+		      nest 2 (ptext (sLit "In scope:") <+> ppr ins), 
+		      nest 2 (ptext (sLit "Env:") <+> ppr env) ]
 \end{code}
 
 %************************************************************************