[project @ 2004-12-21 12:22:22 by simonpj]

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

diff --git a/ghc/compiler/typecheck/TcUnify.lhs b/ghc/compiler/typecheck/TcUnify.lhs
index 87b30c6..179a7db 100644 (file)
--- a/ghc/compiler/typecheck/TcUnify.lhs
+++ b/ghc/compiler/typecheck/TcUnify.lhs
@@ -383,6 +383,9 @@ tcSubPat :: TcSigmaType             -- Pattern type signature
         -> TcM ()
 -- In patterns we insist on an exact match; hence no CoFn returned
 --     See Note [Pattern coercions] in TcPat
+-- However, we can't call unify directly, because both types might be
+-- polymorphic; hence the call to tcSub, followed by a check for
+-- the identity coercion
 
 tcSubPat sig_ty (Infer hole) 
   = do { sig_ty' <- zonkTcType sig_ty