[project @ 2005-02-25 13:06:31 by simonpj]

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

diff --git a/ghc/compiler/typecheck/TcSimplify.lhs b/ghc/compiler/typecheck/TcSimplify.lhs
index 4c0ca4b..dd4d215 100644 (file)
--- a/ghc/compiler/typecheck/TcSimplify.lhs
+++ b/ghc/compiler/typecheck/TcSimplify.lhs
@@ -1984,8 +1984,17 @@ tc_simplify_top is_interactive wanteds
     strangeTopIPErrs bad_ips                           `thenM_`
 
        -- Deal with ambiguity errors, but only if
-       -- if there has not been an error so far; errors often
-       -- give rise to spurious ambiguous Insts
+       -- if there has not been an error so far:
+       -- errors often give rise to spurious ambiguous Insts.
+       -- For example:
+       --   f = (*)    -- Monomorphic
+       --   g :: Num a => a -> a
+       --   g x = f x x
+       -- Here, we get a complaint when checking the type signature for g,
+       -- that g isn't polymorphic enough; but then we get another one when
+       -- dealing with the (Num a) context arising from f's definition;
+       -- we try to unify a with Int (to default it), but find that it's
+       -- already been unified with the rigid variable from g's type sig
     ifErrsM (returnM []) (
        
        -- Complain about the ones that don't fall under