RE: GADT: weird error message

Yes, GHC has a special rule to handle case-of-expression.  (We call it
"smart-app" in the paper.)  But GHC's front end retains all syntax,
including tuple syntax, and I failed to do the special rule for tuple
syntax.  Sorry!

Perhaps you can file it as a Sourceforge bug.  I'm going to do a raft of
changes to GADTs in the autumn, and by filing it you'll ensure I don't
forget.

BTW, what are you using GADTs for?  Any other unexpected surprises?

Thanks

Simon


| -----Original Message-----
| From: glasgow-haskell-bugs-bounces@xxxxxxxxxxx
[mailto:glasgow-haskell-bugs-
| bounces@xxxxxxxxxxx] On Behalf Of Arthur Baars
| Sent: 06 September 2005 16:48
| To: glasgow-haskell-bugs@xxxxxxxxxxx
| Subject: GADT: weird error message
| 
| In the code below the function "trans" is accepted by GHC 6.4, but
| "trans1" is not. I would expect that "(x,y)" is just syntactic sugar
| for "(,) x y", but apparently it isn't. I guess this is a bug; can
| anyone explain what is going on?
| 
| Cheers,
| 
| Arthur
| 
| The Code:
| data Equal a b where
|       Eq :: Equal a a
| 
| trans :: forall a b c. Equal a b -> Equal b c -> Equal a c
| trans = \x -> \y -> case (,) x  y  of
|                      (Eq,Eq ) -> Eq
| 
| trans1 :: forall a b c. Equal a b -> Equal b c -> Equal a c
| trans1 = \x -> \y -> case (x, y)  of
|                      (Eq,Eq ) -> Eq
| 
| The error message:
| Test2.hs:9:0:
|      Quantified type variable `c' is unified with another quantified
| type variable a
|      When trying to generalise the type inferred for `trans1'
|        Signature type:     forall a b c. Equal a b -> Equal b c ->
Equal
| a c
|        Type to generalise: Equal a b -> Equal b a -> Equal a a
|      In the type signature for `trans1'
|      When generalising the type(s) for `trans1'
| 
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