GADTs and functional dependencies in ghc 6.10.1

[Simon Peyton-Jones]

Reid,

Ah yes.  The interaction of functional dependencies and GADTs is flaky and unpredictable in both 6.8 and 6.10.  It's actually rather tricky to get right -- see our ICFP'08 paper.

You may have better luck using type families instead of functional dependencies, but even then 6.10 isn't really right.

We're working very actively on this topic.

Simon

| -----Original Message-----
| From: glasgow-haskell-users-bounces at haskell.org [mailto:glasgow-haskell-users-
| bounces at haskell.org] On Behalf Of Reid Barton
| Sent: 07 January 2009 09:02
| To: glasgow-haskell-users at haskell.org
| Subject: GADTs and functional dependencies in ghc 6.10.1
|
| Hello all,
|
| I think (hope) this question is different from the ones about GADTs
| recently discussed on this list.  The following program compiles under
| ghc 6.8.2 but not under ghc 6.10.1:
|
| > {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, GADTs, KindSignatures,
| ScopedTypeVariables #-}
| >
| > class Foo a fa | a -> fa where
| >   n :: a -> Int
| >
| > data Bar :: * -> * -> * where
| >   Id :: Bar a a
| >
| > baz :: forall a fa b fb. (Foo a fa, Foo b fb) => Bar a b -> Int
| > baz Id = n (undefined :: a)
|
| ghc 6.10.1's error message:
|
| /tmp/fundep.hs:10:0:
|     Couldn't match expected type `fb' against inferred type `fa'
|       `fb' is a rigid type variable bound by
|            the type signature for `baz' at /tmp/fundep.hs:9:21
|       `fa' is a rigid type variable bound by
|            the type signature for `baz' at /tmp/fundep.hs:9:16
|     When using functional dependencies to combine
|       Foo a fa, arising from a use of `n' at /tmp/fundep.hs:10:9-26
|       Foo a fb,
|         arising from the type signature for `baz' at /tmp/fundep.hs:10:0-26
|     When generalising the type(s) for `baz'
|
| I find the message about the `Foo a fb' constraint quite confusing.
| Can anyone explain this error message to me?
|
| If I change the type of Id to Id :: Bar a b, then the program compiles.
|
| Regards,
| Reid
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