Re: [Haskell-cafe] checking types with type families

On Sat, Jul 3, 2010 at 3:32 AM, Kevin Quick wrote:

On Wed, 23 Jun 2010 00:14:03 -0700, Simon Peyton-Jones wrote:

...

I'm interested in situations where you think fundeps work and type families don't.  Reason: no one knows how to make fundeps work cleanly with local type constraints (such as GADTs).

Simon,

I have run into a case where fundeps+MPTC seems to allow a more generalized instance declaration than type families.

The fundep+MPTC case:

{-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE FunctionalDependencies #-} {-# LANGUAGE FlexibleInstances, UndecidableInstances #-}

class C a b c | a -> b, a -> c where    op :: a -> b -> c

instance C Bool a a where op _ = id

main = putStrLn $ op True "done"

In this case, I've (arbitrarily) chosen the Bool instance to be a no-op and pass through the types.  Because the dependent types are part of the declaration header I can use type variables for them.

That's really weird. In particular, I can add this line to your code without problems: foo = putStrLn $ if op True True then "done" else "." but GHC complains about this one: bar = putStrLn $ if op True True then op True "done" else "." fd.hs:14:0: Couldn't match expected type `Bool' against inferred type `[Char]' When using functional dependencies to combine C Bool [Char] String, arising from a use of `op' at fd.hs:14:38-51 C Bool Bool Bool, arising from a use of `op' at fd.hs:14:20-31 When generalising the type(s) for `bar' On the other hand, this is fine, but only with a type signature: baz :: a -> a baz = op True I don't think this is an intended feature of functional dependencies. -- Dave Menendez http://www.eyrie.org/~zednenem/