On Dec 3, 2007, at 4:02 AM, Simon Peyton-Jones wrote:
GHC's new intermediate language, System FC, is specifically designed to do this. Currently we're in transition: equality constraints are starting to work, but fundeps are implemented as they always were. I hope we can eventually switch over to implementing fundeps using equality constraints, and then the above program will work.
Meanwhile, in the HEAD you can write conv :: (a~b) => a -> b conv = id
Which, IHMO, is a much clearer way to say it!
Is it really a good idea to permit a type signature to include equality constraints among unifiable types? Does the above type signature mean something different from a ->a? Does the type signature: foo :: (a~Bar b) => a -> Bar b mean something different from: foo :: Bar b -> Bar b ? I know that System FC is designed to let us write stuff like: foo :: (Bar a ~ Baz b) => Bar a -> Baz b Which is of course what we need for relating type functions. But I'm wondering if there's a subtlety of using an equality constraint vs just substitution that I've missed---and if not why there are so many ways of writing the same type, many of them arguably unreadable! Hoping this will give me a bit of insight into SystemFC, -Jan-Willem Maessen
You may also like to try the paper that Martin and I and others wrote about fundeps: http://research.microsoft.com/%7Esimonpj/papers/fd-chr
Simon _______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users