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
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