Alexey Rodriguez:
On Fri, Jul 4, 2008 at 5:03 AM, Manuel M T Chakravarty <[EMAIL PROTECTED]
> wrote:
The problem is that blah's type is ambiguous, as f does only occur
as an argument to the type family. If you'd define
class Blah f a where
blah :: a -> f -> T f f a
(and change the rest of the program accordingly) then all will be
fine. See this thread for a more in-depth discussion of the problem:
http://www.haskell.org/pipermail/haskell-cafe/2008-April/041385.html
Yes, I was afraid that this was the case. However, the question
remains on whether my functional dependencies encoding is correct. A
correct encoding would help me understand this typing problem a bit
more. Especially, now that Claus showed that adding an equality
constraint makes this program work!
For this particular program, you could argue that the signature is not
ambiguous as T is unary; hence, in (T f f a) only the (T f) is really
a type family application and so the second occurrence of 'f' should
make the signature unambiguous. This is the reason why Claus'
encoding works and why your FD translation works. I will have a look
at whether I can improve the implementation in GHC to be smarter about
handling such higher kinded TFs.
Thanks for the example program!
Manuel
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