Hello, On Tue, May 18, 2010 at 04:47:50PM -0700, Dan Weston wrote:
Unifying those two types by hand, I get:
P (A t -> B a) ~> P (B a)
Maybe the problem is that type families (and associated types, their class cousins) are not injective: P x ~ P y does not imply that x ~ y. Maybe you need a data type (with appropriate wrapping and unwrapping) to ensure injectivity. Cf:
http://www.haskell.org/haskellwiki/GHC/Type_families#Injectivity.2C_type_inf... http://www.mail-archive.com/haskell-cafe@haskell.org/msg63359.html
That's probably it, thanks. I think I'll just stay with functional dependencies this time, a data type with wrapping and unwrapping can't possibly make the code more readable :-). Regards, -- Tomáš Janoušek, a.k.a. Liskni_si, http://work.lisk.in/