On Tue, Jan 10, 2006 at 08:47:32PM +0300, Bulat Ziganshin wrote:
those rank-2 types wil make me mad :)
encodeHelper :: (MRef m r, Binary m a, BitStream m (StringBuffer m r)) => a -> m String encodeHelper x = do h <- newStringBuffer "" stringBufferDefaultCloseFunc put_ h x getStringBuffer h encode x = runST (encodeHelper x)
i can't force `encode` to compile, either with or without any type signatures. actually, signature for `encodeHelper` was developed by Hugs, but now i don't know how to proceed further
just now tried to force Hugs tell me its type. mission impossible :)
DataAltBinaryClass> :t (runST (encodeHelper 1)) ERROR - Cannot justify constraints in application *** Expression : encodeHelper 1 *** Type : ST c [Char] *** Given context : () *** Constraints : (Num b, Binary (ST c) b)
It's telling you that your expression is ill-typed, and that encodeHelper 1 :: (Num b, Binary (ST s) b) => ST s String runST is declared with an argument type in which s is unconstrained, so this is rejected.
Binary class in my library includes monad in its header. May be it's the cause of problems?
class Binary m a where -- | Convert value to sequence of bits and write it to the stream put_ :: (BitStream m h) => h -> a -> m ()
Yes, that's the problem.