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.
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