[Haskell-cafe] Functional dependencies conflict

[Roman Cheplyaka]

Data.ByteString.Lazy.Char8 exports the same lazy bytestring type as
Data.ByteString.Lazy. Only functions and instances differ.

On 05/04/15 15:19, Daniel Trstenjak wrote:
> 
> Hi,
> 
> I'm getting the compile error:
> 
> Gamgine/Image/PNG/Internal/Parser.hs:14:10:
>     Functional dependencies conflict between instance declarations:
>       instance Monad m => Stream LB.ByteString m Word8
>         -- Defined at Gamgine/Image/PNG/Internal/Parser.hs:14:10
>       instance Monad m => Stream LB.ByteString m Char
>         -- Defined in ‘Text.Parsec.Prim’
> 
> 
> 
> The relevant stuff from the parsec 3.1.9 code[1] is:
> 
> {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, FlexibleContexts, UndecidableInstances #-}
> 
> ...
> 
> import qualified Data.ByteString.Lazy.Char8 as CL
> import qualified Data.ByteString.Char8 as C
> 
> ...
> 
> class (Monad m) => Stream s m t | s -> t where
>     uncons :: s -> m (Maybe (t,s))
> 
> instance (Monad m) => Stream CL.ByteString m Char where
>     uncons = return . CL.uncons
> 
> instance (Monad m) => Stream C.ByteString m Char where
>     uncons = return . C.uncons
> 
> 
> 
> And from my code[2] is:
> 
> {-# LANGUAGE BangPatterns, FlexibleInstances, MultiParamTypeClasses, FlexibleContexts #-}
> 
> ...
> 
> import qualified Data.ByteString.Lazy as LB
> 
> ...
> 
> instance (Monad m) => Stream LB.ByteString m Word8 where
>     uncons = return . LB.uncons
> 
> 
> 
> As you can see, the instances are for different ByteString types,
> therefore I don't quite get where GHC sees here any conflicts.
> 
> 
> Greetings,
> Daniel
> 
> 
> [1] https://github.com/aslatter/parsec/blob/master/Text/Parsec/Prim.hs
> [2] https://github.com/dan-t/Gamgine/blob/master/Gamgine/Image/PNG/Internal/Parser.hs
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