[Haskell-cafe] Functional dependencies conflict
Roman Cheplyaka
roma at ro-che.info
Sun Apr 5 12:25:01 UTC 2015
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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