Re: [Haskell-cafe] Functional dependencies conflict

To be precise, the sets of instances differ. Eg. the Char8 module exports the IsString instance, which normal Data.ByteString.Lazy doesn't. On 05/04/15 15:25, Ivan Lazar Miljenovic wrote:

On 5 April 2015 at 22:25, Roman Cheplyaka wrote:

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

Well, *instances* can't differ...

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On 05/04/15 15:19, Daniel Trstenjak wrote:

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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 #-}

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import qualified Data.ByteString.Lazy.Char8 as CL import qualified Data.ByteString.Char8 as C

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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 #-}

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import qualified Data.ByteString.Lazy as LB

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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/Pars... _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe

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