[Haskell-cafe] Re: Re: type class question
jules at jellybean.co.uk
Thu Dec 6 11:57:59 EST 2007
Ben Franksen wrote:
> Ryan Ingram wrote:
>> On 12/5/07, Ben Franksen <ben.franksen at online.de> wrote:
>> You would have to use functional dependencies or associated types to
>> eliminate this error. Alternatively, you can add a dummy argument of type
>> "block" and pass "undefined :: BlockType" in to help choose the instance
> Sounds reasonable, and in fact that was what I tried first. However
> data Command = Skip
> class Java block command | command -> block where
> block_ :: [command] -> block
> compBlock :: [Command] -> block
> compBlock = block_ . map compCommand
> compCommand :: Command -> command
> still gives
> Could not deduce (Java block command1)
> from the context (Java block command)
> arising from use of `block_' at Bla.hs:7:14-19
> Possible fix:
> add (Java block command1)
> to the class or instance method `compBlock'
> In the first argument of `(.)', namely `block_'
> In the expression: block_ . (map compCommand)
> In the definition of `compBlock':
> compBlock = block_ . (map compCommand)
> which is /exactly/ the same error as I get w/o the fundep.
Yes, because command determines block but block doesn't determine command.
So in a usage of 'compBlock' it has no way of deciding which 'command'
to use, although it can choose the block from the return type.
You could have command -> block, block -> command, if that is indeed true.
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