Re: [Haskell-cafe] Re: Re: type class question

Ben Franksen wrote:

Ryan Ingram wrote:

...

On 12/5/07, Ben Franksen 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 declaration.

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. Jule