On Dec 5, 2007 10:38 PM, Ben Franksen
data Command = Skip
class Java block command where block_ :: [command] -> block
compBlock :: [Command] -> block --compBlock = block_ . map compCommand
compCommand :: Command -> command
My guess is that nothing's guaranteeing the calls from block_ and compCommand to be using the same 'command' type as the class head. For example, having
instance Java B C1 instance Java B C2
you can have both
compBlock = (block_ :: [C1] -> B) . map (compCommand :: Command -> C1) compBlock = (block_ :: [C2] -> B) . map (compCommand :: Command -> C2)
Also, there's another problem: from compCommand you can't know the type of block (as it's not appearing in the signature). The modified version below typechecks:
data Command = Skip
class Java block command | command -> block where block_ :: [command] -> block
compBlock :: [Command] -> block compBlock = block_ . map (compCommand :: Command -> command)
compCommand :: Command -> command
(Note that (compCommand :: Command -> command) actually is restricting to a monomorphic type.) So, this seems to me to be a problem with multi-parameter type classes when you prune the types (on compBlock and on compCommand one of the types of the class head is missing). I'm not a wizard on this subject, please anybody correct me if I'm mistaken =). Cheers, -- Felipe.