[Haskell-cafe] Re: type class question

[Ryan Ingram]

On 12/5/07, Ben Franksen <ben.franksen at online.de> wrote:
>
> data Command = Skip
>
> class Java block command where
> block_ :: [command] -> block
>
> compBlock :: [Command] -> block
> --compBlock = block_ . map compCommand
>
> compCommand :: Command -> command
>
> This compiles ok. But when I ask ghci for the type of the body of the
> default definition of compBlock I get
>
> *Main> :t block_ . map compCommand
> block_ . map compCommand :: forall block block1 command.
> (Java block command, Java block1 command) =>
> [Command] -> block


Lets look at the type of "compCommand".

compCommand :: Java block command => Command -> command

The block type is not used at all in this declaration, but the block type
can influence which implementation "compCommand" is chosen.  This means that
it's actually almost impossible to ever call this function, as given a type
for  "command" there could be multiple implementations of "Java block
command" for different block types, and there's no way to ever determine
which function to call unless the instance declaration admits any type.

{-# OPTIONS_GHC -fglasgow-exts #-}
module BrokenTypeClass where

class Broken a b where
    broken :: String -> b

{-
instance Broken Bool String where
    broken = id

instance Broken String String where
    broken = reverse
-} -- which instance of "broken" do you choose in "test" below?

instance Broken a String where
    broken = id

test :: String
test = broken "hello"
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.

Still, I agree with Brent here; whenever I have written code like this I
soon realize that I didn't need a typeclass in the first place, and I would
have been better off not using them; they're not like OO classes.

  -- ryan
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