[Haskell-cafe] Re: The Proliferation of List-Like Types

John Goerzen jgoerzen at complete.org
Wed Feb 20 11:28:16 EST 2008

On 2008-02-20, John Goerzen <jgoerzen at complete.org> wrote:
> I notice that Data.Foldable does some similar things but does not use
> multi-parameter type classes.  I seem to recall that I attempted to do
> this in the same manner, but got tripped up somewhere.  I can't
> remember now exactly what the problem was, but I can go back and look
> if nobody knows off-hand.

I went back and looked.

The problem is that ByteString doesn't work as a member of Foldable,
or of ListLike without it being MPTC.  Trying to do so yields:

    Kind mis-match
    Expected kind `* -> *', but `BS.ByteString' has kind `*'
    In the instance declaration for `F.Foldable BS.ByteString'

Is there any way around that, other than MPTC?

-- John

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