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

David Roundy wrote:

On Wed, Feb 20, 2008 at 11:18:51PM +0100, Ben Franksen wrote:

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John Goerzen wrote:

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On 2008-02-20, Jules Bean wrote:

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Not directly, no.

The point about Foldable, Functor, and Monad, is that they enforce the connection between container and contents. If the contents is of type "a", the container is of type "f a" for a fixed type constructor 'f'. This works for [], Seq, and so on, but fails for ByteString.

Right. In a pure abstract sense, we humans know there is a relationship between container and contents: a ByteString always contains a Word8 (or a Char8 if we choose the alternative implementation).

But that is not expressed in the type of ByteString.

Hm, making a function out of a constant is easy on the value level, just use (const x) instead of (x). So, what about wrapping ByteString in a GADT, like this

data ByteString' a where BS' :: Word8 -> ByteString' Word8

? I probably overlooked something important here...

The problem is that while this would change the kind of ByteString to the same as the kind expected by Functor, you still couldn't define a proper Functor instance, since only ByteString' Word8 can ever actually be created. i.e. how could you implement

fmapBS :: (a -> b) -> ByteString' a -> ByteString' b

Oh yes, indeed. I knew there would be a catch, somewhere... Cheers Ben