[Haskell-cafe] Having a connection between kind * and kind * -> *

Ivan Lazar Miljenovic ivan.miljenovic at gmail.com
Thu Aug 19 18:44:59 EDT 2010

Jason Dagit <dagit at codersbase.com> writes:

> On Thu, Aug 19, 2010 at 6:26 AM, Ivan Lazar Miljenovic <
> ivan.miljenovic at gmail.com> wrote:
>> I'm trying to update container-classes to duplicate the pre-existing
>> classes defined in the Prelude (Functor, etc.) and am trying to get my
>> approach on how to have functions/classes that work on types of kind *
>> (e.g. Bytestring) as well as kind * -> * (e.g. lists), as my previous
>> approach didn't work.
> This is exactly why the iteratee library provides a newtype wrapper around
> ByteString that adds a phantom type.
> If I recall correctly, it looks like this (and probably adds some derived
> instances):
> newtype WrappedByteString a = WBS ByteString
> Haskell isn't friendly when it comes to kind polymorphism and I've never
> really seen a "solution" for it that I like.
> And the above is actually something that may provide a nice addition to
> ByteString.  You could track the encoding in the phantom type,
> potentially.

Except then the value that it contains is expected to be what that
phantom type is, rather than a Word8.  As such, the Foldable instance
will be wrong, etc.

Ivan Lazar Miljenovic
Ivan.Miljenovic at gmail.com

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