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

Ross Paterson ross at soi.city.ac.uk
Wed Feb 20 10:44:37 EST 2008

On Wed, Feb 20, 2008 at 08:39:13AM -0600, John Goerzen wrote:
> I am concerned that the same thing is happening in Haskell.  We now
> have three common list-like types: the regular list, strict
> ByteString, and lazy ByteString.
> This has created some annoying situations.  For instance, a ByteString
> is great for reading data fast, but Parsec doesn't work on
> ByteStrings.  I am glad that someone wrote a Parsec equivalent that
> does[1], which answers a real need.  But this means that all the
> combinators in the hsemail library that implement standard RFC
> conventions won't be usable in my ByteString code, for instance.
> [...]
>   http://software.complete.org/listlike/static/doc/ListLike/Data-ListLike.html

As Henning pointed out, multiple parameter type classes are problematic
for core libraries at present.

An alternative might be explicit dictionaries.  For example, a partial
solution would be to provide coinductive views, i.e. for all these types
to provide functions of a type like

	full -> Maybe (item, full)

(Data.Map, Data.Set and Data.Sequence would each have two such functions),
and to have a library of generalized functions taking such functions as
parameters, like

	splitAt :: (full -> Maybe (item, full)) ->
			Int -> full -> ([item], full)

Parsing libraries could include a similar parameter within their monad.

That only covers the elimination side, of course.

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