Proposal: Export Data.Word.Word from Prelude

[John Lato]

On Wed, Aug 13, 2014 at 4:05 PM, Evan Laforge <qdunkan at gmail.com> wrote:

> On Wed, Aug 13, 2014 at 3:06 PM, Ben Millwood <haskell at benmachine.co.uk>
> wrote:
> > `length :: [a] -> Word` (or things of that ilk) would be even more of a
> > mistake, because type inference will spread that `Word` everywhere, and
> `2 -
> > 3 :: Word` is catastrophically wrong.
>
> This is a pretty convincing argument for me.  I have in the past used
> Word for things that seemed like they should always be positive, and
> pretty quickly reverted back to a signed type.  All you need is a
> subtraction anywhere and the chance of underflow is very high.  And,
> as Ben said, it's easy for the "always positive" type to wind up in a
> domain where subtraction is valid due to type inference.  The
> principled thing might be to make that a different type, but in
> practice that can be a lot of hassle so it often doesn't happen (do
> you use NonEmpty everywhere possible? always have separate types for
> absolute and relative measures? sometimes it's not worth the clutter).


I agree it would be wrong to have `length` return a Word unconditionally,
but I don't think it's a mistake in general to have that option available
(i.e. genericLength).  It just means that the programmer is taking on some
responsibilities manually instead of leveraging the type system, but
sometimes that's the only way to get the desired performance.  Besides,
nobody is actually proposing that `length` return a Word, so I don't find
this argument relevant to the proposal.

Relatively weak +1 from me (I'm more enthusiastic about exporting the
entirely of Intx/Wordx types via the prelude).

John L.
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