Removal candidates in patterns

[Cale Gibbard]

On 26/01/06, Simon Peyton-Jones <simonpj at microsoft.com> wrote:
> I agree that if (n+k) patterns go, then so should k patterns.  Both are
> overloaded, and that's the root of their complexity.

I'm not so sure about that. I don't use (n+k) patterns at all, but I
do get fairly regular use out of k patterns. (n+k) patterns can seem
like an odd special case in everything but Nat, but k patterns are
often the nicest way to handle base cases, and save you from writing
awkward-looking guards. Sure, they're not usually appropriate for
floating point computations, but for integral and rational types, they
work very well. (even if you can't pattern match against fractions)

One possibility is that k patterns could be generalised to arbitrary
members of Eq, rather than just being used for numbers. We could even
make variables bound in the parameter list available. So for a
simplistic example, one could write:

f :: (Eq a, Num a) => a -> a -> a
f x x = x -- if the two parameters match, give their common value
f _ _ = 0 -- otherwise give 0.

The first 'x' would pattern match as usual, and the second would
compare for equality with the first.

We'd have to work out the exact syntax for them -- essentially, it
would just involve detecting an arbitrary expression which was not a
pattern. There's some context sensitivity there with the option of
referring to previously bound variables though.

I'm not sure how often this would be used, and perhaps it's more
trouble than it's worth, but at least it leaves no further room for
generalisation, which makes the feature seem somewhat natural.

Even without previous-variable-binding, it subsumes all pattern
matching on literals, so it would take some thought to determine if it
really makes things more complicated or not.

> Personally I think ~ patterns are great, and we are now talking about !
> patterns, a kind of dual to ~ patterns.  So at least I think we should
> un-couple the two discussions.

I think so too. Removing ~ patterns seems like a fairly poor idea to
me. Sure, they're not very much explicitly used (though everyone uses
them implicitly in pattern bindings), but when you want them, they can
be fairly important. I think perhaps we just need better coverage of ~
in the tutorials. Now that I think about it, I rather like the idea of
! patterns as well. They make ~ patterns seem more natural by
contrast. Strictness can on occasion be just as important as laziness,
and this notation makes it more convenient to obtain in simple cases.
How to get a similarly pretty notation for more structured strictness
annotations is a bit of a concern. I wonder whether some of the
Control.Parallel.Strategies library should be more strategically
located? :)

 - Cale