Bang Patterns

[wren romano]

On Tue, Apr 1, 2014 at 3:02 PM, Dan Doel <dan.doel at gmail.com> wrote:
> Specifically, consider:
>
>     case Nothing of
>       !(~(Just x)) -> 5
>       Nothing -> 12
>
> Now, the way I'd expect this to work, and how I think the spec says it
> works, is that my Nothing is evaluated, and then the irrefutable ~(Just x)
> matches Nothing, giving a result of 5. In fact, GHC warns about overlapping
> patterns for this.

It's sensible to give an overlap warning --that is, assuming we don't
want overlap to be an error-- since the irrefutable pattern matches
everything, and adding bangs doesn't change what values are matched
(it only changes whether we diverge or not).

However, I have no idea how top-level bang in case-expressions is
supposed to be interpreted. If anything, it should be ignored since we
must already force the scrutinee to WHNF before matching *any* of the
clauses of a case-expression. However, I thought bangs were restricted
to (1) immediately before variables, and (2) for top-level use in
let/where clauses...

In any case, following the standard desugaring of the specs:

    case Nothing of !(~(Just x)) -> 5 ; Nothing -> 12

=== { next to last box of
<http://www.haskell.org/ghc/docs/latest/html/users_guide/bang-patterns.html>,
the proposed clause (t) for section 3.17.3, figure 4 }

    Nothing `seq` case Nothing of ~(Just x) -> 5 ; Nothing -> 12

=== { Haskell Report, section 3.17.3, figure 3, clause (d) }

    Nothing `seq` (\x -> 5) (case Nothing of Just x -> x)

Which most definitely does not evaluate to 12. Either the specs are
wrong (dubious) or the implementation is. File a bug report.

-- 
Live well,
~wren