Bang Patterns

[Dan Doel]

Filed. Bug #8952.


On Wed, Apr 2, 2014 at 3:41 PM, wren romano <winterkoninkje at gmail.com>wrote:

> 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
>
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