On Tue, Apr 1, 2014 at 3:02 PM, Dan Doel
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