Erroneous warning

[moran at galconn.com]

Ian Lynagh wrote:

> If Q.lhs is
> 
> > module Q where
> 
> > foo :: Int -> Int
> > foo (i+1) = i
> > foo 0 = -10
> 
> then
> 
> % ghc -c -o Q.o Q.lhs
> 
> Q.lhs:5: Warning: Pattern match(es) are overlapped in the definition of
> function `foo':
>             foo 0 = ...

This is acceptbale behaviour, since the first equation can match with i = -1. 
The type is Int, not Nat.

Note that swapping the order of the equations gets rid of the warning. 
Morally, it shouldn't (since, in a perfect world, equation order shouldn't
matter), but it does in GHC because of the way pattern matching is compiled.
Maybe there should there be a warning in both cases, or not at all (the latter
being Hugs' behaviour).

Cheers,

Andy

-- 
Andy Moran                                              Ph.  (503) 526 3472
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