Pattern guards vs. case (was, unfortunately :Re: interesting example of laziness/ghc optimisation)

[Johannes Waldmann]

> But if you want to be really weird you can write something
> like (I haven't typed this in):
> 
> f x | (Foo _) == x = 
> f x | (Bar _) == x =

no, you can't, e. g. hugs (Feb 2000) says

Prelude> let f x | Just 4 == x = 0 in f (Just 4)
0
Prelude> let f x | Just _ == x = 0 in f (Just 4)
ERROR: Illegal `_' in expression

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