[Haskell-cafe] Is (==) commutative?
mithrandi at mithrandi.net
Tue Jul 24 13:37:28 CEST 2012
Forgot to copy the list. Sorry for the duplicates :(
On Jul 24, 2012 1:35 PM, "Tristan Seligmann" <mithrandi at mithrandi.net>
> On Jul 24, 2012 12:32 PM, "Twan van Laarhoven" <twanvl at gmail.com> wrote:
> > On 2012-07-24 10:10, Christian Sternagel wrote:
> >> Dear all,
> >> with respect to formal verification of Haskell code I was wondering
> whether (==)
> >> of the Eq class is intended to be commutative (for many classes such
> >> requirements are informally stated in their description, since Eq does
> not have
> >> such a statement, I'm asking here). Or are there any known cases where
> >> commutativity of (==) is violated (due to strictness issues)?
> > Strictness plays no role for Eq, since to test for equality both sides
> will have to be fully evaluated.
> I don't think this is necessarily true. For example:
> (==) :: (Eq a) => Maybe a -> Maybe a -> Bool
> Nothing == Nothing = True
> Nothing == Just _ = False
> Just _ == Nothing = False
> Just x == Just y = x == y
> This particular example is still commutative, however (at least I think it
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