[Haskell-cafe] Is (==) commutative?

[Tristan Seligmann]

Forgot to copy the list. Sorry for the duplicates :(
On Jul 24, 2012 1:35 PM, "Tristan Seligmann" <mithrandi at mithrandi.net>
wrote:

> 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
> is).
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.haskell.org/pipermail/haskell-cafe/attachments/20120724/2638aa38/attachment.htm>