[Haskell-cafe] Does GHC compare pointers when eval'ing (==)

[Michael Snoyman]

On Wed, Aug 20, 2014 at 12:15 PM, Erik Hesselink <hesselink at gmail.com>
wrote:

> On Wed, Aug 20, 2014 at 10:35 AM, Michael Snoyman <michael at snoyman.com>
> wrote:
> >
> > On Wed, Aug 20, 2014 at 11:28 AM, Johan Tibell <johan.tibell at gmail.com>
> > wrote:
> >>
> >> On Wed, Aug 20, 2014 at 10:23 AM, Erik Hesselink <hesselink at gmail.com>
> >> wrote:
> >>>
> >>> As I understood, the question was if GHC would first compare pointers,
> >>> and only call the Eq instance if the pointers are not equal. I guess
> >>> this would be safe, but I don't think GHC does such a thing.
> >>
> >> I think the reason it isn't done is that it's not always an
> optimization.
> >> We do it manually in e.g. bytestring.
> >
> > There are two cases I can think of where it would also change the
> semantics
> > of the code:
> >
> > 1. An Eq instance that doesn't obey the reflective property (not
> > recommended):
> >
> > data BadEq = BadEq
> > instance Eq BadEq where
> >     BadEq == BadEq = False
>
> I think this is just a buggy instance, and if you do this, nothing is
> guaranteed. Many functions with an Eq constraint will also not work.
> Interestingly, reflexivity is not a law listed in the haddock
> documentation.
>
> Erik
>

I agree that this is terrible practice, and I hope no one is actually doing
this. It's unfortunate that reflexivity isn't listed in the Haddocks.
Perhaps we should start a separate proposal for making reflexivity an
officially required law of the Eq class.

Michael
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