[Haskell-cafe] Ord methods too strict?

[Clinton Mead]

How would you define Ord on Bools? Like so?

False <= _ = True
_ <= y = y

Note that even with this definition, "undefined <= True = undefined" so
strictness is now not symmetric.

Also, as bool are implemented as just an int of some sort (presumably 0 and
1), the strict definition allows the following implementation in effect:

x <= y = fromEnum x <= fromEnum y

Note "fromEnum" here is zero cost, and there is no branch. Checking the
left hand branch for false first would require a branch that would possibly
hit performance. This probably isn't worth slowing down this function just
so it's lazy in it's right argument (but as a gotcha now, still strict in
it's left argument).

On Wed, Jan 2, 2019 at 10:50 PM V.Liepelt <V.Liepelt at kent.ac.uk> wrote:

> > The implementation of the Ord instance for Bool is derived
>
> So my argument would be—doesn’t this mean that we need to do cleverer
> deriving or at least have a hand-written instance?
>
> > As for the justification, perhaps it's too much of a special case for
> only
> > one value of an enumeration to compare to undefined without crashing
>
> This is not just about crashing. (I’m using `undefined` as a way of making
> strictness explicit.) `False >= veryExpensiveComputation` should return
> `True` immediately without any unnecessary computation.
>
> Also it doesn’t seem like a special case: this makes sense for any
> partially ordered Type with a top and/or bottom element.
>
> > perhaps it inhibits optimisation opportunities.
>
> That doesn’t seem very likely to me, I would rather think the contrary
> (see above): doing unnecessary work can hardly make a program run faster.
>
> V
>
> > On 2 Jan 2019, at 11:37, Tom Ellis <
> tom-lists-haskell-cafe-2017 at jaguarpaw.co.uk> wrote:
> >
> > On Wed, Jan 02, 2019 at 09:47:15AM +0000, V.Liepelt wrote:
> >> I am surprised to find that `False <= undefined = undefined`.
> >>
> >> What justifies (<=) to be strict in both arguments?
> >
> > The implementation of the Ord instance for Bool is derived, as you can
> see
> > here:
> >
> >
> https://www.stackage.org/haddock/lts-12.1/ghc-prim-0.5.2.0/src/GHC-Classes.html#Ord
> >
> > As for the justification, perhaps it's too much of a special case for
> only
> > one value of an enumeration to compare to undefined without crashing, and
> > perhaps it inhibits optimisation opportunities.
> >
> > Tom
> > _______________________________________________
> > Haskell-Cafe mailing list
> > To (un)subscribe, modify options or view archives go to:
> > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe
> > Only members subscribed via the mailman list are allowed to post.
>
> _______________________________________________
> Haskell-Cafe mailing list
> To (un)subscribe, modify options or view archives go to:
> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe
> Only members subscribed via the mailman list are allowed to post.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/haskell-cafe/attachments/20190102/ed924ee1/attachment-0001.html>