Re: Enum and bounded instances for (,) and Either

Note that it is probably just missing some `error` calls in the right place.

On Wed, May 12, 2021, 13:27 chessai <[email protected]> wrote:

> I am +1 on these instances existing, but I just discovered via
> quickcheck-classes that the given code does not uphold succ-pred or
> pred-succ identity:
>
> ```
> Enum: Succ Pred Identity *** Failed! Falsified (after 2 tests):
>   Description: succ (pred x) = x
>   a = (0,1)
>   succ (pred x) = (-1,1)
> Enum: Pred Succ Identity *** Failed! Falsified (after 1 test):
>   Description: pred (succ x) = x
>   a = (0,0)
>   pred (succ x) = (1,0)
> *** Failed! Falsified (after 2 tests)..
>        Enum: Succ Pred Identity *** Failed! Falsified (after 2 tests):
>   Description: succ (pred x) = x
>   a = Left 0
>   succ (pred x) = Right (-9223372036854775808)
> Enum: Pred Succ Identity *** Failed! Falsified (after 1 test):
>   Description: pred (succ x) = x
>   a = Left 0
>   pred (succ x) = Right (-9223372036854775808)
> ```
>
> Perhaps this is indicative of something wrong with the proposed instance?
>
> Thanks
>
> On Wed, May 12, 2021 at 11:01 AM Henning Thielemann
> <[email protected]> wrote:
> >
> >
> > On Wed, 12 May 2021, Sandy Maguire wrote:
> >
> > > Found myself puzzled the other day when I wanted an (Enum a, Enum b) =>
> > > Enum (a, b) instance, and was distraught that it didn't exist.
> > >
> > > The following is a reasonable implementation:
> >
> > This is certainly an instance that fulfills some laws, but do we really
> > want it or is it more likely an accident if someone writes, say, succ
> > (a,b)?
> >
> > We already allow "max (a,b) (c,d)", which can be pretty counterintuitive.
> > I think the Ord instance for pairs is currently mostly used for Set and
> > Map, but a separate class for this purpose would have been better.
> > _______________________________________________
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> > [email protected]
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>

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