Questions regarding Bounded

[David Feuer]

The Bounded class is broken because it combines two distinct concepts:
BoundedAbove and BoundedBelow. If Maybe is to have instances similar
to what it has now, they should surely look like

instance BoundedBelow (Maybe a) where
  minBound = Nothing
instance BoundedAbove a => BoundedAbove (Maybe a) where
  maxBound = Just maxBound

On Wed, May 6, 2020 at 1:52 PM Simon Jakobi via Libraries
<libraries at haskell.org> wrote:
>
> Dear Henning,
>
> I've re-read your email now and thought about it a bit more.
>
> > Unfortunately, Ord is dual use:
> >
> > 1. Comparison for magnitude as suggested by the mathematical symbols (<)
> > and (>).
> >
> > 2. Some arbitrary but total order for use in Set and Map.
> >
> > E.g. Set (Complex a) makes total sense, but 1:+0 > 0:+1 is unexpected. Ord
> > instances on pairs is sensible for Set and Map, but (max (1,0) (0,1) ==
> > (1,0)) is strange.
>
> Personally, I find nothing strange about (max (1,0) (0,1) == (1,0)),
> but I started wondering how other languages handle these examples. I
> tried Python:
>
> $ python3
> Python 3.5.2 (default, Apr 16 2020, 17:47:17)
> [GCC 5.4.0 20160609] on linux
> Type "help", "copyright", "credits" or "license" for more information.
> >>> max((1,0),(0,1))
> (1, 0)
> >>> complex(1,0) > complex(0,1)
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> TypeError: unorderable types: complex() > complex()
>
> I also find the Ord Maybe instance quite intuitive: "Nothing is less
> than something (Just)"
>
> It is also consistent with the interpretation of Maybe as a list of at
> most one element:
>
> $ ghci
> > [] < [()]
> True
>
> So in my view the Bounded Maybe instance still seems like a good idea.
>
> Cheers,
> Simon
> _______________________________________________
> Libraries mailing list
> Libraries at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries