Constraints on definition of `length` should be strengthened

[Nathan Bouscal]

It *is* actually a useful instance and it is used in practice. It's not
that better Haskell wouldn't have an biased pair type with these instances,
it's that it would *also* have an unbiased one with the instances that such
a type could support. The issue seems to be that people don't like the
biased type having special syntax that wrongly gives an unknowing reader
the impression that the type is unbiased. This is a reasonable position,
but getting rid of the tuple instances isn't a reasonable way to act on
that position: 1) they're going to be defined anyway, but also 2) it's not
helpful to just pretend the type is unbiased when it isn't. It would be
coherent to argue for the removal of the special tuple syntax (though
coherent doesn't mean reasonable; this would break everything), but it's
not coherent to argue for crippling tuples so we can pretend they're
something they aren't.


On Thu, Apr 6, 2017 at 11:17 AM <amindfv at gmail.com> wrote:

>
>
> > El 6 abr 2017, a las 08:52, Ivan Lazar Miljenovic <
> ivan.miljenovic at gmail.com> escribió:
> >
> >> On 6 April 2017 at 23:56,  <amindfv at gmail.com> wrote:
> >> I don't totally understand this viewpoint. It sounds like what you're
> saying
> >> is it's unfortunate that tuples (and everything else) are biased in
> Haskell,
> >> but because they are we're obligated to make all the legal instances we
> can
> >> for them.
> >>
> >> E.g. if I define a datatype "data Foo x y z", I'm powerless and sort of
> >> obligated to define "instance Functor (Foo x y)" if there's a legal one,
> >> regardless of if that's what I want Foo to mean.
> >
> > Is Foo going to be widely used or only an internal data type to your own
> code?
> >
>
> For the sake of comparison, let's say it's going to be widely used. It's
> also a structure which isn't (conceptually) biased.
>
> If we're starting from a place of feeling that it's a shame Haskell is
> unable to have unbiased structures, then probably an "if we knew then what
> we know now" version of Haskell would have them. So then why knowingly
> create instances we think a "better Haskell" wouldn't have?
>
> Is the argument that if it's public-facing, someone's going to define the
> instance and so we should do it canonically? If so, this feels to me a
> little like "you can't fire me, I quit!" - doing what we don't want before
> someone else has a chance to.
>
> Tom
>
>
> >>
> >> Tom
> >>
> >>
> >> El 3 abr 2017, a las 15:29, Nathan Bouscal <nbouscal at gmail.com>
> escribió:
> >>
> >> I expect most people probably agree that it'd be nice to have tuples be
> an
> >> unbiased cartesian product, but the actual fact of the matter is that
> tuples
> >> as they exist in Haskell are biased. We can't just ignore that and
> pretend
> >> they're unbiased. It definitely sucks that the answer people would
> naively
> >> give to "what is a tuple in Haskell" is not the correct answer, but
> we're
> >> stuck in that situation. The question is how to make the best of it.
> >>
> >> On Mon, Apr 3, 2017 at 12:56 PM, Henning Thielemann
> >> <lemming at henning-thielemann.de> wrote:
> >>>
> >>>
> >>>> On Mon, 3 Apr 2017, Sven Panne wrote:
> >>>>
> >>>> Of course such an interpretation is possible, but let's remember
> >>>> Abelson's famous quote:
> >>>>
> >>>>   "Programs must be written for people to read, and only incidentally
> >>>> for machines to execute."
> >>>>
> >>>> When you show somebody a pair and ask "What is this?", how many
> people do
> >>>> you *seriously* expect to say "Oh, yeah, I've seen that: It's a value
> on the
> >>>> right decorated by another one on the left!" compared to people
> telling you
> >>>> something about e.g. cartesian products (which are totally symmetric
> with no
> >>>> bias to the right or left)? The point is: Using a pair for a decorated
> >>>> one-element container is completely miscommunicating your intent,
> even if
> >>>> you find a sensible mathematical interpretation for it.
> >>>
> >>>
> >>> That's what I am saying all the time.
> >>> _______________________________________________
> >>> Libraries mailing list
> >>> Libraries at haskell.org
> >>> http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries
> >>
> >> _______________________________________________
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> >>
> >>
> >> _______________________________________________
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> >
> >
> >
> > --
> > Ivan Lazar Miljenovic
> > Ivan.Miljenovic at gmail.com
> > http://IvanMiljenovic.wordpress.com
>
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