Haskell Foldable Wats (Was: Add conspicuously missing Functor instances for tuples)

Wed Feb 24 13:43:08 UTC 2016

I'm not trying to say that a pair is not a container of two things. I'm
saying that that description is insufficiently specific to be useful for
the purposes of the discussion. There are many ways to be a container of
two things, and if we are to have functions whose behavior depends on the
structure of the data they're working on, it's inevitable that those
functions will behave differently for different of those ways of being a
container. If the issue is that "containers" don't always behave the way
one might naively expect containers to behave, then I'm just pointing out
that this isn't the only place that holds, and that in other areas we've
already accepted this.

On Wed, Feb 24, 2016 at 1:38 PM, Augustsson, Lennart <
Lennart.Augustsson at sc.com> wrote:

> Of course a pair is a container of two things (which can have different
> types).
>
> You can come up with some different definition of what it means to be a
> container, so that a pair is no longer a container of two things, but this
> is just obfuscation.
>
>
>
> *From:* Libraries [mailto:libraries-bounces at haskell.org] *On Behalf Of *Nathan
> Bouscal
> *Sent:* 24 February 2016 13:29
> *To:* Haskell Libraries
> *Subject:* Re: Haskell Foldable Wats (Was: Add conspicuously missing
> Functor instances for tuples)
>
>
>
> On Wed, Feb 24, 2016 at 1:04 PM, Henrik Nilsson <
> Henrik.Nilsson at nottingham.ac.uk> wrote:
>
> Hi,
>
> On 02/24/2016 11:08 AM, Fumiaki Kinoshita wrote:
>
> Thinking tuples of as multi-element containers is not recommended. A
> tuple (a, b) is, a pair of one 'a' and one 'b';
>
>
> Which, to me, at least, very much sounds like a container of two
> elements?
>
>
>
> You can use essentially the same argument to say that [a] sounds like a
> container of any number of elements, therefore there shouldn't be anything
> wrong with [1, 'foo']. It's not uncommon in programming for "what a thing
> naively sounds like" to be quite different from "what a thing actually is". *Tuples
> are not lists*.
>
>
>
> I agree that there's room for confusion, but there is room for confusion
> in *a lot* of parts of Haskell, especially for people who bring a lot of
> preconceived notions with them. We should try to make the transition easier
> for them, but to me that looks a lot more like "really good error messages"
> and less like pointedly ignoring the structure of types that might be
> confusing.
>
>
>
>
>
>
> Seriosuly, if, as a result of tuples being instances of Functor and
> Foldable etc., the end result is confusion to the point that
> many no longer understand a tuple simply as a container of a certain
> number of elements, then that's another case in point against
> this whole design. (In particular the Foldable part: while I personally
> don't find the functor instances particularly compelling or useful,
> they seem less likely to seriously bite.)
>
> as Foldable works on
> values pointed by the rightmost type argument, 1 should be the only
> reasonable result of 'length'.
>
>      data TwoThree a b = TwoThree a a b b b
>
> What should 'length (TwoThree "Foo" "Bar" 0 1 2)' be?
>
>
> A static type error, perhaps?
>
> (As indeed it will be unless the appropriate instances are made
> for TwoThree. But I am guessing we should understand TwoThree
> as a tuple here.)
>
> Looking at only
> the expression, 5 might seem to make sense, but is not meaningful
> considering the type.
>
>
> Best,
>
> /Henrik
> --
> Henrik Nilsson
> School of Computer Science
> The University of Nottingham
> nhn at cs.nott.ac.uk
>
>
>
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