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

Wed Feb 24 14:34:09 UTC 2016

A pair is a certainly a container of two things, but an (a, b) is a
container of only one b. The length function is not named sufficiently
descriptively to explain it's behavior here, but that doesn't make the
behavior nonsensical.

On Wed, Feb 24, 2016, 9:20 AM Henrik Nilsson <
Henrik.Nilsson at nottingham.ac.uk> wrote:

> I am totally with Lennart here: something that used to be conceptually
> very simple, clear, and, not the least, easy to teach, has become complex
> and muddled for little if any good reason at all. What is that if not
> obfuscation?
>
> And I am not just drawing on my own experience here, but also from that of
> many colleagues with years and years of teaching experience.
>
> /Henrik
>
>
> Henrik Nilsson
> School of Computer Science
> The University of Nottingham
> nhn at cs.nott.ac.uk
>
>
>
> -------- Original message --------
> From: Nathan Bouscal
> Date:2016/02/24 13:43 (GMT+00:00)
> To: Haskell Libraries
> Subject: Re: Haskell Foldable Wats (Was: Add conspicuously missing Functor
> instances for tuples)
>
> 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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