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

Wed Feb 24 15:04:41 UTC 2016

Were any of these objections put forward in 1993 when Mark P. Jones
published on constructor classes [1] ? Everything about this follows
directly and uniquely from that design.

[1]: http://web.cecs.pdx.edu/~mpj/pubs/fpca93.pdf

On Wed, Feb 24, 2016 at 7:38 AM, 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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-- 
Chris Allen
Currently working on http://haskellbook.com
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