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

Wed Feb 24 14:20:46 UTC 2016

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<mailto: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<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<mailto: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<mailto:nhn at cs.nott.ac.uk>

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