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

Nathan Bouscal nbouscal at gmail.com
Wed Feb 24 13:29:13 UTC 2016

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

> 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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