On Wed, Feb 24, 2016 at 1:04 PM, Henrik Nilsson < Henrik.Nilsson@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@cs.nott.ac.uk
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