Thinking tuples of as multi-element containers is not recommended. A tuple
(a, b) is, a pair of one 'a' and one 'b'; 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? Looking at only the
expression, 5 might seem to make sense, but is not meaningful considering
the type.
2016-02-17 20:02 GMT+09:00 Henning Thielemann : On Mon, 18 Jan 2016, Ryan Scott wrote: * The Not-A-Wat in Haskell: I see his examples and draw the opposite conclusions. What he presents are
perfect Wats and they have eventually moved Haskell to the MatLab league
where everything is allowed and the programming system accepts almost
everything the programmer enters. Sure, length (2,3) = 1 product (2,3) = 3
sum (2,3) = 3
or (True,False) = False are all consistent but consistently useless, unintuitive (not only to
novices) and dangerous. There are alternatives: There was no need to
generalize 'Prelude.length' using Foldable. I always opposed to the
argument "put the most general variant to Prelude", because there is no
clear most general variant or there is one like "length :: Length f => f"
and you won't like it. We could reasonably have the Haskell 98 class class Length a where
length :: a -> Int instance Length [a] where
length = List.length instance Length (a,b) where
length _ = 2 This would yield the intuitive
length (2,3) = 2 I do not propose to implement this class, because I never encountered a
situation where I could equally choose between lists and pairs. If at all,
I can see value in a special TupleSize class. However, the Length class
proves that the suggestion that the only reasonable result of 'length
(2,3)' is 1, is plain wrong. How did we get there? There were three steps that made this Wat possible:
1. Foldable.length added
2. instance Foldable ((,) a)
3. export Foldable.length from Prelude. For me
1. was correct
2. was wrong because a programmer should better define a custom type
like "data AdornedSingleton a b = AS a b"
3. Was wrong because there are multiple ways to generalize 'length'.
Without 3. you would have to use explicitly 'length' from Foldable
and this way you would have been warned, that strange things may
happen.
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