Haskell Foldable Wats (Was: Add conspicuously missing Functor instances for tuples)
lemming at henning-thielemann.de
Wed Feb 17 11:02:23 UTC 2016
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.
> 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.
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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