length is part of the Foldable class:

length = foldl' (\c _ -> c+1) 0

It is probably defined this way s.t. it satisfies:

length = length . toList

Not the actual definition, but you can read toList as:

toList = foldr (:) []

So as soon as one defines foldr for pairs:

foldr f z (_, y) = f y z

We get:

toList (a,b) = [b]

Using the 'natural' property: length = length . toList

We obtain: length (a,b) = length [b] = 1

On Mon, Nov 23, 2020 at 10:22 AM Manuel Schneckenreither <manuel.schnecki@gmail.com> wrote:

>>>>> "HT" == Henning Thielemann <lemming@henning-thielemann.de> writes:

HT> "length (a,b) == 1" is only a consequence of defining instance
HT> Foldable for pairs. It was not the argument to implement that
HT> instance, at all.

What was the argument to use 1, not 2 for instance?
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