Add consecutivePairs and pairwise

[Janiczek]

consecutivePairs : List a -> List ( a, a )
pairwise : (a -> a -> b) -> List a -> List b

consecutivePairs [ 1, 2, 3, 4 ]
--> [ ( 1, 2 ), ( 2, 3 ), ( 3, 4 ) ]

pairwise (-) [100, 10, 1]
--> [ 90, 9 ]

[gampleman]

Not really sure that consecutivePairs is really worth the space is takes given that it's trivially pairwise Tuple.pair. I think there was some good wisdom in the design of the core libraries when map2 was preferred over Haskell's zip...

[Chadtech]

Do we have some use cases associated with these? @Janiczek did you run into anything recently that lead you to using functions like this

I like pairwise and I think I have been in the situation where I wanted that function.

But for consecutivePairs, my hunch is that no one wants (a, a) as the final step in whatever they are doing. Getting consecutive elements paired up is always an intermediate step in something bigger, so it isn't clear why it needs to be tupled up to begin with. Does that make sense?

[Janiczek]

Makes sense! We can probably go without consecutivePairs. I was mostly copying the existing API of uniquePairs : List a -> List (a,a).

My hunch is that I run into the need for pairwise a few times a year. This PR was prompted by one of these but I can't remember what it was for :joy: