Functional programming fans, a challenge:

Here are two problems I've needed to solve lately where I don't see any reasonable way to do them with your average set of basic functional tools and immutable data, and instead I've ended up having to write very procedural loops-and-mutable-arrays type of code.

That code is generally not very satisfying to look at, but I just don't know any other reasonable solution, so, I'd love to hear how you would solve these:

1. Given a list like: [(1, a), (2, a), (3, a), (4, b), (6, b), (9, c)], transform it into:

[([1, 2, 3], a), ([4, 6], b), ([9], c)]

This is straight-forward enough in Haskell:

groupSnd :: Eq t => [(a, t)] -> [([a], t)]
groupSnd = map f . groupBy (\x y -> snd x == snd y)
where f group = (map fst group, snd $ head group)

@cthulahoops That fails the requirement I forgot and put in a later reply, I think?

@WAHa_06x36 I'm not seeing that. Unless you mean that ordering? In which case it's fine.


*Main> groupSnd [(1, 'a'), (2, 'b'), (3, 'a')]

Not quite unchanged, but the type of the function would be weird if it was.

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