[Haskell-beginners] lost a typeclass maybe?

[Silent Leaf]

well, i sent once more my message too early by mistake.
when i say invent IO a b, i don't actually mean an IO type, i meant just,
any type you can't manually unbox via pattern matching or otherwise.

2017-06-29 20:36 GMT+02:00 Silent Leaf <silent.leaf0 at gmail.com>:

> hi,
>
> i keep trying to find something that feels terribly obvious but i can't
> make any link.
>
> say i have a function of the following type:
>
> foo :: (a, b) -> ([a], [b]) -> ([a], [b])
> or perhaps more generally:
> foo :: SomeClass f => f a b -> f [a] [b] -> f [a] [b]
>
> is SomeClass supposed to be BiFunctor or something else?
> clearly, what i want to do is to combine the elements of the first pair
> into the elements of the second, preferrably without pattern matching, that
> is, merely in function of (:).
>
> i think the problem with bifunctor is that it seems to only allow the
> application of both arguments in a separate fashion. but here the first
> argument is in one block, that is (a,b).
> i know, ofc we could do something like:
> foo pair pairList = bimap (fst pair :) (snd pair:) pairList
> or maybe use curry or whatever. but i'd like my pair to not need to be
> unboxed!
>
> is there not a way to not have to manually call fst and snd? are both of
> these functions typeclass methods by any chance? then we could write a
> generalized function that could work for any f = (:) or any kind of
> pair-like thingy. mind you i'm not sure to which extent it would keep the
> opacity of the type constructor (,).
>
> especially, it's a bit like unboxing the Maybe type constructor: you can
> do it manually by pattern matching, but when you have the exact same issue
> but with IO, it's not possible anymore to unbox the underlying type
> equally, i bet one could invent IO a b, in a way that you could not just
> get a and b, but you could somehow implement
> opaqueBimap :: (i -> k i) -> f a b -> f (k a) (k b) -> f (k a) (k b)
> with here of course f = (,), k = [] or List, and (i -> k i) = (:)
>
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