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@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) = (:)