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haskellhaskell-lens

Elegant way to apply curried function to indexed traversal

I want to compose a curried function

f :: i -> a -> b

With an indexed traversal

l :: IndexedTraversal' i s a

apply it to an s and get back a [b].

I came up with

s ^.. l . withIndex . to (uncurry f)

But I would like a combinator itoListByOf that would allow me to do this

s & itoListByOf l f

Like

itoListByOf :: IndexedGetting i (Endo [b]) s a -> (i-> a -> b) -> s -> [b]
itoListByOf l f = ifoldrOf l (\i a -> (f i a :)) []

But I want to make sure I'm not missing this combinator hiding in the library with a more general type.

Solution

  • It looks like you have correctly defined the itoListByOf function to achieve your goal. While there are many combinators in the lens library that provide similar functionality, I don't think there is a more general combinator with exactly the type you are looking for.

    The ifoldrOf function you use in your definition of itoListByOf is itself a very general function that can be used to traverse any IndexedFold and accumulate results. The itoListByOf function simply specializes ifoldrOf to the case where the accumulation function is a function that applies the f function to each element of the fold and appends the result to a list.

    In summary, I believe your implementation of itoListByOf is a good one, and there is not a more general combinator with the same type in the lens library.