Validation<'a, 'b> list -> Validation<'a list, 'b list>

I have a scenario the user creates n objects. If all objects are valid I want to get all the objects, if any one of them is invalid, I want to get all the errors, something like: Validation<'a, 'b> list -> Validation<'a list, 'b list>.

I’m posting this here as it might be a common scenario.

Based on @gusty’s comment on a previous issue, I ended up with:

let f (vs : Validation<_, _> list): Validation<_ list, _> = traverse (first result) vs

(it needed a little more help with the types.)

Which gives me the behaviour I would expect:

let private f (vs : Validation<_, _> list): Validation<_ list, _> = traverse (first result) vs

[<Fact>]
let ``My test`` () =
    let x1 : Validation<string, _> list = [ Success   1; Success   2; Success   3 ]
    let e1 : Validation<string list, _> = Success [   1;           2;           3 ]
    let x2                              = [ Success   1; Failure "a"; Success   3 ]
    let e2 : Validation<_, int list>    = Failure [              "a"              ]
    let x3 : Validation<_, int> list    = [ Failure "a"; Failure "b"; Failure "c" ]
    let e3 : Validation<_, int list>    = Failure [ "a";         "b";         "c" ]

    test <@ f x1 = e1 @>
    test <@ f x2 = e2 @>
    test <@ f x3 = e3 @>

Thank you for you help @gusty, @DunetsNM. One slight follow-up, is there an appropriate name for this function?

Originally posted here: https://github.com/fsprojects/FSharpPlus/issues/51#issuecomment-586202997

[gusty]

One more tip:

If you don’t write a generic result F# is able to infer the whole function.

traverse (first List.singleton) vs so you don’t need to pre-create the function.

We might add Bitraversable and Biapplicative to the library.

Bitraversable is almost there as many extensions have bisquence and bitraverse already.

[NickDarvey]

Perfect, thank you @gusty!