Turn list of Result into Result of list inside a computation expression?

I have a Result<'T, 'E> list that I would like to turn into a single Result<'T list, 'E> following these rules:

If any Result is an Error then the result should be an Error
If the result is an Error it should be the first Error in the list
If every result is an OK then the result should be an Ok and the list order should be maintained

So I had a go and implemented this as follows:

let all xs = 
  let folder = fun state next -> 
    match (state, next) with 
    | (Result.Ok ys, Result.Ok y) -> ys |> List.append [ y ] |> Result.Ok
    | (Result.Error e, _) -> Result.Error e 
    | (_, Result.Error e) -> Result.Error e 
  Seq.fold folder (Result.Ok []) xs

However, this seems like something that might already have been implemented in the standard library. Has it?

Second, I have a computation expression for Result like this:

type ResultBuilder () = 
  member this.Bind(x, f) = 
    match x with 
    | Result.Ok o -> f o
    | Result.Error e -> Result.Error e
  member this.Return(value) = Result.Ok value
  member this.ReturnFrom(value) = value

let result = new ResultBuilder()

I can use all inside of a result { ... } but is further integration possible? For example by implementing ResultBuilder.For?

Solution

You have a Result<'a, 'e> list and want a Result<'a list, 'e>. That sounds like the sequence function described in https://fsharpforfunandprofit.com/posts/elevated-world-4/ (which has nothing to do with seqs, despite what the name sounds like). A quick check of https://github.com/fsharp/fsharp/blob/master/src/fsharp/FSharp.Core/result.fs suggests that this function hasn't been implemented in the standard FSharp.Core library, so you'll need to implement it yourself.

BTW, if you haven't already read Scott Wlaschin's "Elevated World" series, I don't recommend starting in the middle with the article I linked. Start with this article instead, as it builds the background knowledge you need to understand what the "traverse" and "sequence" functions do. Then you'll know the general pattern for implementing one of those functions.

As for your second question, could you provide a few more specifics? For example, what behavior do you want out of ResultBuilder.For? The normal behavior expected of a for expression would be to take a Result<'a, 'e> list (or seq or array) and run the inner block once for every Result<'a, 'e> that's in the list or seq or array. If you tried to use your all function here, you'd have a type mismatch between Result<'a, 'e> (which is what F# would expect a CE's .For method to produce) and Result<'a list, 'e> which is what your all method is returning. What specifically do you want your ResultBuilder.For method to do?