ResultBuilder.fs

namespace FSharp.Control.Results

[<AutoOpen>]
module Builder = 
    type ResultBuilder () =
        member this.Bind(x, f) = 
            match x with
            | Ok c -> f c
            | Error c -> Error c

        member this.Delay f = f ()
        member this.Return x : Result<'a, 'c> = Ok x
        member this.Zero () = Ok ()

        member this.For(cs : Result<'a, 'b> seq, f : 'a -> Result<'a, 'b>) : Result<'a list, 'b list> =
            let candidates = cs |> Seq.map (function | Ok c -> f c | Error c -> Error c) |> Seq.toList
            match candidates |> List.filter (function | Error _ -> true | Ok _ -> false) with
            | [] -> candidates |> List.map (function Ok c -> c | Error _ -> failwith "impossible") |> Ok
            | errors -> errors |> List.map (function Error c -> c | Ok _ -> failwith "impossible") |> Error

        member this.For(cs : 'a seq, f : 'a -> Result<'b, 'c>) : Result<'b list, 'c list> = 
            let candidates = cs |> Seq.map f |> Seq.toList
            match candidates |> List.filter (function | Error _ -> true | Ok _ -> false) with
            | [] -> candidates |> List.map (function Ok c -> c | Error _ -> failwith "impossible") |> Ok
            | errors -> errors |> List.map (function Error c -> c | Ok _ -> failwith "impossible") |> Error

        member this.Yield x = Ok x
        member this.YieldFrom (c : Result<'a, 'b> seq) : Result<'a list, 'b list> = 
            let c = c |> Seq.toList
            match c |> List.filter (function | Error _ -> true | Ok _ -> false) with
            | [] -> c |> List.map (function Ok c -> c | Error _ -> failwith "impossible") |> Ok
            | errors -> errors |> List.map (function Error c -> c | Ok _ -> failwith "impossible") |> Error
            

    let result = ResultBuilder ()