fs5-applicatives.fsx

// First, define a 'zip' function
module Result =
    let zip x1 x2 = 
        match x1,x2 with
        | Ok x1res, Ok x2res -> Ok (x1res, x2res)
        | Error e, _ -> Error e
        | _, Error e -> Error e

// Next, define a builder with 'MergeSources' and 'BindReturn'
type ResultBuilder() = 
    member _.MergeSources(t1: Result<'T,'U>, t2: Result<'T1,'U>) = Result.zip t1 t2
    member _.BindReturn(x: Result<'T,'U>, f) = Result.map f x

let result = ResultBuilder()

let run r1 r2 r3 =        
    // And here is our applicative!
    let res1: Result<int, string> =
        result { 
            let! a = r1 
            and! b = r2
            and! c = r3
            return a + b - c 
        }

    match res1 with
    | Ok x -> printfn "%s is: %d" (nameof res1) x
    | Error e -> printfn "%s is: %s" (nameof res1) e

let printApplicatives () =
    let r1 = Ok 2
    let r2 = Ok 3 // Error "fail!"
    let r3 = Ok 4

    run r1 r2 r3
    run r1 (Error "failure!") r3