F# computation expression practice.

option_early_return.fs

type OptionReturnBuilder() =
    member __.Bind(opt, expr) = opt |> function
        | Some x -> Some x
        | None -> expr opt
    member __.Return(x) = x
    member __.ReturnFrom(x) = x
    member __.Delay(f): option<_> = f ()

let optionr = OptionReturnBuilder()

option_early_return_test.fs

let f x = x |> function
    | x when x < 0 ->
        printfn "%d: None" x
        None
    | x ->
        printfn "%d: Some" x
        Some x

// ---
optionr {
    let! r1 = f 2
    let! r2 = f -2
    let! r3 = f 4
    return r3
}
// 2: Some

// ---
optionr {
    let! r1 = f -2
    let! r1 = f -3
    return! f 4
}
// -2: None
// -3: None
// 4: Some

partial_active_early_return.fs

let (|FOO|_|) x y = x |> function
    | x when x < y ->
        printfn "%d < %d: None" x y
        None
    | x ->
        printfn "%d >= %d: Some" x y
        Some x

match 10 with
    | FOO 5 n -> n
    | FOO 15 n -> n
    | _ -> -1
// 15

match 20 with
    | FOO 5 n -> n
    | FOO 15 n -> n
    | _ -> -1
// -1