Solution to Scott Wlaschin's "Introducing bind" chapter from "F# for fun and profit"

OptionBuilder.fs

module MyBind

open System

let strToInt (str: string) =
    try
        str |> int |> Some
    with
    | :? FormatException -> None

let strAdd (str: string) i =
    match strToInt str with
    | None -> 0 + i
    | Some str -> str + i
    |> Some

let (>>=) m f = Option.bind f m

type StrToIntMonadBuilder() =
    member this.Bind(m, f) =
        match m with
        | None -> None
        | Some a -> f a

    member this.Return(m) = Some m

let stringWorkflow = StrToIntMonadBuilder()

let stringAddWorkflow x y z =
    stringWorkflow {
        let! a = strToInt x
        let! b = strToInt y
        let! c = strToInt z
        return a + b + c
    }