F# maybe monad

gistfile1.fs

type Maybe() =
    member __.Bind(value, rest) =
        if value = null then null else rest value

    member __.Return(value) =
        value

let maybe = new Maybe()

let substring (str: string) (from: int) =
    maybe {
        let! x = str
        let! substr = x.Substring from
        return "[" + substr + "]"
    }

// test it with not null string
> substring "1234" 2;;
> val it : string = "[34]"

// and now with null string
> substring null 2;;
> val it : string = null