Implement Writer monad with custom keyword.

Writer.fs

type Writer<'Result, 'Output> =
    | W of 'Result * seq<'Output>

type WriterBuilder() =
    member this.Zero() = W ((), Seq.empty)
    member this.Yield(x) = W (x, Seq.empty)
    member this.Return(x) = this.Yield(x)
    member this.Bind(W (a, xs), k) =
        let (W (b, ys)) = k a
        W (b, Seq.append xs ys)

    [<CustomOperation("write", MaintainsVariableSpace=true)>]
    member this.Write(W (a, xs), x) =
        W (a, Seq.append xs (Seq.singleton x))

let writer = WriterBuilder()

let logger = writer {
    write "Hello"
    write "World"
    write "David"
}

let runWriter (W (a, xs)) = xs

[<EntryPoint>]
let main argv = 
    printfn "Wrote out: %A" (runWriter logger)
    0