Forkis-monadic-challenge

type OptionBuilder() =

    member b.Delay(f) = f()

    member b.Return(x) =
        match x with
         | None -> None
         | Some r -> Some(r)

    member b.Bind(p,rest) =
        match p with
            | None -> None
            | Some r -> rest r

let maybe = OptionBuilder()

//Sample dictionaries
let fullNamesDb = Map.ofSeq(seq{
        yield ("Bill Gates", "billg@microsoft.com")
        yield ("Bill Clinton", "bill@hope.ar.us")
        yield ("Michael Jackson", "mj@wonderland.org")
        yield ("No Pref Guy", "guy@nopref.org")
    })

let nickNamesDb = Map.ofSeq(seq{
        yield ("billy", "billg@microsoft.com")
        yield ("slick willy", "bill@hope.ar.us")
        yield ("jacko", "mj@wonderland.org")
    })

let prefsDb = Map.ofSeq(seq{
        yield ("billg@microsoft.com", "HTML")
        yield ("bill@hope.ar.us", "Plain")
        yield ("mj@wonderland.org", "HTML")
    })

let mplus m1 m2 = if m1 <> None then m1 else m2  // TODO: As combine function in Builder?
let (+) = mplus

let lookUp name = maybe{
    let! combined = fullNamesDb.TryFind name + nickNamesDb.TryFind name
    return prefsDb.TryFind combined
}

let billGatesPref = lookUp "Bill Gates" |> printfn "%A"     // Some "HTML"
let billyPref = lookUp "billy" |> printfn "%A"              // Some "HTML"
let billClintonPref = lookUp "Bill Clinton" |> printfn "%A" // Some "Plain"
let steffenPref = lookUp "Steffen" |> printfn "%A"          // None
let noPref = lookUp "No Pref Guy" |> printfn "%A"           // None

System.Console.ReadKey() |> ignore

Lazy alternative: http://gist.github.com/458504

Maybe we should build a lazy maybe for this sample.