Implementing the Haskell-MaybeMonad in F# - how can we get this lazy?

Question

we are trying to build the Haskell-MaybeMonad sample from http://www.haskell.org/all_about_monads/html/maybemonad.html in F#.

The idea is to search for a mailaddress in

Answer 1

There's always Lazy, which is effectively what you have here but with different syntax:

let mplus m1 (m2 : Lazy<'a option>) =
    match m1 with
    | Some _ as m -> m
    | None -> m2.Force()

let (+) = mplus

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