ApplicativeBuilder.fsx

type 'a Foo = Foo of 'a

module Foo =

    let ofValue (a : 'a) : 'a Foo = Foo a

    let apply (a : 'a Foo) (f : ('a -> 'b) Foo) : 'b Foo =
        let (Foo f) = f
        let (Foo a) = a
        Foo (f a)

    let map (f : 'a -> 'b) (a : 'a Foo) : 'b Foo =
        ofValue f |> apply a

    let map2 (f : 'a -> 'b -> 'c) (a : 'a Foo) (b : 'b Foo) : 'c Foo =
        map f a |> apply b


type FooBuilder () =

    member __.For (foo, f) =
        Foo.map f foo

    member __.Yield (a : 'a) : 'a =
        a

    member __.Return (a : 'a) : 'a =
        a

    [<CustomOperation("apply", IsLikeZip = true)>]
    member __.Zip ((), foo : 'a Foo, f : unit -> 'a -> 'b) : 'b Foo =
        Foo.map (f ()) foo

    [<CustomOperation("also", IsLikeZip = true)>]
    member __.Zip2 (foo1 : 'a Foo, foo2 : 'b Foo, f : 'a -> 'b -> 'c) : 'c Foo =
        Foo.map2 f foo1 foo2


let foo = FooBuilder ()

let myFoo = Foo.ofValue 1234
let myFoo2 = Foo.ofValue false
let myFoo3 = Foo.ofValue "Hello"

let test : string Foo =
    foo {
        apply i in myFoo
        also b in myFoo2
        also s in myFoo3
        return if b then sprintf "%i" i else s
    }

Yes, that's right - we've been using this as a workaround for the lack of support for applicative computation expression builders in F#. However, we won't need to in the future! Take a look at dotnet/fsharp#7756 - applicative computation expression builders have now been implemented and should be available as part of F# 5.