FSharp_apply.fs

let apply optionF optionStr =
  match optionF,optionStr with
  | Some f, Some str -> Some (f str)
  | _ -> None

let repeat count (str:string) = String.replicate count str
let mul x y = x * y
let optionMul = Option.map mul
let retn x = Some x // official naming for upgrading a value

let (<!>) f x = Option.map f x
let (<*>) fo xo = apply fo xo
let lift1 f x = Option.map f x // same as map!!
let lift2 f x y     = f <!> x <*> y
let lift3 f x y z   = f <!> x <*> y <*> z
let lift4 f x y z w = f <!> x <*> y <*> z <*> w

let resultMul = apply (Option.map mul (Some 7)) (Some 3) // ugly
let resultMul2 = mul <!> retn 7 <*> retn 3 // better
let resultMul3 = retn mul <*> retn 7 <*> retn 3