https://twitter.com/benkyrlach/status/448540208263208961

gistfile1.txt

// broken fmap for option
let fmap f =
    function
    | None -> None
    | Some a -> 
        let b = f a
        if box b = null then None else Some b

// second functor law, see http://www.haskell.org/haskellwiki/Functor
let fmapL p q = fmap (p << q)
let fmapR p q = fmap p << fmap q
let test (p: string -> string) (q: string -> string) (x: string option) = fmapL p q x = fmapR p q x

test (fun _ -> "") (fun _ -> null) (Some "")