module Monad:sig..end
type'am ='a option
val return : 'a -> 'a mreturn x puts a value in the Option monad, that is, returns Some x.val bind : 'a m -> ('a -> 'b m) -> 'b mbind m f combines the calculation result m with the function f.
E.g, in the Option monad :
bind (Some 1) (fun x -> if x = 1 then Some 4 else None) returns Some 4.