ReasonML monad proof

monad.re

/* monad */
type box('t) = Foo('t);
let return = (x) => Foo(x);
let bind = (b, f) => switch b {
 | Foo(v) => f(v)
};

/* proof helpers */
let a = 1;
let m = Foo(a);
let f = (x) => Foo(x + 1);
let g = (x) => Foo(x * 2);

/* proof */
let left = bind(return(a), f) == f(a);
let right = bind(m, return) == m;
let assoc = (m |> bind(_, f) |> bind(_, g)) == (m |> bind(_, x => bind(f(x), g))); 

/* box is a monad */
assert(left==right==assoc==true);