package travesty

then_m x y sequentially composes the actions x and y as with >>=, but, rather than using the returned value of x, it instead just returns y.

x >> y is then_m x y.

compose_m f g is the Kleisli composition of f and g.

x >=> y is compose_m x y.

map_when_m ?otherwise condition ~f a is f a when condition is true, and otherwise a (by default, return) otherwise.

when_m ?otherwise condition ~f is f () when condition is true, and otherwise () (by default, return) otherwise.

map_unless_m ?otherwise condition ~f a is f a when condition is false, and otherwise a (by default, return) otherwise.

unless_m ?otherwise condition ~f is f () when condition is false, and otherwise () (by default, return) otherwise.

tee_m val ~f executes f val for its monadic action, then returns val.

Example (using an extended Or_error):

let fail_if_negative x =
  On_error.when_m (Int.is_negative x) ~f:(fun () ->
      Or_error.error_string "value is negative!" )
in
Or_error.(42 |> tee_m ~f:fail_if_negative >>| fun x -> x * x)

(* Ok (1764) *)

tee val ~f behaves as tee_m, but takes a non-monadic f.

Example (using an extended Or_error):

let print_if_negative x =
  if Int.negative x then Stdio.print_string "value is negative!"
in
Or_error.(
  try_get_value () >>= tee ~f:print_if_negative >>= try_use_value () )