either.ml

open Pervasiveext
open Listext

type ('a,'b) t = Left of 'a | Right of 'b

module Monad = Monad.M2.Make (struct

	type ('a, 'b) m = ('b, 'a) t

	let bind value f =
		match value with
			| Left value -> Left value
			| Right value -> f value

	let return value = Right value

end)

let left x = Left x
let right x = Right x
let is_left = function
	| Left _ -> true
	| Right _ -> false
let is_right x = not ++ is_left $ x
let to_option = function
	| Right x -> Some x
	| Left _ -> None

let cat_right l = List.unbox_list ++ List.map to_option $ l

let join = function
	| Right (Right x) -> Right x
	| Left x -> Left (Left x)
	| Right (Left x) -> Left (Right x)

let swap = function
	| Right x -> Left x
	| Left x -> Right x

let of_exception f =
	try Right (f ())
	with e -> Left e