# MIT License, Copyright (c) 2023 Marvin Borner
# a box can store a single item very efficiently
# similar structure to std/Option

:import std/Logic .
:import std/Combinator .

# a empty box
empty true ⧗ (Box a)

# returns true if the box is empty
empty? [0 true [false]] ⧗ (Box a) → Boolean

:test (empty? empty) (true)

# builds a box out of a value
box [[[0 2]]] ⧗ a → (Box a)

# returns true if the box is set
set? [0 false [true]] ⧗ (Box a) → Boolean

:test (set? (box [0])) (true)
:test (set? empty) (false)

# sets the value of a empty box, ignores argument if already set
store! [[empty? 1 (box 0) 1]] ⧗ (Box a) → a → (Box a)

:test (store! (box [[0]]) [[1]]) (box [[0]])
:test (store! empty [[1]]) (box [[1]])

# sets/overrides the value of a box
set! [[(box 0)]] ⧗ (Box a) → a → (Box a)

:test (set! (box [[0]]) [[1]]) (box [[1]])
:test (set! empty [[1]]) (box [[1]])

# extracts value from a box or returns first argument if none
get [[0 1 i]] ⧗ a → (Box b) → c

:test (get [[0]] (box [[1]])) ([[1]])
:test (get [[0]] empty) ([[0]])