How to mke this code with match

You could use a match by doing something like this, but I don't know if it's an improvement:

var action: int = int(Input.is_action_just_pressed("ui_right")) + 2 * int(Input.is_action_just_pressed("ui_left")) + 4 * int(Input.is_action_just_pressed("ui_up")) + 8 * int(Input.is_action_just_pressed("ui_down"))

match action:
	0: pass
	1: spawn_bullet(Vector2.RIGHT)
	2: spawn_bullet(Vector2.LEFT)
	4: spawn_bullet(Vector2.UP)
	8: spawn_bullet(Vector2.DOWN)
	(other values, if desired, for combinations of inputs)

The int() calls convert the bools into ints that are 0 or 1. The factors are powers of 2 (2, 4, 8) to avoid ambiguous results.

Here's another way to do it.

func _input(event):
	if event.is_pressed():
		var v = Input.get_vector('ui_left', 'ui_right', 'ui_up', 'ui_down')
		match v:
			Vector2(1, 0):
				print('right')
			Vector2(-1, 0):
				print('left')
			Vector2(0, -1):
				print('up')
			Vector2(0, 1):
				print('down')

It has the advantage that you could just use the vector "v" as the argument to spawn_bullet.

func _input(event):
	if event.is_pressed():
		var v = Input.get_vector('ui_left', 'ui_right', 'ui_up', 'ui_down')
		spawn_bullet(v)

However, if two buttons are pressed at the same time, you'll get a vector like "(0.7, 0.7)", so you'll have to check that only one coordinate is set [ (1, 0), etc ] in your spawn_bullet() function. Or just use:

func _input(event):
	if event.is_pressed():
		var v = Input.get_vector('ui_left', 'ui_right', 'ui_up', 'ui_down')
		if v and v == v.floor():
			spawn_bullet(v)

Edit: Checking for "v and ..." ensures that unrelated mouse-clicks won't fire a bullet.