Climbing example of 2d platformer coroutine use case

climb.lua

function Player:climb()
	self.climbing = true
	self:set_facing(self.climbable_wall_facing)

	-- Initially zero out all y-vel to "latch" onto the wall.
	if not self.climb_sliding then
		self.vy = 0
	end
	self.vx = 0
	self.climbing_needs_reset = true
	joypad_rumble(1, 0.15, 0.15, 0.25)
	yield()

	-- Handles all cases to exit the climbing state.
	local was_climbing_on = collide_at(self, self.climbable_wall_facing * climb_pixel_grace, 0)
	local function should_stop_climbing()
		if self.input_jump:just_pressed() then
			self:wall_jump()
			return true
		end

		-- Exit if there's no more wall.
		local dx = self.climbable_wall_facing * climb_pixel_grace
		local climb_on = collide_at(self, dx, 0)
		if not climb_on then
			-- Start a little hop to just above the ledge for convenience if you've climbed up something.
			self.vy = self.vy + 15
			self.vx = self.vx + 75*sign(dx)
			self.climbing = false
			self.climbing_fall_grace_timer = fall_grace_time
			assert(was_climbing_on)
			self:climb_onto_ledge(was_climbing_on)
			return true
		else
			was_climbing_on = climb_on
		end

		-- Exit if let go of climbing trigger.
		if not self.input_climb:down() then return true end
	end

	-- This happens if you slide down a wall and touch the ground.
	::climb_again::

	-- Climbing state.
	while true do
		yield()

		if should_stop_climbing() then
			goto stop_climbing
		end

		if not self.on_ground and on_interval(0.4) then
			joypad_rumble(1, 0.15, 0.15, 0.1)
		end

		if self.on_ground then
			self.climbing_elapsed = 0
		end

		-- Climbing.
		local sign_x, sign_y = self.input_move:sign()
		if sign_y >= 0 or sign_y <= 0 then
			local down_boost = 1
			if sign_y < 0 then down_boost = 1.5 end
			self.vy = approach(self.vy, climb_speed * sign_y * down_boost, climb_accel * DELTA_TIME)
		else
			self.vy = approach(self.vy, 0, climb_accel * DELTA_TIME)
		end

		-- Grace period to hang on the wall before sliding.
		self.climbing_elapsed = self.climbing_elapsed + DELTA_TIME
		if self.climbing_elapsed > climb_time then break end
	end

	self.climb_sliding = true
	self.vy = min(self.vy, 0) -- Prevent weird upwards sliding.

	-- Sliding down the wall state.
	while true do
		if should_stop_climbing() then
			goto stop_climbing
		end

		if self.on_ground then
			goto climb_again
		end

		-- Slide down wall.
		self.vy = approach(self.vy, -wall_slide_speed, wall_slide_accel * DELTA_TIME)

		if on_interval(0.2) then
			joypad_rumble(1, 0.25, 0.25, 0.2)
		end

		::continue::
		yield()
	end

	::stop_climbing::
	self.climbing_fall_grace_timer = fall_grace_time
	self.climbing = false
end