fn apply_command {
	"Takes a turtle state and a command and calculates a new state."
	(state, command) -> match command with {
		(:goto, (x, y)) -> assoc (state, :position, (x, y))
		(:home) -> do state > 
			assoc (_, :position, (0, 0)) >
			assoc (_, :heading, 0)
		(:clear) -> do state >
			assoc (state, :position, (0, 0)) >
			assoc (_, :heading, 0)
		(:right, turns) -> update (state, :heading, add (_, turns))
		(:left, turns) -> update (state, :heading, sub (_, turns))
		(:forward, steps) -> {
			let #{heading, position, ...} = state
			let unit = heading/vector (heading)
			let vect = mult (steps, unit)
			update (state, :position, add (vect, _))
		}
		(:back, steps) -> {
			let #{heading, position, ...} = state
			let unit = heading/vector (heading)
			let vect = mult (steps, unit)
			update (state, :position, sub (_, vect))
		}
		(:penup) -> assoc (state, :pendown?, false)
		(:pendown) -> assoc (state, :pendown?, true)
		(:penwidth, pixels) -> assoc (state, :penwidth, pixels)
		(:pencolor, color) -> assoc (state, :pencolor, color)
		(:setheading, heading) -> assoc (state, :heading, heading)
		(:loadstate, position, heading, visible?, pendown?, penwidth, pencolor) -> #{position, heading, visible?, pendown?, penwidth, pencolor}
		(:show) -> assoc (state, :visible?, true)
		(:hide) -> assoc (state, :visible?, false)
		(:background, _) -> state
	}
}

apply_command (turtle_init, (:penup))