@node Actions
@section Actions

Actions are composable procedures that perform an operation over a
period of game time. Action objects have two properties: an arbitrary
procedure and a duration in game ticks. Action procedures accept one
argument: a time delta in the range [0, 1]. Use actions in combination
with coroutines for things that are a function of time, such as moving
a sprite across the screen.

@anchor{2d actions make-action}@defun make-action proc duration
Create a new action object that takes DURATION updates to complete.  PROC
is a procedure that takes a value in the range [0, 1] as its only
argument.  An error is thrown if DURATION is 0.

@end defun

@anchor{2d actions action?}@defspec action?
@end defspec

@anchor{2d actions null-action}@defvar null-action
[unbound!]
@end defvar

@anchor{2d actions null-action?}@defvar null-action?
[unbound!]
@end defvar

@anchor{2d actions action-duration}@defspec action-duration
@end defspec

@anchor{2d actions action-proc}@defspec action-proc
@end defspec

@anchor{2d actions perform-action}@defun perform-action action
Execute ACTION.  `perform-action` must be called from within a
coroutine, as it yields back to the agenda after each step.

@end defun

@anchor{2d actions schedule-action}@defun schedule-action action
Schedules a coroutine in the current agenda that will perform ACTION on
the next update.

@end defun

@anchor{2d actions action-cons}@defun action-cons a1 a2
Return an action that performs A1 first, followed by A2.

@end defun

@anchor{2d actions action-list}@defun action-list . actions
Return an action that performs every action in the list ACTIONS.

@end defun

@anchor{2d actions action-parallel}@defun action-parallel . actions
Perform every action in the list ACTIONS in parallel.

@end defun

@anchor{2d actions action-repeat}@defun action-repeat n action
Return an action that will perform ACTION N times.

@end defun

@anchor{2d actions idle}@defun idle duration
Return an action that does nothing.

@end defun

@anchor{2d actions lerp}@defun lerp proc start end duration
Linearly interpolate a number from START to END that takes DURATION
updates.  Apply PROC to the linearly interpolated at each step.

@end defun