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+;;; guile-2d
+;;; Copyright (C) 2013 David Thompson <dthompson2@worcester.edu>
+;;;
+;;; Guile-2d is free software: you can redistribute it and/or modify it
+;;; under the terms of the GNU Lesser General Public License as
+;;; published by the Free Software Foundation, either version 3 of the
+;;; License, or (at your option) any later version.
+;;;
+;;; Guile-2d is distributed in the hope that it will be useful, but
+;;; WITHOUT ANY WARRANTY; without even the implied warranty of
+;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+;;; Lesser General Public License for more details.
+;;;
+;;; You should have received a copy of the GNU Lesser General Public
+;;; License along with this program.  If not, see
+;;; <http://www.gnu.org/licenses/>.
+
+;;; Commentary:
+;;
+;; Actions are composable procedures that perform an operation over a
+;; period of game time.
+;;
+;;; Code:
+
+(define-module (2d actions)
+  #:use-module (2d coroutine)
+  #:export (lerp))
+
+(define (lerp proc start end duration)
+  "Linearly interpolate a number from start to end. Calls proc with the
+interpolated value every frame."
+  (let ((delta (- end start)))
+    (define (lerp-iter time)
+      (if (< time duration)
+          (begin
+            (proc (+ start (* delta (/ time duration))))
+            (wait)
+            (lerp-iter (1+ time)))
+          (proc end)))
+    (lerp-iter 0)))