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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:
-;;
-;; Simple functional reactive programming API.
-;;
-;;; Code:
-
-(define-module (2d signals)
-  #:use-module (srfi srfi-1)
-  #:use-module (srfi srfi-9)
-  #:use-module (srfi srfi-26)
-  #:export (<signal>
-            signal?
-            root-signal?
-            make-signal
-            make-root-signal
-            signal-ref
-            signal-ref-maybe
-            signal-receiver
-            signal-inputs
-            signal-outputs
-            signal-connect!
-            signal-disconnect!
-            signal-clear!
-            signal-set!
-            signal-merge
-            signal-combine
-            signal-do
-            signal-map
-            signal-fold
-            signal-filter
-            signal-reject
-            signal-constant
-            signal-count))
-
-;;;
-;;; Signals
-;;;
-
-;; Signals are time-varying values. For example, a signal could
-;; represent the mouse position at the current point in time. The
-;; signals API provides an abstraction over regular event-based
-;; programming. State mutation is hidden away and a functional,
-;; declarative interface is exposed.
-(define-record-type <signal>
-  (%make-signal value proc inputs outputs)
-  signal?
-  (value signal-ref %signal-set!)
-  (proc signal-proc)
-  (inputs signal-inputs set-signal-inputs!)
-  (outputs signal-outputs set-signal-outputs!))
-
-(define (make-signal init proc inputs)
-  "Create a new signal with initial value INIT, a procedure PROC
-to transform incoming signal values and one or more signals to connect
-to."
-  (let ((signal (%make-signal init proc inputs '())))
-    (for-each (cut signal-connect! <> signal) inputs)
-    signal))
-
-(define (make-root-signal init)
-  "Create a new root level signal with initial value INIT."
-  (%make-signal init #f '() '()))
-
-(define (root-signal? signal)
-  "Returns true if a signal has no receiver procedure or false
-otherwise."
-  (not (signal-proc signal)))
-
-(define (signal-ref-maybe object)
-  "Retrieves the signal value from OBJECT if it is a signal and or
-simply returns OBJECT otherwise."
-  (if (signal? object)
-      (signal-ref object)
-      object))
-
-(define (signal-connect! signal-in signal-out)
-  "Attach SIGNAL-OUT to SIGNAL-IN. When the value of SIGNAL-IN
-changes, the value will be propagated to SIGNAL-OUT."
-  (if (root-signal? signal-out)
-      (error 'root-signal-error
-             "Cannot connect to a root signal"
-             signal-out)
-      (let ((outputs (signal-outputs signal-in)))
-        (set-signal-outputs! signal-in  (cons signal-out outputs)))))
-
-(define (signal-disconnect! signal-in signal-out)
-  "Detach SIGNAL-OUT from SIGNAL-IN."
-  (let ((inputs (signal-inputs signal-out))
-        (outputs (signal-outputs signal-in)))
-    (set-signal-inputs!  signal-out (delete signal-in  inputs  eq?))
-    (set-signal-outputs! signal-in  (delete signal-out outputs eq?))
-    ;; Disconnect all inputs when the input signal has no remaining
-    ;; outputs in order to prevent memory leaks and unnecessary
-    ;; computation.
-    (when (null? (signal-outputs signal-in))
-      (signal-clear-inputs! signal-in))))
-
-(define (signal-clear-outputs! signal)
-  "Disconnect all output signals from SIGNAL."
-  (for-each (cut signal-disconnect! signal <>)
-            (signal-outputs signal))
-  (set-signal-outputs! signal '()))
-
-(define (signal-clear-inputs! signal)
-  "Disconnect all inputs signals from SIGNAL."
-  (for-each (cut signal-disconnect! <> signal)
-            (signal-inputs signal))
-  (set-signal-inputs! signal '()))
-
-(define (signal-update! signal from)
-  "Re-evaluate the signal procedure for the signal SIGNAL as ordered
-by the signal FROM."
-  ((signal-proc signal) signal from))
-
-(define (signal-propagate! signal)
-  "Notify all output signals about the current value of SIGNAL."
-  (for-each (cut signal-update! <> signal)
-            (signal-outputs signal)))
-
-(define (signal-set! signal value)
-  "Change the current value of SIGNAL to VALUE and propagate SIGNAL to
-all connected signals."
-  (%signal-set! signal value)
-  (signal-propagate! signal))
-
-(define (splice-signals! old new)
-  "Remove the inputs and outputs from the signal OLD, connect the
-outputs to the signal NEW, and return NEW."
-  (when (signal? old)
-    (let ((outputs (signal-outputs old)))
-      (signal-clear-inputs! old)
-      (signal-clear-outputs! old)
-      (for-each (cut signal-connect! new <>) outputs))
-    (signal-propagate! new))
-  new)
-
-(define-syntax define-signal
-  (lambda (x)
-    (syntax-case x ()
-      ;; Splice in new signal if a signal with this name already
-      ;; exists.
-      ((_ name (signal ...))
-       (defined? (syntax->datum #'name))
-       #'(define name (splice-signals! name (signal ...))))
-      ((_ name (signal ...))
-       #'(define name (signal ...))))))
-
-;;;
-;;; Higher Order Signals
-;;;
-
-(define (signal-merge signal1 signal2 . rest)
-  "Create a new signal whose value is the that of the most recently
-changed signal in SIGNALs.  The initial value is that of the first
-signal in SIGNALS."
-  (let ((signals (append (list signal1 signal2) rest)))
-    (make-signal (signal-ref (car signals))
-                 (lambda (self from)
-                   (signal-set! self (signal-ref from)))
-                 signals)))
-
-(define (signal-combine . signals)
-  "Create a new signal whose value is a list of the values stored in
-the given signals."
-  (define (update signals)
-    (map signal-ref signals))
-
-  (make-signal (update signals)
-               (lambda (self from)
-                 (signal-set! self (update (signal-inputs self))))
-               signals))
-
-(define (signal-map proc signal . signals)
-  "Create a new signal that applies PROC to the values stored in one
-or more SIGNALS."
-  (define (update signals)
-    (apply proc (map signal-ref signals)))
-
-  (let ((signals (cons signal signals)))
-    (make-signal (update signals)
-                 (lambda (self from)
-                   (signal-set! self (update (signal-inputs self))))
-                 signals)))
-
-(define (signal-fold proc init signal)
-  "Create a new signal that applies PROC to the values stored in
-SIGNAL. PROC is applied with the current value of SIGNAL and the
-previously computed value, or INIT for the first call."
-  (make-signal init
-               (let ((previous init))
-                 (lambda (self from)
-                   (let ((value (proc (signal-ref from) previous)))
-                     (set! previous value)
-                     (signal-set! self value))))
-               (list signal)))
-
-(define (signal-filter predicate default signal)
-  "Create a new signal that keeps an incoming value from SIGNAL when
-it satifies the procedure PREDICATE.  The value of the signal is
-DEFAULT when the predicate is never satisfied."
-  (make-signal (if (predicate (signal-ref signal))
-                   (signal-ref signal)
-                   default)
-               (lambda (self signal)
-                 (when (predicate (signal-ref signal))
-                   (signal-set! self (signal-ref signal))))
-               (list signal)))
-
-(define (signal-reject predicate default signal)
-  "Create a new signal that does not keep an incoming value from
-SIGNAL when it satisfies the procedure PREDICATE.  The value of the
-signal is DEFAULT when the predicate is never satisfied."
-  (signal-filter (lambda (x) (not (predicate x))) default signal))
-
-(define (signal-constant constant signal)
-  "Create a new signal whose value is always CONSTANT regardless of
-what the value received from SIGNAL."
-  (signal-map (lambda (value) constant) signal))
-
-(define (signal-count signal)
-  "Create a new signal that increments a counter every time a new
-value from SIGNAL is received."
-  (signal-fold + 0 (signal-constant 1 signal)))
-
-(define (signal-do proc signal)
-  "Create a new signal that applies PROC when a new values is received
-from SIGNAL.  The value of the new signal will always be the value of
-SIGNAL.  This signal is a convenient way to sneak a procedure that has
-a side-effect into a signal chain."
-  (signal-map (lambda (x) (proc x) x) signal))