Potluck dish - Simple functional reactive programming

[David Thompson <dthompson2@worcester.edu>]

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Hello Guilers,

I didn't have time to put together a proper potluck dish, but I wanted
to find something to share anyway.

Lately I've been playing around with functional reactive programming
(FRP) applied to video games.  This style of programming allows for a
declarative, functional way of describing time-varying values.  Contrast
this method of programming with more traditional hooks and callbacks.
My FRP module can be used on top of hooks to escape callback hell.

And now for a simple example.  This morning I was writing a program
using my game engine, guile-2d, and I wanted to display the number of
times the GC has been run in the game window.  Without FRP I could have
done something like:

(define gc-label-position (vector2 0 40))
(define gc-counter 0)
(define (make-gc-label)
  (let ((text (format #f "GCs: ~d" counter)))
    (make-label font text gc-label-position)))
(define gc-label (make-gc-label))

(add-hook! after-gc-hook
           (lambda ()
             (set! gc-counter (1+ gc-counter))
             (set! gc-label (make-gc-label))))

This code isn't terrible, but wouldn't it be nice to declare that
'gc-label' will always contain a string with the number of GC runs in
it instead?  Enter FRP:

(define gc-label-position (vector2 0 40))
(define gc-counter (make-root-signal 0))
(define gc-label
  (signal-map (lambda (counter)
                (let ((text (format #f "GCs: ~d" counter)))
                  (make-label font text gc-label-position)))
              gc-counter))

(add-hook! after-gc-hook
           (lambda ()
             (signal-set! gc-counter (1+ (signal-ref gc-counter)))))

'gc-counter' and 'gc-label' both become 'signals', or time-varying
values.  Now, when the GC runs, the 'gc-counter' signal is incremented
by 1.  The act of setting 'gc-counter' triggers propagation of the
counter to the 'gc-label' signal which maps the counter to a new label
that prints the current number of GC runs.  Magic!  Note that
'after-gc-hook' is still needed to bootstrap the signal graph, but once
that is out of the way it's signals all the way down.

This example was fairly trivial, but what if the desired chain reaction
was more complicated?  Writing the logic using regular callback
procedures would become a nightmare.  The nightmare that JavaScript
programmers constantly find themselves in.

And that's my potluck dish!  I'm currently working on a new version of
this API that will allow the signal graph to handle the dynamic
environment of the REPL, but it's not ready yet.

Thanks to all of the Guile maintainers and contributors for the great
work these past 3 years!

- David Thompson

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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?
            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))