Sly: Signals

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4.3.1 Signals

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(use-modules (sly signal))
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Game state is a function of time. The player’s score, the current -stage, an enemy’s hit points, etc. all change in response to events -that happen at discrete points in time. Typically, this means that a -number of callback procedures are registered to respond to events -which mutate the relevant data structures. However, this approach, -while simple and effective, comes at the price of readability, -reproducibility, and expression. Instead of explicitly mutating data -and entering “callback hell”, Sly abstracts and formalizes the -process using a functional reactive programming style. -

In Sly, time-varying values are called “signals”, and they are -defined in a declarative and functional manner. Rather than -describing the process of mutation procedurally, one describes the -relationships between signals instead. The result is a “signal -graph”, a directed acyclic graph of event responses. -

(define-signal position
-  (signal-fold v+ (vector2 320 240)
-               (signal-map (lambda (v) (v* v 4))
-                           (signal-sample 1 key-arrows))))
-

- -

This signal describes a relationship between the arrow keys on the -keyboard and the position of the player. signal-sample is used -to trigger a signal update upon every game tick that provides the -current state of the arrow keys. key-arrows is a 2D vector -that maps to the current state of the arrow keys, allowing for 8 -directional movement. This vector is then scaled 4x to make the -player move faster. Finally, the scaled vector is added to the -previous player position via signal-fold. The player’s -position is at (320, 240) initially. As you can see, there are no -callbacks and explicit mutation needed, and the position seems to -magically change with the passage of time. -

Scheme Procedure: signal? obj: Return #t if obj is a signal. -

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Scheme Procedure: make-signal value: Wrap value in a signal. -

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Scheme Syntax: define-signal name value

Create a top-level signal variable called name. If the variable -already exists and refers to a signal then its outputs will be spliced -into the new signal. If the given value is not a signal then it will -be put into one via make-signal. -

define-signal is particularly useful when working at the REPL. -A top-level signal variable defined by define-signal can be -redefined at runtime, and the signals that depended on the old signal -will continue to work with the new signal. -

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Scheme Procedure: signal-ref signal: Return the value stored within signal. -

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Scheme Procedure: signal-ref-maybe object: Return the value stored within object if object is a -signal. Otherwise, return object. -

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Scheme Syntax: signal-let ((var signal) …) body …: Evaluate body in the context of the local bindings defined by -the two-element lists ((var signal) …). -signal-let works like regular let, except that it -derefences signal before binding to var. -

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Scheme Syntax: signal-let* ((var signal) …) body …: Similar to signal-let, but the variable bindings are performed -sequentially. This means that all initialization expressions are -allowed to use the variables defined to the their left in the binding -list. -

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Scheme Procedure: signal-set! signal-box value: Change the contents of signal to value. This procedure -should almost never be used, except to bootstrap a root node of a -signal graph. -

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Scheme Procedure: hook->signal hook init proc: Create a new signal whose initial value is init and whose future -values are calculated by applying proc to the arguments passed -when hook is run. -

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Scheme Procedure: signal-merge signal1 signal2 . rest: Create a new signal whose value is the that of the most recently -updated signal in signal1, signal2, etc. The initial -value is that of signal1. -

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Scheme Procedure: signal-zip . signals: Create a new signal whose value is a list of the values stored in -signals. -

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Scheme Procedure: signal-map proc signal . rest: Create a new signal that applies proc to the values of -SIGNAL. More than one input signal may be specified, in which -case proc must accept as many arguments as there are input -signals. -

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Scheme Procedure: signal-sample-on value-signal sample-signal: Create a new signal that takes on the value of value-signal -whenever sample-signal receives a new value. -

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Scheme Procedure: signal-negate signal: Create a new signal whose value is the negation of signal by -applying not to each value received. -

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Scheme Procedure: signal-fold proc init signal . rest: Create a new signal that applies proc with the value received -from signal and the past value of itself, starting with -init. Like signal-map, more than one input signal may be -given. -

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Scheme Procedure: signal-filter predicate default signal: Create a new signal that takes on the value received from signal -when it satisfies the procedure predicate. The value of the -signal is default in the case that the predicate is never -satisfied. -

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Scheme Procedure: signal-drop predicate default signal: Create a new signal that takes on the value received from signal -when it does not satisfy the procedure predicate. The -value of the signal is default in the case that the predicate is -never satisfied. -

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Scheme Procedure: signal-drop-repeats signal [equal?]: Create a new signal that drops the value received from signal -when it is equivalent to the current value. By default, equal? -is used for testing equivalence. -

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Scheme Procedure: signal-switch predicate on off: Create a new signal whose value is that of the signal on when -the signal predicate is true, or the value of the signal -off otherwise. -

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Scheme Procedure: signal-constant constant signal: Create a new signal whose value is always constant no matter the -value received from signal. -

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Scheme Procedure: signal-count signal [start] [step]: Create a new signal that increments a counter by step when a -value from signal is received, starting from start. By -default, start is 0 and step is 1. -

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Scheme Procedure: signal-tap proc signal: Create a new signal that applies proc for side-effects when a -value from signal is received. The value of the new signal will -always be the value of signal. This signal is a convenient way -to sneak in a procedure that with a side-effect into a signal graph. -Such a signal might write text to a file, or play a sound. -

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Scheme Procedure: signal-timestamp signal: Create a new signal whose value is a pair, the car of which is the -time that the value of signal was received and the cdr of which -is the received value. -

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Scheme Procedure: signal-time signal: Create a new signal whose value is the time that the value of -signal was received. -

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Scheme Procedure: signal-sample step signal: Create a new signal that takes on the value of signal every -step ticks. -

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Scheme Procedure: signal-every step: Create a new signal that emits step every step ticks. -

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Scheme Procedure: signal-since step signal: Create a new signal that emits the time since signal was updated -ever step ticks. -

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Scheme Procedure: signal-delay delay signal: Create a new signal that delays propagation of signal by -delay ticks.. -

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Scheme Procedure: signal-throttle delay signal: Create a new signal that propagates signal at most once every -delay ticks. -

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Scheme Syntax: signal-generator body …: Create a new signal whose value is the most recently yielded value of -the coroutine defined by body. A special yield syntax is -available within body to specify which values are passed to the -signal. -

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