CS 330 Lecture 37 – Promises and Futures
Agenda
- what ?s
- what does this do?
- easier concurrency without side effects
- call by value, name, need
- the dangers of the C preprocessor
- delayed/lazy evaluation
- promises in Ruby
- futures in Java
What Does This Do?
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#define PRINT_SMALL(r) \
if (r < 1000000) \
printf("%d\n", r)
int main(int argc, char **argv) {
srand(time(NULL));
for (int i = 0; i < 100000; ++i) {
PRINT_SMALL(rand());
}
return 0;
}
Code
compromise.rb
#!/usr/bin/env ruby
require 'open-uri'
class Promise
def initialize &block
@block = block
@cache = nil
end
def get
if not @cache
@cache = @block.call
end
@cache
end
end
p = Promise.new do
puts "The block is executing!"
open('http://www.twodee.org/tests/slowimage/slowimage.php').read;
end
# puts p.get.length
# puts p.get.length
# puts p.get.length
Thunk.java
package foo;
public interface Thunk<T> {
public T call();
}
Future.java
package foo;
public class Future<T> {
private T cache;
private Thread thread;
public Future(Thunk<T> thunk) {
thread = new Thread(new Runnable() {
@Override
public void run() {
cache = thunk.call();
}
});
thread.start();
}
public T get() {
while (cache == null) {
try {
thread.join();
} catch (InterruptedException e) {
}
}
return cache;
}
}
Main.java
package foo;
import java.awt.Color;
import java.awt.image.BufferedImage;
import java.io.File;
import java.io.IOException;
import javax.imageio.ImageIO;
public class Main {
public static void main(String[] args) throws IOException {
Thunk<BufferedImage> thunk = new Thunk<BufferedImage>() {
@Override
public BufferedImage call() {
try {
BufferedImage in = ImageIO.read(new File("/Users/johnch/Desktop/plasma.bmp"));
Thread.sleep(5000);
BufferedImage out = new BufferedImage(in.getWidth(), in.getHeight(), in.getType());
for (int r = 0; r < out.getHeight(); ++r) {
for (int c = 0; c < out.getWidth(); ++c) {
Color color = new Color(in.getRGB(c, r));
Color swappedColor = new Color(color.getBlue(), color.getGreen(), color.getRed());
out.setRGB(c, r, swappedColor.getRGB());
}
}
return out;
} catch (Exception e) {
throw new RuntimeException(e);
}
}
};
System.out.println("starting future");
Future<BufferedImage> future = new Future<>(thunk);
System.out.println("constructed future");
BufferedImage image = future.get();
ImageIO.write(image, "bmp", new File("/Users/johnch/Desktop/future_plasma.bmp"));
}
}
Haiku
No, that arrow zinged!
He’ll love the first thing he sees
Hit him with a thunk
He’ll love the first thing he sees
Hit him with a thunk