Reactive programming

Derive from web front-end programming, Melang also supports reactive programming.

Reactive programming is based on two functions:

• watch

  Watch(var, func, userdata);
  

• unwatch

  unWatch(var);
  

But these functions are bound with interpreter tightly, so they won’t be achieved in library.

Calling watch to trace a variable, a callback function will be called after the variable’s value changed.

e.g.

@handler (newValue, userData)
{
  sys = Import('sys');
  sys.print(newValue);
  sys.print(userData);
  userData = 'world';
}
a = 10;
userData = 'hello';
Watch(a, handler, userData);
a = 11;
a = 12;

The result of this code is:

11
hello
12
hello

As we can see in this example, if watch is called, handler will be called when a’s value changed.

We should note that handler is a function, its parameter supports reference. In this example, newValue and userData are not references. So every modification on userData can not affect on the outer scope variable as we wish.

So let’s fix this problem.

@handler (newValue, &userData)
{
  sys = Import('sys');
  sys.print(newValue);
  sys.print(userData);
  userData = 'world';
}
a = 10;
userData = 'hello';
Watch(a, handler, userData);
a = 11;
a = 12;

Now, the output is:

11
hello
12
world

If we don’t want to trace this variable any more, we can use Unwatch to stop tracing.

Unwatch(a);
a = 13;

handler won’t be called any more, and nothing will be printed on terminal.