More declarative style of declaring computed values

[Rich Harris]

Possibly a bit blue-sky, but this syntax would be neat:

state.compute( 'width', '<% x2 %> - <% x1 %>' );

// as opposed to
state.compute( 'width', {
  triggers: [ 'x1', 'x2' ],
  fn: function ( x1, x2 ) {
    return x2 - x1;
  }
});

Probably best to use different delimiters, to avoid confusion with templating languages.

At any rate, something like this would be very compact and fairly straightforward to a) identify dependencies, and b) compile to a function.

[Rich Harris]

Done. Example usage:

model = new Statesman({
  foo: 1
});

model.compute( 'bar', '2 * ${foo}' );
model.get( 'bar' ); // 2

model.observe( 'bar', function ( newValue, oldValue ) {
  console.log( 'bar changed from %s to %s', oldValue, newValue );
}, { init: false });

model.set( 'foo', 2 ); // logs 'bar changed from 2 to 4'