10.1 Simple generator definition

A generator is a function that may have side effects. Alternatively you can think of it as a procedure that returns a value.

  proc prompt (x:string) {
    write$ stdout, x; 
    fflush stdout;
  }
  
  gen getnum():int = {
    prompt "Input number : ";
    var x = readln stdin;
    var y = int (strip x);
    return y;
  }

You may ask, why not use generators all the time, instead of functions?

The answer is two-fold. The first is readability. When you write a function, the reader knows an application shouldn't have side-effects. This makes it possible to write cleaner code, and it specifies semantics that can be verified by examining the function definition.

The second answer is performance. Generator applications are lifted out of expressions to ensure they're only evaluated once, and in a determinate order. Lifting purely functional code like this is not necessary and interferes with optimisation.