A generator is a special function like construction which is permitted to have an internal effect. The prototypical generator is the rand() function.

An internal effect is one which changes some state on each call and which my modify the result returned by a subsequent call, the state, may not be observed except in the return value of the function.

A generator is defined by using the binder “gen” instead of “fun”.

Note that the type of a generator is the same as function type.


Felix has a special role for generators named iterator: they’re used to traverse a data structure such as a list, array, tree or other type, visiting certain values in some order, and returning each such value in sequence on each call. In C++ terminology an iterator is an input iterator.

Yielding Generators

Most generators maintain internal mutable state in local variables. In order to preserve modification between applications, as well as the current location of the program counter, they execute a yield statement to return a value. Subsequent calls to the generator continue after the yield statement.

To use a yielding generator, a closure must be assigned to a variable to hold the state. For example:

gen iterator(xs:T^N) () : opt[T] =
  if xs.len > 0uz do
    for var j in 0uz upto xs.len - 1uz do
      yield Some (xs,j).unsafe_get;
  return None[T];

proc check() {
  var a = 1,2,3,4;
  var it = iterator a;
  var v = it ();
  match v with
  | Some x =>
    println$ x;
    goto next;
  | None => ;

defines and uses an array iterator.