In many programming languages today, there is a special instruction usually called yield which allows a procedure to return a value, suspending its execution state in such a way it can resume where it left off.

gen producer () : int = {
 var i = 0;
 yield i;
 goto again;

This is a simple example of what is called a yielding generator in Felix. A generator is a function like construction which may return a different value on each invocation, depending on some mutable state. The prototypical generator is rand which returns a random number.

The yield instruction seen above is similar to return, however the producer above does not loose its current location or local variables when it provides a value, instead it suspends so that it may be resumed and continue on where it left off.

You can use the generator above like this:

var next = producer;
var current = next();
while current < 10 do
  println$ current;
  current = next();

The key here is that the variable next is used to store the suspended state of the producer as a closure, which is resumed by each call.


This particular kind of construction is also known as an iterator in Python. In C++ it is called an input iterator although the use is slightly different, and the definition is via a class object.

In general, iterators can be constructed in any object oriented language using an object with mutable state and a get method which simultaneously returns a fresh value and also updates the state so the next value can be calculated.

However, OO based iterators are weak compared to yielding generators because the yield instruction automatically saves the current location in the generator.


I want you to see that an iterator is roughly a function turned inside out and therefore deserves the technical name cofunction. The natural output of an iterator is a stream, but there is more: it a temporal stream.

Functional programming models have a very serious weakness which is that they attempt to be atemporal. Advocates laud the fact that an FPL is primarily declarative. Data structure are indeed spatial, but there is more to programming that space, and more to programming than data and functions.

Cofunctions provide a space time transform. You can take a list and produce a stream. A purely spatial, linear data structure has been converted actively into a temporaly linear sequence of codata.

Where data lives at addresses, some of which may be adjacent, and others linked indirectly by pointers, codata has temporal coordinates, marked by a clock.

The world of space and time provide the coordinate system of a program, and the flow of control explains how an algorithm looks at one location at one time, but progresses the construction of new data by simultaneous spatial and temporal sequencing. You move down the list, physically, and you do so in time.

A cofunction, therefore, is an iterator over an abstract data structure, which produces a stream of values. The stream has no natural end and this is a fundamental property which is entirely misunderstood.

Many people think streams are infinite lists but this is in fact completely and utterly wrong. It is hard to comprehend how such fundamentals are so badly misunderstood.

Contrary to popular belief, inductive data types like lists are infinite, whereas streams are finite! It is not hard to understand when one realises that computing is like science not mathematics, it requires a concept of observation.

Suppose I give you a list and ask you how long it is:

 fun len (x: list[int]) => match x with
  | Empty => 0
  | Cons (head, tail) => 1 + len tail

You would use that algorithm and say that

[B] -> [C] -> [D] -> *

was length 3. But, you would be wrong! You see, I didn’t show you the whole list:

[A] -> [B] -> [C] -> [D] -> *

I only showed you the tail starting at B. Now you realise your answer should have been at least 3. That is the correct answer because the algorithm for the length counts in time by following pointers to the end, but it is a singly linked list so you cannot go backwards!

Let me say that again another way: irrespective of what exists in space, or not, the only thing that matters is what you can observe by an effective procedure which is also called an algorithm.

So we can observe only a lower bound of the size of a list because we only ever see the tail of the list. You can never tell, or, measure if the first element you see is the head of the list.

So we must emphasise again the relativistic nature of computing: it is all about what you can observe, not about what is. So inductive data types, like lists, all have the same structural property that observations are finite, but are necessarily only lower bounds.

Because the list could be longer than any calculated bound we have to assume it is, because no observation can contradict that assumption, in other words, lists are infinite!

Now it is vital to understand that a functional observation of some property, is intrinsically bounded. Suppose you write a function and make a mistake and write an infinite loop. The function never returns, so it is not, in fact able to be used to make an observation: it is not an algorithm.

So I am now going to blow your minds, by claiming that due to duality, streams are finite, and, in fact, when you make an observation on a stream, you are producing an upper bound.

Suppose you have a an iterator producing a stream of all the integers. You might think, this is an infinite stream but you could not be more wrong! If it were infinite, an program using the stream to perform a calculation would never terminate!

Let us see how to measure the length of a stream:

gen observer () : int = {
  var next = producer;
  var current = next();
  println$ current; // observation
  yield current;
  goto again;

Now here is the critical thing: to actually use a stream and calculate some value, we have to impose a bound on our observations:

gen sum () : int = {
  var it = observer;
  for i in 0 ..< 10 perform
     x += it();
  return x;

Now if we call sum, how many prints do you see? Did you say 10? So you think, the stream is at least 10 long but, you have it arse about.

Suppose you only saw two:

gen producer () : int = {
 var i = 0;
 yield i;
 yield i + 1;
 yield i + 2;
 again: goto again;

If you see this producer it produces 3 values, then it goes into an infinite loop. So you can write code that reads the first 4 values from it, and that code will never return. It does not make any observation. If you reduce the number to 3,2,1, or 0, you get an observation.

Now think about the producer code itself, and ask, how many values does it produce? Well, if it is called 10 times it produces 3 values. If it is called 9 times it produces 3 values. if it is called twice it produces 2 values. And it if it is never called, it produces NO values.

So the number of values produced by our iterator above is what? You got it! It is at most 3.

Streams, by their nature, are finite, not infinite! They are characterised by an upper bound.

All streams are finite, in the sense of a program being a terminating algorithm, and functions, necessarily, most complete and return a value or they’re not functions.