I first met yield and coroutines many years ago, when I was learning Python. I quickly realized that this construct adds both power and elegance to a language; and since then, one of the first things I check when working with a new language, is if it has support for yield.

I am happy to say that most modern languages offer it - but there's a catch: not all of them support it in the same way. This may lead to... unexpected results.

To demonstrate, I will use Python, C# and F#, in implementing the Fibonacci sequence.

In Python...

#!/usr/bin/env python


def fib():
    a0, a1 = 1, 1
    yield a0
    yield a1
    while a0<1000000:
        yield a0+a1
        a0, a1 = a1, a0+a1

fibonacciNumbersUpToMillion = fib()
for i in fibonacciNumbersUpToMillion:
    print(i)
print(any(i==10946 for i in fibonacciNumbersUpToMillion))
for i in fibonacciNumbersUpToMillion:
    print(i)

• We initialize a0 and a1 to the first two terms of the sequence
• Yield them back in sequence
• Then do a loop, calculating the next element and yielding it back

But mathematicians will be surprised by the output of this program...

bash$ python fib.py
1
1
2
3
5
8
13
...
1346269
2178309
False

bash$

But only once!

When we call any to check if 10946 is part of the sequence, we get... False. And the second iteration prints... nothing.

Why?

Because Python has "exhausted" the generator with the print that was done before - the loop has ended, there's no more data returned, StopIteration was raised, and the generator is now empty...

In C#

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication {
    class Program {
        static IEnumerable<int> fib() {
            int a0 = 1;
            int a1 = 1; 
	    yield return a0;
            yield return a1;
            while (a0<1000000) {
                yield return a0+a1;
                var a2 = a0+a1; 
		a0 = a1; 
		a1 = a2;
            }
        }
        static void Main(string[] args) {
            var fibonacciNumbersUpToMillion  = fib();
            foreach (var x in fibonacciNumbersUpToMillion)
                Console.WriteLine(x);
            Console.WriteLine(fibonacciNumbersUpToMillion.Contains(10946));
            foreach (var x in fibonacciNumbersUpToMillion)
                Console.WriteLine(x);
        }
    }
}

bash$ csc fib.cs
bash$ ./fib.exe
1
1
2
3
...
1346269
2178309
True
1
1
2
3
...
2178309

bash$

How can C# do it? Why does Python fail to do so?

Because C# paid the price that this demands: When implementing yield, the C# compiler was instructed to perform a code transformation (generating a "magic" class) whenever it meets the construct. This transformation, in plain words, arranges so that the iteration state be kept at the *caller* side, not the *callee* side. In terms of our example above, you can think of iteration state as being managed in the "for" loop that prints - not in the fib() itself, as is done in Python.

If you want to avoid this in Python, make sure you NEVER assign the generator to a variable (in this case, fibonacciNumbersUpToMillion) - because the variable's state will be modified upon use. Instead, use the generator creator every time:

...

for i in fib():
    print(i)

print(any(i==10946 for i in fib()))

In F#

let fib () =
    seq {
        let mutable a0 = 1
        let mutable a1 = 1
        yield a0
        yield a1
        while a0<1000000 do
            yield a0+a1
            let a2 = a0+a1
            a0 <- a1
            a1 <- a2
    }

bash$ fsc.exe fib.fs
          yield a1
  --------^^^^^^^^

fib.fs(6,9): error FS0407: The mutable variable 'a0' is used in an invalid way. 
Mutable variables cannot be captured by closures. Consider eliminating this use of mutation 
or using a heap-allocated mutable reference cell via 'ref' and '!'.

F# is the descendant of a functional language (OCaml), and it wants its closures to be state-free. In plain words, it wants readers of your code to reason about it in the same way mathematicians do :‑)

But F#, like her mother OCaml (and unlike her crazy aunt Haskell :‑), is practical. She knows that imperative constructs are sometimes the way to go. And it allows you - if you want to - to do the dirty deed and keep state inside:

let fib () =
    seq {
        let a0 = ref 1
        let a1 = ref 1
        yield !a0
        yield !a1
        while !a0<1000000 do
            yield !a0 + !a1
            let a2 = !a0 + !a1
            a0 := !a1
            a1 := a2
    }

Conclusion