Editing Corecursion (section)

=== Fibonacci sequence ===
In the same way, the [[Fibonacci sequence]] can be represented as:
:<math>a, b = (0, 1) : (b, a+b)</math>
Because the Fibonacci sequence is a [[recurrence relation]] of order 2, the corecursive relation must track two successive terms, with the <math>(b, -)</math> corresponding to shift forward by one step, and the <math>(-, a+b)</math> corresponding to computing the next term. This can then be implemented as follows (using [[parallel assignment]]):
<syntaxhighlight lang="python">
def fibonacci_sequence():
    a, b = 0, 1
    while True:
        yield a
        a, b = b, a + b
</syntaxhighlight>
In Haskell, <syntaxhighlight lang=haskell> map fst ( (\(a,b) -> (b,a+b)) `iterate` (0,1) )</syntaxhighlight>