F# 4.0 Design Patterns

Continuation passing style

This sophisticated technique of arranging recursion allows you to avoid stack consumption by putting all function calls into the tail position with continuation, that is, a function that performs the remaining computations instead of returning result to the caller. Let me demonstrate this technique by refactoring the factorial implementation one more time as shown in the following snippet (Ch7_6.fsx):

let rec ``factorial (cps)`` cont = function 
  | z when z = 0I -> cont 1I 
  | n -> ``factorial (cps)`` (fun x -> cont(n * x)) (n - 1I)  

Although slightly mind-bending, the code consists of all tail calls:

• A recursive call to itself ``factorial (cps)`` is a tail call
• A new continuation anonymous function also makes a tail call to ...