Recursion

constant Rate { value: 50 }
signal N { default: 0 rate: Rate }
signal FactorialResult { default: 0 rate: Rate }

N >> Factorial () >> FactorialResult;

module Factorial {
        input:          Input
        output:         Output
        propertyName:   none
        propertyBlock:  none
        internalBlock:  [
                signal Input { default: 0 rate: none },
                signal Output { default: 0 rate: none },
                signal Result { default: 0 rate: none reset: Reset },
                signal Count { default: Input rate: none reset: Reset },
                switch End { default: off reset: Reset },
                switch FlipFlop { default: off reset: none },
                trigger Reset {},
                reaction FactorialReaction {
                        output:         Result
                        onExecution:    Reset
                        terminateWhen:  Done
                        propertyName:   'value'
                        propertyBlock:  signal Value { default: 0 }
                        internalBlock:  [
                                signal Result { default: 0 rate: none },
                                switch Done { default: off reset: Reset },
                                trigger Reset {}
                        ]
                        streams:                [
                                Value >> Factorial () >> Result;
                                on >> Done;
                        ]
                }
        ]
        streams:                [

                FlipFlop >> Select ( whenOn: off whenOff: on ) >> FlipFlop >> OnEdge (edge: 'Both' ) >> Reset;
                Count >> Compare ( value: 0 operator: 'Equal' ) >> End;
                not End >> FactorialReaction ( value: Count - 1 ) >> Result;
                End >> Select ( whenOn: 1 whenOff: Count * Result ) >> Output;
        ]
}

---

Python

def factorial(n):
        if n == 0:
                return 1
        else:
                return n * factorial(n-1)

C / C++

unsigned int factorial (unsigned int f) {
    if (f == 0) {
        return 1;
    } else {
        return f * factorial(f - 1);
    }
}