Recursion

Handlers can call themselves recursively.

Example 014 of the basic Soma programs: a small, commented program that teaches one thing. Every output below was printed by soma 3.2.4 when this page was built.

How it is built

The cell Recursive has no face and no memory. Handlers: factorial (line 15), fibonacci (line 25) and digit_sum (line 38).

Generated from soma describe on this file.

The source

014_recursion.cell

// ============================================================================
// 014 — Recursion
// ============================================================================
// Handlers can call themselves recursively.
//
// Run:   soma run 014_recursion.cell factorial 6
// Expect: 720
//
// Run:   soma run 014_recursion.cell fibonacci 10
// Expect: 55

cell Recursive {

    // Classic factorial: n! = n * (n-1)!
    on factorial(n: Int) {
        // Base case
        if n <= 1 {
            return 1
        }
        // Recursive case: handler calls itself
        return n * factorial(n - 1)
    }

    // Fibonacci: fib(n) = fib(n-1) + fib(n-2)
    on fibonacci(n: Int) {
        if n <= 0 {
            return 0
        }
        if n == 1 {
            return 1
        }
        return fibonacci(n - 1) + fibonacci(n - 2)
    }

    // Sum of digits: 123 -> 1 + 2 + 3 = 6
    // Run: soma run 014_recursion.cell digit_sum 123
    // Expect: 6
    on digit_sum(n: Int) {
        if n < 10 {
            return n
        }
        return (n % 10) + digit_sum(n / 10)
    }
}

What the toolchain says

$ soma check 014_recursion.cell
✓ All checks passed.

$ soma run 014_recursion.cell factorial 6
720

$ soma run 014_recursion.cell fibonacci 10
55