Course of Raku / Functional, concurrent, reactive, and web programming / Functional programming / Recursion / Exercises / Triangular number

Solution: Triangular number

Here is a possible solution to the task.

Code

multi tri(0)  { 0 }
multi tri($n) { $n + tri($n - 1) }

say tri(5);

🦋 You can find the source code in the file triangular-number.raku.

Output

15

Comments

1. The base case is a candidate of its own: multi tri(0) matches only when the argument is exactly 0 and returns 0 without recursing.

2. Every other call lands in multi tri($n), which adds $n to the triangular number of $n - 1. The calls descend 5 + 4 + 3 + 2 + 1 + 0, and when the argument reaches 0 dispatch switches to the base-case candidate and the sum unwinds to 15.

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