ReasonML Quick Start Guide

Recursion

Since list is a recursive data structure, we typically use recursion when working with it.

To get warmed up, let's write a (naive) function that sums a list of integers:

let rec sum = list => switch(list) {  | [] => 0  | [hd, ...tl] => hd + sum(tl)};

• This is a recursive function and therefore requires the rec keyword (that is, let rec instead of just let)
• We can pattern match on a list (just like any other variant and many other data structures)
• From the example type definition, Empty is represented as [] and Head is represented as [hd, ...tl] where hd is the head of the list and tl is the rest (that is, the tail) of the list
• tl could be [] (that is, Empty) and when it is, recursion stops

Passing sum the list [1, 2, 3] results in ...