LYR-SEM0220

a string grown with '+' in a loop

warning · spec 08-expressions · lint string-concat-in-loop, group perf, warn unless said

In a loop, a string kept across its rounds is assigned itself with something added: s = s + x or s += x.

Explanation

A string never changes, so each + makes a new one and copies all that came before: n rounds copy on the order of n² bytes.

Append to a StringBuilder in the loop and take its toString() after it.

Wrong

fn main(): void {
    var line = "";
    for (i in 0..3) {
        line = line + f"{i}";
    }
    let _ = line;
}

Right

import std.string { StringBuilder };

fn main(): void {
    let line = StringBuilder.new();
    for (i in 0..3) {
        line.append(i);
    }
    let _ = line.toString();
}