E098: That write would only change a copy

When you write bs[0], the [] operator hands back the element by value: a copy. Assigning into a field of that copy would change the copy and throw it away, leaving the list untouched. Turf rejects it so the mistake can’t silently do nothing:

import list::List;struct Big { a: int }fn main(): int {    mut bs: List<Big> = List::default();    bs[0].a = 50;               // changes a copy of bs[0]    return 0;}

Read the element into a variable, change that, and store it back:

mut e: Big = bs[0];e.a = 50;bs[0] = e;

Writing through a pointer or reference element is fine, and so is writing into a nested list (grid[i][j] = v), because there the write lands on the real thing.

Run turf --explain E098 in a terminal to read this again.