E142: A range slice [a..b] was used on something that isn't an array

x[1..3] takes part of an array: a view of the elements from 1 up to (not including) 3. It only makes sense for a built-in array (int[5], a row of a 2D array, an int[] parameter or another slice). Anything else is refused:

immut s: string = "hello";immut t = s[1..3];                 // E142 -- a string is not an arraymut l: List<int> = List::default();immut u = l[0..1];                 // E142 -- a List keeps its elements behind its own bufferimmut n: int = 5;immut v = n[0..1];                 // E142 -- an int has no elements

A string is text that ends at a zero byte, so a view into its middle would have no end marker. To take part of a string, build a new string with strings::substring(s, start, end). To take part of a List, copy the elements you need in a loop (or into an array).

The same applies to a struct: p[0..1] on a struct value is E142. Indexing a single element, l[i], still works on a List; only the range form is limited to arrays.

Run turf --explain E142 in a terminal to read this again.