E001: You tried to reach past the end of an array

Turf arrays have a fixed size you choose when you declare them, and their valid indices always start at 0 and stop at size - 1 – never at size itself. So an array of 3 things has valid indices 0, 1, and 2, not 3.

immut primes: int[3] = [2, 3, 5];mut first: int = primes[0];   // correct -- the first elementmut last: int = primes[2];    // correct -- the last element (size - 1)

A negative index counts back from the end, so primes[-1] is that same last element and primes[-3] is the first. The valid range is -3 to 2 – an index below -3 is out of bounds just like one above 2:

mut also_last: int = primes[-1];   // correct -- the last elementmut oops2: int = primes[-4];       // there is no primes[-4]!

If you ask for one index too many:

mut oops: int = primes[3];   // there is no primes[3]!

Turf catches this at compile time and tells you exactly how big the array actually is, so you don’t have to go count the brackets yourself.

This mistake almost always comes from an off-by-one: thinking of “3 elements” as meaning “indices 1 through 3” (like counting objects) instead of “indices 0 through 2” (like counting positions). It’s one of the most common trips in programming in general, not just in Turf – you’ll stop making it once counting from 0 feels natural, which usually doesn’t take long.

If you actually need more room in the array, the fix isn’t to index further – it’s to make the array bigger and give it the extra value:

immut primes: int[4] = [2, 3, 5, 7];mut last: int = primes[3];   // now valid -- the array really has 4 slots

Run turf --explain E001 in a terminal to read this again.