Credence.Pattern.NoListDeleteAtLength (credence v0.7.0)

Detects List.delete_at(list, length(list) - 1) — computing the length of a list just to index its last element for deletion.

Why this matters

length/1 is O(n) and List.delete_at/2 is another O(n), so the combination walks the list twice. List.delete_at/2 already accepts negative indexes counted from the end, so List.delete_at(list, -1) deletes the last element in a single pass — the length call is pure overhead.

Bad

middle = List.delete_at(tail, length(tail) - 1)

Good

middle = List.delete_at(tail, -1)

Detection scope

Matches List.delete_at(x, length(x) - 1) where x is the same variable in both positions and the subtracted literal is exactly 1.

Larger offsets such as List.delete_at(x, length(x) - 2) are not flagged: length(x) - K differs from -K for short lists (when length(x) < K the left form yields a negative index that still deletes an element from the end, while -K is out of range and deletes nothing), so no constant-index rewrite is behaviour-preserving for them.