Credence.Pattern.NoLengthComparisonForEmpty (credence v0.4.0)

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Detects length(list) comparisons with small integers (0–5) that can be replaced with O(1) pattern matching.

length/1 is O(n) on linked lists — it traverses every element to count them. LLMs use it freely because Python's len() is O(1). In Elixir, pattern matching can answer the same questions in O(1).

Bad

length(list) == 0
length(list) > 0
length(list) < 2
length(list) >= 3

Good

list == []
list != []
!match?([_, _ | _], list)
match?([_, _, _ | _], list)

What is flagged

Any comparison of length(expr) with a literal integer 0–5 using ==, !=, >, >=, <, or <=. Reversed operands like 0 < length(list) are also detected. Comparisons with larger integers are not flagged since the match pattern becomes unwieldy.

Auto-fix

Rewrites to == [], != [], match?/2, or !match?/2 depending on the comparison. Only fixes when the argument to length/1 is a simple variable name.