Credence.Pattern.NoLengthBasedIndexing (credence v0.6.0)

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Detects n = length(list) followed by Enum.at(list, n - K) — a Python list[len(list) - 1] idiom. Elixir's Enum.at/2 natively supports negative indices, so Enum.at(list, -1) is the idiomatic equivalent.

Detection constraints

Only flags when ALL of:

  • var = length(list) or var = Enum.count(list) exists
  • Enum.at(list, var - K) appears in the same block (K is a positive integer literal)
  • Same list variable in both calls
  • No rebinding of either variable between the two calls

Bad

n = length(sorted)
largest = Enum.at(sorted, n - 1)
second_largest = Enum.at(sorted, n - 2)

Good

largest = Enum.at(sorted, -1)
second_largest = Enum.at(sorted, -2)

Auto-fix

Replaces Enum.at(list, n - K) with Enum.at(list, -K). If the length variable is only used for indexing, the n = length(list) line is removed.