Credence.Pattern.NoManualFrequencies (credence v0.7.1)

Readability rule: Detects manual frequency counting with Enum.reduce(list, %{}, fn x, acc -> Map.update(acc, KEY, 1, &(&1 + 1)) end).

When the counted key is the element itself, Enum.frequencies/1 does exactly this in one call. When the key is a function of the element (e.g. String.downcase(word)), Enum.frequencies_by/2 is the value-for-value equivalent — the key expression is carried over verbatim, so both forms produce byte-identical maps on every input (both available since Elixir 1.10).

Bad

list
|> Enum.reduce(%{}, fn item, counts ->
  Map.update(counts, item, 1, &(&1 + 1))
end)

Enum.reduce(words, %{}, fn word, acc ->
  Map.update(acc, String.downcase(word), 1, &(&1 + 1))
end)

Good

Enum.frequencies(list)

Enum.frequencies_by(words, fn word -> String.downcase(word) end)