Credence.Pattern.NoMapKeysEnumLookup (credence v0.4.0)

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Detects Map.keys(var) piped into an Enum function whose callback also looks up values from the same map variable.

Why this matters

LLMs frequently port the Python idiom for key in dict: ... dict[key] into Elixir as Map.keys(map) |> Enum.xxx(fn k -> ... map[k] ... end). This creates an unnecessary intermediate list and performs redundant lookups. In Elixir, maps are directly enumerable as {key, value} pairs:

# Flagged — extra allocation + redundant lookups
Map.keys(freqs)
|> Enum.all?(fn char -> Map.get(other, char, 0) >= freqs[char] end)

# Idiomatic — single traversal, values already in hand
Enum.all?(freqs, fn {char, count} ->
  Map.get(other, char, 0) >= count
end)

Detection scope (strict)

Only flagged when all three conditions hold:

  1. Map.keys(var) is called on a simple variable,
  2. The result is passed to one of: Enum.all?, Enum.any?, Enum.each, Enum.map, Enum.filter, Enum.reject, Enum.flat_map, and
  3. The callback body references var via var[key], Map.get(var, ...), Map.fetch(var, ...), or Map.fetch!(var, ...).

Patterns where only keys are needed (no value lookup in the callback) are not flagged.