Credence.Rule.UseMapJoin (credence v0.2.0)

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Detects Enum.map/2 chained into Enum.join/1 or Enum.join/2, and suggests Enum.map_join/3 instead.

Why this matters

Enum.map/2 followed by Enum.join creates a throwaway intermediate list. Enum.map_join/3 maps and joins in a single pass with no intermediate allocation:

# Flagged — intermediate list
list
|> Enum.map(&to_string/1)
|> Enum.join(", ")

# Idiomatic — single pass
Enum.map_join(list, ", ", &to_string/1)

LLMs generate the two-step version frequently because they decompose "transform then combine" into separate operations.

Flagged patterns

  • Enum.map(enum, f) |> Enum.join() (pipeline, any separator)
  • Enum.join(Enum.map(enum, f)) (nested call)
  • Longer pipelines where map and join are adjacent steps

Only adjacent map→join is flagged. Intervening steps like Enum.map(f) |> Enum.filter(g) |> Enum.join() are left alone.

Severity

:warning