Credence.Rule.NoUnderscoreFunctionName (credence v0.3.2)

Detects function names that use a leading underscore to indicate privacy, a convention borrowed from Python that is non-idiomatic in Elixir.

Why this matters

In Python, _private_method signals "internal use." In Elixir, defp is the privacy mechanism, and a leading underscore on a name signals "unused variable" — not "private function." LLMs frequently generate helper functions like _factorial, _do_find, or _fibonacci because their training data mixes Python and Elixir conventions.

The Elixir convention for recursive helpers is the do_ prefix:

# Flagged — Python convention
defp _factorial(0, acc), do: acc
defp _factorial(n, acc), do: _factorial(n - 1, n * acc)

# Idiomatic — Elixir convention
defp do_factorial(0, acc), do: acc
defp do_factorial(n, acc), do: do_factorial(n - 1, n * acc)

Detection scope

Flags any def or defp clause where the function name starts with a single underscore. Names starting with double underscores __) are excluded — those are legitimate Elixir/Erlang callbacks such as __using__/1, __before_compile__/1, and __info__/1.

Auto-fix

Renames the function definition and all call sites from _name to do_name throughout the module.