Credence.Pattern.NoIsPrefixForNonGuard (credence v0.5.0)

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Detects def/defp functions with an is_ prefix, which in Elixir is reserved for guard-safe functions defined with defguard or Erlang BIFs.

Why this matters

Elixir has a clear naming convention for boolean-returning functions:

  • is_foo/1 → must be usable in guard clauses (defguard) or Erlang BIFs
  • foo?/1 → regular boolean function (def / defp)

LLMs generate is_valid, is_palindrome, etc., on virtually every boolean function because Python and JavaScript use is_ freely. In Elixir, this misleads readers into thinking the function is guard-safe:

Bad

def is_palindrome(str), do: str == String.reverse(str)

defp is_valid_email(str), do: String.contains?(str, "@")

Good

def palindrome?(str), do: str == String.reverse(str)

defp valid_email?(str), do: String.contains?(str, "@")

Exceptions

Guard-safe BIFs from Erlang that legitimately use the is_ prefix (like is_list/1, is_binary/1) are ignored so that user-defined wrapper functions shadowing them are not mistakenly flagged.

Auto-fix

The fix renames the function definition and all bare (unqualified) call sites within the same source file. is_valid_foo becomes valid_foo?.

Pass auto_fix_public: false to restrict the rename to defp only. Useful when running file-by-file: external callers of a public function live in other files and aren't visible to a per-file rule, so renaming a def can leave imports / qualified calls referencing the old name.