SephiaCredo

Credo checks for common Elixir pitfalls.

SephiaCredo catches performance anti-patterns, incorrect operator usage, and dead code in your test setups — issues that the compiler and standard Credo rules miss.

Installation

SephiaCredo requires Credo to already be installed in your project.

With Igniter (recommended)

If your project uses Igniter, a single command will add the dependency and register all checks in your .credo.exs:

mix igniter.install sephia_credo --only dev,test

Manual

Add sephia_credo to your list of dependencies in mix.exs:

def deps do
  [
    {:sephia_credo, "~> 0.1", only: [:dev, :test], runtime: false}
  ]
end

Then fetch the dependency and add the checks to the extra section of your .credo.exs:

mix deps.get

# .credo.exs
%{
  configs: [
    %{
      name: "default",
      checks: %{
        extra: [
          {SephiaCredo.Checks.AppendInLoop, []},
          {SephiaCredo.Checks.NoDateTimeOperatorCompare, []},
          {SephiaCredo.Checks.UnusedSetupKeysInTests, []},
          {SephiaCredo.Checks.UnusedSetupKeysPerTest, []}
        ]
      }
    }
  ]
}

Checks

AppendInLoop

Appending to a list with ++ inside a loop (Enum.reduce, Enum.flat_map_reduce, for/reduce, or a recursive function) creates a new copy of the left-hand list on every iteration, turning an O(n) traversal into O(n²). This check flags those call sites and suggests prepending with [head | acc] and reversing at the end, or collecting into a different data structure.

NoDateTimeOperatorCompare

Elixir's comparison operators (<, >, ==, etc.) use structural comparison on Date, Time, DateTime, and NaiveDateTime structs, which can produce incorrect results. For example, ~D[2024-02-01] > ~D[2024-01-31] happens to work, but ~T[09:00:00] > ~T[10:00:00] does not behave as expected in all cases. This check enforces the use of Date.compare/2, Time.compare/2, DateTime.compare/2, or NaiveDateTime.compare/2 instead.

UnusedSetupKeysInTests

Detects keys returned from setup blocks that are never destructured by any test in the same describe block (or module top-level). Dead setup keys add noise and slow down test comprehension — they should be removed from the setup return value.

UnusedSetupKeysPerTest

A more granular companion to UnusedSetupKeysInTests. Instead of checking whether any test uses a key, it checks each test individually and flags tests that don't destructure all in-scope setup keys. This helps keep tests focused by surfacing unnecessary fixtures.

Contributing

1. Fork the repository
2. Create your feature branch (git switch -c my-new-check)
3. Apply formatting and make sure tests pass (mix format, mix test)
4. Commit your changes
5. Open a pull request

License

MIT - see LICENSE for details.