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.AshCodeInterfaceReadWithArgs, []},
          {SephiaCredo.Checks.AssertWithoutAssertion, []},
          {SephiaCredo.Checks.ProcessSleepInTests, []},
          {SephiaCredo.Checks.RawRuntimeError, []},
          {SephiaCredo.Checks.StructComparisonOperator, []},
          {SephiaCredo.Checks.UnusedSetupKeysInTests, []},
          {SephiaCredo.Checks.UnusedSetupKeysPerTest, []}
          # Opt-in (not enabled by default):
          # {SephiaCredo.Checks.SysGetStateWithoutTimeoutInPoll, []}
        ]
      }
    }
  ]
}

Upgrading from 0.1

NoDateTimeOperatorCompare has been replaced with the more general StructComparisonOperator (now also covers Decimal and Version, with a configurable extra_modules list). Update your .credo.exs: replace the old tuple with {SephiaCredo.Checks.StructComparisonOperator, []}.

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.

AshCodeInterfaceReadWithArgs

Inside an Ash code_interface do ... end block, define :name, action: :read, args: [...] registers a code interface against Ash's generic :read action, which declares no inputs. Calling the resulting function raises Ash.Error.Invalid.NoSuchInput at runtime. The bug typically ships silently — LiveView callers wrap the call in else {:error, _} -> ... and the page just "doesn't do anything." Define a custom read action that declares the args, or remove args:.

AssertWithoutAssertion

assert x = expr (or any pattern with fresh bindings on the left) succeeds vacuously: the pattern always matches a bare variable, so the assertion tests nothing about expr. If the bound variables are never referenced afterward, the assertion is dead. Reference them in subsequent assertions, or use assert match?(pattern, expr). Test files only (*_test.exs).

ProcessSleepInTests

Process.sleep/1 in test bodies, setup blocks, or setup_all blocks causes timing-dependent flakes and slows the suite linearly. Prefer assert_receive, assert_eventually, or a polling helper. Test files only (*_test.exs).

RawRuntimeError

raise "msg" and raise RuntimeError, ... both lower to a RuntimeError exception. Error trackers (Appsignal, Sentry, etc.) group exceptions by module name — every distinct RuntimeError message becomes its own issue, hiding the signal in noise. Define a defexception module with a descriptive name and raise that instead.

StructComparisonOperator

Elixir's comparison operators (<, >, ==, etc.) use Erlang's term order on structs, which walks fields in declaration order. For most calendar/numeric structs this produces silently incorrect results — for example, Decimal.new("1.0") == Decimal.new("1.00") returns false, and Decimal.new("1.5") > Decimal.new("2") returns true. This check enforces the use of Date.compare/2, DateTime.compare/2, Decimal.compare/2, Version.compare/2, etc. instead. Built-in coverage: Date, Time, DateTime, NaiveDateTime, Decimal, Version. Configurable via extra_modules.

SysGetStateWithoutTimeoutInPoll (opt-in)

Inside a polling fn (configurable via poll_functions:, defaults to [:wait_until]), :sys.get_state(pid) without an explicit timeout uses the default 5-second timeout. If the GenServer is blocked (e.g. by cascading PubSub), the call raises :exit — which rescue doesn't catch — and the test crashes. Pass a short explicit timeout AND wrap in try ... catch :exit, _ -> false. Add this check manually to .credo.exs if you use poll-style test helpers.

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.