Smell

Credo checks for the anti-patterns described in the official Elixir documentation.

The catalog comes from the research of Lucas Vegi and Marco Tulio Valente (ASERG/DCC/UFMG). Smell turns the anti-patterns that can be found by static analysis into Credo checks, and links each check to the matching section of the documentation.

Installation

Add smell to your dependencies in mix.exs:

def deps do
  [
    {:smell, "~> 0.2", only: [:dev, :test], runtime: false}
  ]
end

Then enable the plugin in your .credo.exs:

%{
  configs: [
    %{
      name: "default",
      plugins: [{Smell, []}]
    }
  ]
}

If your .credo.exs declares an explicit checks: %{enabled: [...]} list (as the one generated by mix credo.gen.config does), Credo only runs the checks in that list. Add Smell's checks to it:

checks: %{
  enabled:
    [
      # ...your existing checks...
    ] ++ Enum.map(Smell.checks(), &{&1, []})
}

Checks

Check	Anti-pattern	Reports
`Smell.Check.Design.AppConfigInLibrary`	Using application configuration for libraries	In a library (a `mix.exs` with `:package` and no `:releases`), a read of the library's own application environment (`Application.get_env/3`, `fetch_env!/2`, `compile_env/3`, ...), except keys that swap a component (`:adapter`, `:json_library`, ...) and the `:otp_app` pattern
`Smell.Check.Design.UseInsteadOfImport`	`use` instead of `import`	A `__using__/1` whose clauses only inject `import`, `alias` and `require` directives (unless it is deprecated)
`Smell.Check.Readability.ComplexElseClauses`	Complex `else` clauses in `with`	A `with` with two or more `<-` clauses whose `else` block has more than `max_else_clauses` (default 1) clauses
`Smell.Check.Warning.NamespaceTrespassing`	Namespace trespassing	A top-level module defined inside a namespace owned by Elixir (`Mix`, `Logger`, ...) or by a dependency listed in `mix.lock` (`Plug`, `Phoenix`, ...)
`Smell.Check.Warning.NonAssertivePatternMatching`	Non-assertive pattern matching	A `case` that matches `{:ok, value}` and then everything else with `_`, with no clause for `:error` or `{:error, ...}`

Run mix credo explain on an issue to see the full explanation and the parameters of each check.

Parameters

AppConfigInLibrary:

library (default :auto): :auto checks the project when its mix.exs has a :package and no :releases; true always checks it, false never does.
allowed_keys (default []): keys the library may read, as atoms or regexes, for example settings used to generate code from data files: [:statuses, ~r/^debug_/]. For a key path such as [MyLib.Repo, :timeout], the last key is matched.
exclude_test_files (default true): skip files under test/.

{Smell.Check.Design.AppConfigInLibrary, [allowed_keys: [:statuses]]}

UseInsteadOfImport:

exclude_test_files (default true): skip files under test/.

ComplexElseClauses:

max_else_clauses (default 1): the maximum number of clauses allowed in the else block.

NamespaceTrespassing:

allowed_namespaces (default []): namespaces the project may define modules in, for example when you maintain both packages: [Phoenix.LiveView].
lockfile (default "mix.lock"): where to read dependencies from, relative to where Credo runs.
exclude_test_files (default true): skip files under test/.

{Smell.Check.Warning.NamespaceTrespassing, [allowed_namespaces: [Phoenix.LiveView]]}

NonAssertivePatternMatching:

exclude_test_files (default true): skip files under test/.

Anti-patterns Credo already covers

Some anti-patterns from the documentation already have a check in Credo itself, so Smell does not duplicate them:

Anti-pattern	Credo check
Dynamic atom creation	`Credo.Check.Warning.UnsafeToAtom`
Long parameter list	`Credo.Check.Refactor.FunctionArity`
Structs with 32 fields or more	`Credo.Check.Warning.StructFieldAmount` (Credo 1.7.14+)
`with` with a single `<-` clause and an `else`	`Credo.Check.Readability.WithSingleClause`

License

MIT. See LICENSE.