Code Quality: Formatter, Credo, Dialyzer and Types

Tools for maintainable Elixir

mix format formats code according to the community standard, configured in .formatter.exs, and mix format --check-formatted enforces it in CI. Credo is a static analysis tool that flags consistency issues, refactoring opportunities and likely bugs (mix credo --strict). Dialyzer, usually run through dialyxir (mix dialyzer), uses @spec typespecs and success typing to find type inconsistencies and unreachable code; it reports only what it can prove wrong, so it has no false positives in theory but can miss errors and its messages take practice to read. Elixir is also gaining a built-in gradual, set-theoretic type system: since version 1.17 the compiler performs type inference on patterns and guards and warns about type violations, such as calling a function with a value that can never match, without requiring annotations, with more capabilities arriving release by release. Other useful tools: mix xref for dependency graphs between modules, Sobelow for security scanning of Phoenix apps, ex_doc for documentation and mix_audit for vulnerable dependencies. A typical CI pipeline runs format check, compile with --warnings-as-errors, Credo, tests and Dialyzer.

A CI quality pipeline

Formatting, warnings as errors, static analysis, tests and security checks.

# .github/workflows/ci.yml (steps excerpt, expressed as shell commands)
mix deps.get
mix format --check-formatted
mix compile --warnings-as-errors
mix credo --strict
mix test --warnings-as-errors
mix sobelow --config                 # Phoenix security checks
mix deps.audit                       # from the mix_audit package
mix dialyzer                         # cache the PLT files between CI runs

# typespecs help both Dialyzer and readers:
# @spec place_order(Customer.t(), map()) :: {:ok, Order.t()} | {:error, Ecto.Changeset.t() | atom()}

Spellcheck, grammar check and a proofreader

The formatter is spellcheck for layout, Credo is a grammar checker with style advice, and Dialyzer plus the compiler's type checks are a careful proofreader who points out sentences that cannot possibly make sense.

Quick check: What does Dialyzer use to find bugs?

• Runtime logs
• Typespecs and success typing to detect provable type inconsistencies
• Unit tests
• Code formatting rules