elixir-dev

# Elixir Dev

## Running Mix Commands

See [references/mix-commands.md](references/mix-commands.md) for full command reference.

### Test

```bash
# Run all tests
mix test

# Specific file or line
mix test test/my_app/accounts_test.exs:42

# By tag
mix test --only integration

# Failed only (requires --failed flag from prior run)
mix test --failed

# With coverage
mix test --cover
```

**Interpreting failures:**
- `** (MatchError)` — Pattern match failed; check return value shape.
- `** (Ecto.NoResultsError)` — `Repo.get!` with non-existent ID; use `Repo.get` or seed data.
- `** (DBConnection.OwnershipError)` — Missing `async: true` or sandbox setup.
- `no function clause matching` — Wrong arity or unexpected arg type.

### Credo

```bash
mix credo --strict
mix credo suggest --format json
mix credo explain MyApp.Module  # Explain issues for specific module
```

**Common Credo fixes:**
- `Credo.Check.Readability.ModuleDoc` — Add `@moduledoc`.
- `Credo.Check.Refactor.CyclomaticComplexity` — Extract helper functions.
- `Credo.Check.Design.TagTODO` — Address or remove TODO comments.

### Dialyzer

```bash
mix dialyzer
mix dialyzer --format short
```

**Common Dialyzer warnings:**
- `The pattern can never match` — Dead code or wrong type in pattern.
- `Function has no local return` — Crashes on all paths; check internal calls.
- `The call will never return` — Calling a function that always raises.
- Fix: Add `@spec` annotations; use `@dialyzer {:nowarn_function, func: arity}` as last resort.

### Format

```bash
mix format
mix format --check-formatted  # CI mode — exit 1 if unformatted
```

## Module Generation

Always include `@moduledoc`, `@doc`, and `@spec` on public functions.

### Context Module

```elixir
defmodule MyApp.Notifications do
  @moduledoc """
  Manages notification delivery and preferences.
  """
  import Ecto.Query
  alias MyApp.Repo
  alias MyApp.Notifications.Notification

  @doc "List notifications for a user, most recent first."
  @spec list_notifi...