# Code Quality

> MANDATORY for all code quality and refactoring work for Elixir. Use when analyzing or refactoring Elixir code. Covers duplication detection, ABC complexity, unused private functions, template duplication, and Credo integration. Provides thresholds and fix patterns for each quality issue. Trigger words: code quality, duplication, complexity, unused functions, Credo, refactoring, analysis, mix credo, abc complexity, function length, module length, refactor, extract function, shared code, code smell, technical debt, clean code.

- Skill: `igmarin/code-quality` (Agent Skill, multi-file: 2 files)
- Install (CLI): `npx skillmds@latest add igmarin/code-quality`
- Raw SKILL.md: https://api.skillmd.com/api/skills/igmarin/code-quality/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Integrations & APIs
- License: MIT
- Author: igmarin (https://skillmd.com/u/igmarin)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/igmarin/code-quality

---


# Code Quality


Canonical FP bar: [`docs/fcis-engineering-rules.md`](../../docs/fcis-engineering-rules.md) — **Functional Core, Imperative Shell**: pure domain modules; side effects at edges. Enforce FCIS in reviews/refactors: fat LiveViews and mixed Repo+math are defects.

## RULES — Follow these with no exceptions

**1.** **Duplicated functions must be extracted** — when 2+ modules share >70% similar implementations, create a shared module
**2.** **Functions must stay below ABC complexity 30** — break complex functions into smaller helpers
**3.** **Remove unused private functions after refactoring**
**4.** **Duplicated templates must become components** — when 2+ HEEx files share >40% identical markup, extract to a function component
**5.** **Address duplication before complexity** — extracting shared code first reduces overall complexity
**6.** **Run `mix credo --strict` before any PR**
**7.** **Run `mix sobelow` for security** — check after quality checks


## End-to-End Workflow

1. **Run analysis** — `mix credo --strict` to identify all issues
2. **Fix by priority** — address duplication first (Rule 5), then complexity, then unused functions
3. **Verify fixes** — re-run `mix credo --strict` to confirm all issues are resolved
4. **Security check** — run `mix sobelow` before committing
5. **Commit** — only after both Credo and Sobelow pass cleanly

See [`assets/refactoring_checklist.md`](assets/refactoring_checklist.md) for copy-paste complexity thresholds and a before/during/after refactoring checklist.


## What Gets Detected

### Code Duplication

**How to fix:**
```elixir
# Create: lib/app_web/live/helpers.ex
defmodule AppWeb.Live.Helpers do
  def format_time(%Decimal{} = seconds) do
    seconds |> Decimal.to_float() |> format_time()
  end

  def format_time(seconds) when is_number(seconds) do
    # shared formatting logic
  end
end

# In each LiveView:
import AppWeb.Live.Helpers, only: [format_time: 1]
```

### ABC Complexity

**How to fix:**
```elixir
# Before: one large function (complexity 41)
def calculate_trend_line(data) do
  # 50 lines of assignments, branches, conditions
end

# After: composed smaller functions (complexity <20 each)
def calculate_trend_line(data) do
  sums = calculate_regression_sums(data)
  slope = calculate_slope(sums)
  intercept = calculate_intercept(sums, slope)
  build_trend_points(data, slope, intercept)
end
```

### Unused Private Functions

Remove any private functions no longer referenced after refactoring.

### Template Duplication

**How to fix:**
```elixir
# Create a function component for the shared markup
defmodule AppWeb.Live.Components do
  use Phoenix.Component

  def metric_filters(assigns) do
    ~H"""
    <div class="filters">
      <!-- shared filter markup -->
    </div>
    """
  end
end
```


## Running Analysis

### Credo (Static Analysis)

```bash
# Run with strict mode (recommended)
mix credo --strict

# Focus on a specific file
mix credo lib/my_app/accounts.ex
```

### Sobelow (Security)

```bash
# Run security analysis
mix sobelow

# With configuration
mix sobelow --config
```

### Dependency Auditing

```bash
# All three in sequence
mix deps.audit && mix hex.audit && mix sobelow
```


## Common Pitfalls

| ❌ Don't | ✅ Do |
|----------|-------|
| Copy-paste similar logic across modules | Extract a shared module when 2+ share >70% implementation |
| Let a function grow past ABC complexity 30 | Break it into named private helper functions |
| Leave unused private functions after refactoring | Remove dead private functions |
| Duplicate HEEx markup across templates | Extract a function component when 2+ files share >40% markup |
| Reduce complexity before removing duplication | Extract shared code first, then reduce complexity |
| Open a PR without a security pass | Run `mix credo --strict` and `mix sobelow` before committing |

---

## Integration

| Predecessor | This Skill | Successor |
|-------------|------------|-----------|
| code-review | code-quality | refactor-code |
| refactor-code | code-quality | testing-essentials |

**Companion skills:**
- `credo-config` — configure Credo checks and CI integration
- `refactor-code` — safe structural changes backed by characterization tests
- `code-review` — PR review that surfaces the quality issues fixed here