Code Quality

Code Quality

Canonical FP bar: 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 for copy-paste complexity thresholds and a before/during/after refactoring checklist.

What Gets Detected

Code Duplication

How to fix:

# 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:

# 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:

# 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)

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

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

Sobelow (Security)

# Run security analysis
mix sobelow

# With configuration
mix sobelow --config

Dependency Auditing

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

Common Pitfalls

Integration

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