---

name: Test Coverage Guardian description: Analyze test coverage, identify gaps, detect dead code, and improve test quality. Use when user asks to check coverage, review tests, find untested code, or improve test robustness.

Test Coverage Guardian

Analyze and improve test coverage for the AILANG codebase.

Quick Start

Most common usage:

# Quick coverage check
make test-coverage-badge  # Shows: "Coverage: 29.9%"

# Detailed coverage report
make test-coverage        # Generates HTML report

# Coverage for specific package
go test -cover ./internal/parser/...

When to Use This Skill

Invoke when user says:

• "Check test coverage"
• "What's untested?"
• "Review the test suite"
• "Find dead code"
• "Tests keep breaking" (brittleness analysis)

Coverage Commands

Quick Check

make test-coverage-badge
# Output: Coverage: 29.9%

Detailed HTML Report

make test-coverage
# Opens: coverage.html in browser

Per-Package Coverage

go test -cover ./internal/parser/...
go test -cover ./internal/types/...
go test -cover ./internal/eval/...

Coverage with Profile

go test -coverprofile=coverage.out ./...
go tool cover -func=coverage.out | grep -E "^total:|0\.0%"

Coverage Standards

Module	Target	Priority
lexer	80%+	High
parser	80%+	High
types	80%+	High
eval	70%+	Medium
effects	70%+	Medium
repl	50%+	Low

Critical paths (100% target):

• Type inference
• Effect checking
• Pattern matching exhaustiveness

Analysis Workflow

Step 1: Get Current Coverage

# Overall coverage
make test-coverage-badge

# Per-package breakdown
go test -cover ./internal/... 2>&1 | grep -E "coverage:|ok"

Step 2: Identify Gaps

# Find 0% coverage functions
go test -coverprofile=coverage.out ./...
go tool cover -func=coverage.out | grep "0.0%"

# Visual inspection
go tool cover -html=coverage.out

Step 3: Categorize Gaps

Category	Action
Core logic uncovered	Write tests (high priority)
Error paths uncovered	Add error case tests
Dead code (never called)	Consider removal
Experimental code	Accept lower coverage

Step 4: Report Findings

## Coverage Report

### Summary
- Overall: 29.9%
- Core modules: 45%
- Target: 50%+ overall, 80%+ core

### Critical Gaps
1. internal/types/unify.go - row polymorphism (0%)
2. internal/eval/effects.go - effect handlers (15%)

### Dead Code Candidates
- internal/legacy/old_parser.go (0%, no references)

### Recommendations
1. Add unification tests for row types
2. Add effect handler integration tests
3. Remove legacy parser code

Test Quality Checklist

Anti-Brittleness

• Tests public APIs, not private implementation
• Uses interface-based testing where appropriate
• Tests error types, not exact messages
• Mocks external dependencies at boundaries
• Tests behavior, not call sequences

Good Patterns

• Table-driven tests for comprehensive inputs
• Property-based tests for invariants
• Error path testing
• Integration tests at module boundaries

AILANG-Specific

• Expression-based tests (everything returns value)
• Effect tracking tests
• Pattern matching exhaustiveness tests
• Deterministic execution tests

CI/CD Monitoring

# View recent CI runs
gh run list --limit 10

# Check failed run
gh run view <run-id> --log-failed

# Watch run in progress
gh run watch <run-id>

Common CI failures:

• Example validation failures
• Module path issues
• Coverage regressions

Example Session

User: "Check test coverage"

Response:

Running coverage analysis...

## Coverage Report

### Overall: 29.9%

### Per-Module Breakdown
  internal/lexer:    85.2%  (good)
  internal/parser:   42.1%  (needs work)
  internal/types:    38.5%  (needs work)
  internal/eval:     25.3%  (low)
  internal/effects:  18.7%  (low)

### Critical Gaps (0% coverage)
1. internal/types/row.go:UnifyRows() - row polymorphism
2. internal/eval/match.go:matchADT() - ADT pattern matching
3. internal/effects/budget.go - entire file

### Recommendations
1. Priority: Add tests for UnifyRows (critical for type system)
2. Add ADT matching tests
3. Review effects/budget.go - possibly dead code

Would you like me to generate test stubs for any of these?

Quality Gates

• No PR should decrease overall coverage
• New features must include tests
• Bug fixes must include regression tests
• Breaking changes require test updates

Tips

• Focus on branch coverage, not just line coverage
• Test the sad path - error conditions matter
• Golden tests for complex outputs (parser, codegen)
• Property tests for mathematical invariants