DRY — Don't Repeat Yourself

What it detects

Two functions can look different in naming and literal values yet share identical structure. This skill finds those structural duplicates by normalising each function's AST — replacing variable names with :symbol, literals with :literal, and preserving only the skeleton (control flow, collection shapes, call patterns). It then compares every pair of functions via Jaccard similarity over their fingerprint sets.

A score of 1.0 means two functions are structurally identical despite different names and values. The default threshold of 0.82 catches candidates close enough to warrant review.

Supported languages: Python, JavaScript, TypeScript (including JSX/TSX).

Workflow

Phase 1 — Scan

• Detect project language(s) from file extensions
• Run python3 scripts/drycheck.py <src> -f json --base-dir <projectroot>
• Parse JSON output; sort by worst similarity score

Phase 2 — Review

For each candidate pair above the threshold:

• Read both functions side by side
• Classify the duplication:

- Exact structural clone (score ~1.0): identical shape, only names differ - Near clone (score 0.90–0.99): one has a small extra branch or binding - Fuzzy match (score 0.82–0.89): shared core with divergent details

• Determine if the duplication is accidental (coincidence, leave alone) or essential (same concept expressed twice, should unify)
• For essential duplicates, propose a refactoring:

- Extract shared logic into a common function with parameters for the differing parts - Use higher-order functions or strategy pattern when behaviour varies - For near-clones, identify the delta and parameterise it

Phase 3 — Report

Present a table to the user:

| # | Score | Left | Right | Classification | Proposed action |

Include totals: functions scanned, duplicate pairs found, score distribution. Ask user to confirm which pairs to refactor (all, subset, or skip).

Phase 4 — Refactor

After user confirms:

• Extract common function (one pair at a time, commit-ready)
• Replace both call sites with the new unified function
• Run existing tests to confirm nothing broke
• Re-run dry_check.py to verify the pair no longer appears
• Present before/after comparison

.dryignore

The script respects a .dryignore file in the project root (same directory as --base-dir). Syntax is gitignore-like:

# Ignore generated code
generated/
*.gen.ts

# Ignore test fixtures
__fixtures__/
*_fixture.py

# But keep this one
!__fixtures__/important_fixture.py

Blank lines and # comments are ignored. ! negates a previous match.

Key rules

• Never refactor without running tests to confirm nothing broke
• Keep refactoring minimal — only unify what's genuinely duplicated
• If two functions score high but serve intentionally different domains, flag but don't force unification — some duplication is acceptable when coupling would be worse
• Accidental similarity (e.g. two simple CRUD handlers) is not a DRY violation — use judgement
• Default thresholds are conservative; lower --threshold for broader sweeps, raise it for precision
• The script auto-skips node_modules, .git, pycache, venv, dist, build, and similar directories

Script

The bundled scripts/dry_check.py handles: Python AST parsing (via stdlib ast), JS/TS token-based structural parsing, normalisation, fingerprinting, Jaccard similarity, .dryignore filtering, and text/JSON output. Zero external dependencies. See [REFERENCE.md](REFERENCE.md) for algorithm details and CLI usage.