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Pattern Critic

Systematically review pattern code for violations of Common Tools documentation rules and gotchas.

Workflow

1. Read the pattern file to review
2. Check each category below against the code
3. Output results in the checklist format (see Output Format)
4. For any [FAIL], include the line number and fix

Violation Categories

1. Module Scope Violations

Check that these are NOT inside the pattern body:

Allowed inside patterns: computed(), action(), .map() callbacks, JSX event handlers.

2. Reactivity Violations

3. Conditional Rendering

Note: Ternaries work fine in JSX - the transformer auto-converts them to ifElse(). Both {show ? <Element /> : null} and {ifElse(show, ...)} are valid.

4. Type System

5. Binding

6. Style Syntax

7. Handler Binding

8. Stream/Async

9. LLM Integration

10. Performance

11. Action vs Handler Choice

Key Principle: Prefer action() by default. Use handler() only when you need to bind different data to different handler instantiations.

When to useaction() (the default):

• Handler is specific to this pattern
• Closes over pattern-scope variables (inputs, Writables, computeds, wish results)
• All instantiations use the same closed-over data
• Examples: button clicks, form submissions, modal open/close, state toggles

When to usehandler():

• You need different data bound per instantiation
• Common scenarios:
  • .map() loops where each item needs its own binding
  • Reusable handlers shared across multiple patterns
  • Same handler definition called with different bindings in different places
• Examples: list item clicks, per-row actions in tables, delete buttons per item

Decision Question: Does this handler need different data bound to different instantiations?

• YES → Use handler() at module scope, bind with item-specific data
• NO → Use action() inside pattern body, close over what you need

12. Design Review

Check the domain model quality:

13. Regression Check (for updates only)

When reviewing changes to existing code:

Output Format

## Pattern Review: [filename]

### 1. Module Scope
- [PASS] No handler() inside pattern
- [FAIL] lift() immediately invoked (line 23)
  Fix: Use computed() or move lift to module scope

### 2. Reactivity
- [PASS] [NAME] properly wrapped
- [FAIL] Writable.of(deck.name) uses reactive value (line 15)
  Fix: Initialize empty, set in action()

### 3. Conditional Rendering
- [PASS] Using ifElse() correctly
- [N/A] No conditional rendering

[...continue for all categories...]

### 11. Action vs Handler Choice
- [PASS] Actions used for pattern-specific handlers
- [FAIL] handler() used but not needed for multi-binding (line 45)
  Fix: Convert to action() inside pattern body

### 12. Design Review
- [PASS] Clear entity boundaries
- [WARN] Handler names could be clearer (moveCard vs reorderCard)
- [PASS] Unidirectional data flow

### 13. Regression Check (if updating)
- [PASS] Existing tests pass
- [N/A] No type signature changes

## Summary
- Passed: 22
- Failed: 3
- Warnings: 1
- N/A: 2

## Priority Fixes
1. [Line 15] Writable.of() with reactive value
2. [Line 23] lift() inside pattern
3. [Line 45] Missing $ prefix on binding

Documentation References

• docs/development/debugging/README.md - Error reference table
• docs/development/debugging/gotchas/ - Individual gotcha files
• docs/common/components/COMPONENTS.md - UI components and binding
• docs/common/capabilities/llm.md - LLM integration

Quick Patterns

Correct action() Usage (Default Choice)

// action() inside pattern body - closes over pattern variables
export default pattern<MyInput, MyOutput>(({ items, title }) => {
  const menuOpen = Writable.of(false);

  // Action closes over menuOpen - no binding needed
  const toggleMenu = action(() => menuOpen.set(!menuOpen.get()));

  // Action closes over items - no binding needed
  const addItem = action(() => items.push({ title: title.get() }));

  return {
    [UI]: (
      <>
        <ct-button onClick={toggleMenu}>Menu</ct-button>
        <ct-button onClick={addItem}>Add</ct-button>
      </>
    ),
    items,
  };
});

Correct handler() Usage (Only for Multi-Binding)

// handler() at module scope - will be bound with different items in .map()
const deleteItem = handler<void, { item: Writable<Item>; items: Writable<Item[]> }>(
  (_, { item, items }) => {
    const list = items.get();
    items.set(list.filter(i => i !== item));
  }
);

export default pattern<MyInput, MyOutput>(({ items }) => ({
  [UI]: (
    <ul>
      {items.map((item) => (
        <li>
          {item.name}
          {/* Each item gets its own binding */}
          <ct-button onClick={deleteItem({ item, items })}>Delete</ct-button>
        </li>
      ))}
    </ul>
  ),
  items,
}));

Correct Reactive [NAME]

export default pattern<Input>(({ deck }) => ({
  [NAME]: computed(() => `Study: ${deck.name}`),
  // ...
}));

Correct Conditional Rendering

// Both are valid - ternaries auto-transform to ifElse()
{showDetails ? <div>Details content</div> : null}
{ifElse(showDetails, <div>Details content</div>, null)}

Correct Style Syntax

<div style={{ display: "flex", gap: "1rem" }}>
  <ct-vstack style="flex: 1; padding: 1rem;">
    Content
  </ct-vstack>
</div>

Weekly Installs

43

Repository

commontoolsinc/labs

GitHub Stars

30

First Seen

Jan 21, 2026

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