Type Safety Issue: Preset interface allows invalid states (filters and symbols both optional)

[bearmug]

Problem

The current Preset interface has a critical type safety issue where both filters and symbols are optional:

export interface Preset {
  name: string;
  description: string;
  filters?: Array<{...}>;
  symbols?: string[];
  // ...
}

This design allows invalid states:

• ❌ Both filters and symbols could be undefined (invalid preset)
• ❌ Both could be defined simultaneously (ambiguous behavior)
• ❌ No TypeScript enforcement of mutual exclusivity

Impact

• Type safety degradation: Loses compile-time guarantees about preset validity
• Runtime errors: Code must defensively check which field is present
• Maintenance burden: Easy to create invalid presets accidentally
• Unclear intent: Not obvious which presets are filter-based vs symbol-based

Proposed Solution

Use a discriminated union pattern to enforce mutual exclusivity:

export type Preset = {
  name: string;
  description: string;
  markets?: string[];
  sort_by?: string;
  sort_order?: "asc" | "desc";
  columns?: string[];
} & (
  | { type: 'filter'; filters: Array<{...}> }
  | { type: 'lookup'; symbols: string[] }
);

Benefits:

• ✅ TypeScript enforces exactly one of filters or symbols is present
• ✅ Type narrowing works automatically based on type discriminator
• ✅ Clear intent in code (preset.type === 'filter' vs preset.type === 'lookup')
• ✅ Industry best practice for representing mutually exclusive options

Additional Considerations

• Verify PresetsTool correctly routes symbol-based presets to lookupSymbols function
• Update tests to reflect discriminated union pattern
• Migration path for existing preset definitions

References

• Discovered in PR Feat/multi asset screening #19 review: Feat/multi asset screening #19 (comment)
• Related files:
  • src/resources/presets.ts - Preset interface definition
  • src/tests/presets.test.ts - Preset validation tests

Priority

High - Type safety issues can lead to runtime errors and maintenance problems. Should be addressed before merging lookup_symbols feature.