fp-refactor

What it does

This skill provides comprehensive patterns and strategies for migrating existing imperative TypeScript code to fp-ts functional programming patterns.

How it works

• When refactoring, it suggests converting try/catch blocks to Either or TaskEither, including synchronous and asynchronous examples, and provides a step-by-step refactoring guide:
  1. Identify the error type
  2. Change return type to Either<E, T>
  3. Replace throw statements with E.left(...)
  4. Replace returns with E.right(...)
  5. Remove try-catch blocks
  6. Update callers using pipe with E.flatMap
• It shows converting null/undefined checks to Option, including wrapping nullable values with fromNullable, changing return types to Option, and using O.map/O.flatMap, with explicit boundary handling via getOrElse or match.
• It covers converting callbacks to Task, including Node-style callbacks to Task, and wrapping fs/promises in TaskEither with examples for reading and writing files, and composing multiple files.
• It includes patterns for converting class-based DI to Reader, and converting imperative loops to functional operations, though the provided excerpt focuses on the above three areas.
• It provides concrete code patterns: functions like parseJSON to Either, fetchUser to TaskEither, getDatabaseUrl to Option, findUserById to Option, readFile/writeFile to TaskEither, and multi-file processing via traverse.

When to use it

• You are refactoring an existing imperative TypeScript codebase toward fp-ts patterns.
• The task involves converting try/catch, null checks, callbacks, DI, or loops into functional equivalents.
• You need migration guidance and tradeoffs, not just isolated fp-ts examples.

What it can touch

• The skill references and demonstrates touchpoints with:
  • fp-ts/Either, fp-ts/TaskEither, fp-ts/Option
  • pipe from fp-ts/function
  • Specific utilities: TE.tryCatch, TE.flatMap, TE.map, E.left, E.right, O.fromNullable, O.map, O.flatMap, O.match, A.findFirst, A.traverse
  • File system APIs: fs.readFile, fs.writeFile (demonstrated via fs/promises in examples)

Caveats

• Declared risk: critical
• The skill provides patterns and examples; outcomes depend on correct application to actual codebases
• It emphasizes not to speculate beyond the coded patterns and to follow the step-by-step guides shown for refactoring