fp-refactor_skill

Overview

This skill is a practical guide for refactoring imperative TypeScript into fp-ts functional patterns. It focuses on concrete migrations—try/catch to Either/TaskEither, null checks to Option, callbacks to Task, class DI to Reader, and Promise/loop refactors—so you get safer, composable code without academic jargon.

How this skill works

The guide inspects common imperative idioms and provides step-by-step patterns and code transforms using fp-ts primitives (Either, TaskEither, Option, Task, Reader, Array helpers). It shows how to change return types, replace throws/nulls/callbacks, and compose operations with pipe and flatMap. It also includes helper utilities and migration strategies for gradual adoption.

When to use it

• Replacing nested try-catch and thrown errors with explicit error types
• Making nullable or optional data explicit and composable
• Converting Node-style callbacks and Promise chains into composable tasks
• Refactoring class-based dependency injection to pure Readers for testability
• Rewriting imperative loops and finds into fp-ts Array/Option pipelines

Best practices

• Start at boundaries: convert incoming nullable values and I/O to Option/Either/TaskEither
• Define small error types and propagate them explicitly rather than throwing generic Errors
• Prefer pipe + flatMap/map for composition so errors and None propagate automatically
• Provide thin adapters at non-fp boundaries (getOrElse, match, toNullable) instead of spreading conversions
• Migrate incrementally: wrap existing functions with tryCatch helpers and progressively replace call sites

Example use cases

• Turn JSON.parse + validate + nested try-catch into Either pipelines with explicit error types
• Wrap fetch and multiple network calls into TaskEither with automatic error propagation and sequencing
• Replace deep optional property checks in config objects with nested Option combinators and getOrElse fallbacks
• Convert Node callback-based file operations to Task/TaskEither and compose batch processing without callback hell
• Replace imperative array.find + null checks with fp-ts Array.findFirst and Option.map chains