fp-taskeither-ref_skill

Overview

This skill is a compact quick reference for using TaskEither from fp-ts to handle async operations that can fail. It highlights creation, transformation, execution patterns, and common use cases like API calls, parallel requests, and typed error recovery. Designed for practical, copy-paste-ready patterns without jargon.

How this skill works

TaskEither represents a lazy async computation that yields either an error (Left) or a value (Right), effectively Promise<Either<E, A>>. The skill shows how to construct TaskEither values (right, left, tryCatch), transform them (map, mapLeft, flatMap, orElse), and execute them by calling the TaskEither or using pattern matching helpers. It also demonstrates sequencing in parallel and recovering from failures while keeping errors typed and composable.

When to use it

• Performing HTTP or database calls where failures must be tracked explicitly
• Composing multiple async steps while preserving typed error information
• Recovering from failures with fallback values or alternative tasks
• Running several independent async tasks in parallel and combining results
• Avoiding lost error context that often happens with try/catch and null returns

Best practices

• Use TE.tryCatch to wrap Promises and provide a deterministic error type
• Prefer TE.flatMap for sequencing tasks that return TaskEither, TE.map for pure transforms
• Keep error shapes consistent across your domain to simplify handling and mapping
• Use TE.orElse to handle recoverable errors and TE.getOrElse at the edge to convert to a plain value
• Leverage ApplyPar or sequenceT(TE.ApplyPar) for parallel independent requests

Example use cases

• Wrap fetch calls and parse JSON with separate network and parse error types
• Chain user fetch → posts fetch → transform posts count with TE.flatMap and TE.map
• Perform user, posts, and comments requests in parallel and combine the results
• Provide a default object on API failure using TE.orElse(() => TE.right(defaultValue))
• Convert an existing Either into a TaskEither when mixing sync validation and async IO