fp-ts Quick Reference

When to Use

You need help choosing between Option, Either, Task, TaskEither, or related fp-ts types.
The task is about imports, decision guidance, or selecting the right abstraction for a TypeScript flow.
You want a compact reference for common fp-ts type choices and patterns.

Which Type Should I Use?

Is the operation async?
├─ NO: Does it involve errors?
│   ├─ YES → Either<Error, Value>
│   └─ NO: Might value be missing?
│       ├─ YES → Option<Value>
│       └─ NO → Just use the value
└─ YES: Does it involve errors?
    ├─ YES → TaskEither<Error, Value>
    └─ NO: Might value be missing?
        ├─ YES → TaskOption<Value>
        └─ NO → Task<Value>

Common Imports

// Core
import { pipe, flow } from 'fp-ts/function'

// Types
import * as O from 'fp-ts/Option'      // Maybe exists
import * as E from 'fp-ts/Either'      // Success or failure
import * as TE from 'fp-ts/TaskEither' // Async + failure
import * as T from 'fp-ts/Task'        // Async (no failure)
import * as A from 'fp-ts/Array'       // Array utilities

One-Line Patterns

Need	Code
Wrap nullable	`O.fromNullable(value)`
Default value	`O.getOrElse(() => default)`
Transform if exists	`O.map(fn)`
Chain optionals	`O.flatMap(fn)`
Wrap try/catch	`E.tryCatch(() => risky(), toError)`
Wrap async	`TE.tryCatch(() => fetch(url), toError)`
Run pipe	`pipe(value, fn1, fn2, fn3)`

Pattern Match

// Option
pipe(maybe, O.match(
  () => 'nothing',
  (val) => `got ${val}`
))

// Either
pipe(result, E.match(
  (err) => `error: ${err}`,
  (val) => `success: ${val}`
))

Limitations

Use this skill only when the task clearly matches the scope described above.
Do not treat the output as a substitute for environment-specific validation, testing, or expert review.
Stop and ask for clarification if required inputs, permissions, safety boundaries, or success criteria are missing.

fp-types-ref

Documentation

fp-ts Quick Reference

When to Use

Which Type Should I Use?

Common Imports

One-Line Patterns

Pattern Match

Limitations

Use Cases

Quick Info

Tags

Related Skills

acceptance-orchestrator

accessibility-compliance-accessibility-audit

accesslint-diff