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.
