fp-ts Quick Reference

---
name: fp-types-ref
description: Quick reference for fp-ts types. Use when user asks which type to use, needs Option/Either/Task decision help, or wants fp-ts imports.
risk: safe
source: community
date_added: "2026-03-06"
version: 1.0.0
tags: [fp-ts, typescript, quick-reference, option, either, task]
---

# 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

```typescript
// 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'        // Lazy async; no typed error channel
import * as A from 'fp-ts/Array'       // Array utilities
```

## One-Line Patterns

| Need | Code |
|------|------|
| Wrap nullable | `O.fromNullable(value)` |
| Default value | `O.getOrElse(() => fallbackValue)` |
| 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

```typescript
// 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.

## Reference scope

These are fp-ts 2.x fragments. Supply application variables, types, validators and imports, and check the installed version. `map` does not catch thrown exceptions. `Task` has no typed error channel; an improperly wrapped promise can still reject. Use `tryCatch` at fallible boundaries.

Official API: [fp-ts modules](https://gcanti.github.io/fp-ts/modules/).