fp-types-ref – Other skill for AI agents

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'        // 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

// 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/).