Guide fp-ts Type Choices and Patterns

This skill is a compact decision reference for fp-ts types in TypeScript. It gives a decision tree for choosing between Option, Either, Task, TaskEither, and TaskOption based on whether an operation is async and whether it can fail or return a missing value: synchronous with possible errors uses Either<Error, Value>, synchronous with a possibly-missing value uses Option<Value>, async with possible errors uses TaskEither<Error, Value>, async with a possibly-missing value uses TaskOption<Value>, and plain async with neither uses Task<Value>.

Use it when deciding which fp-ts type to reach for, when the task is about fp-ts imports or picking the right abstraction for a TypeScript flow, or when you want a compact reference for common fp-ts type choices and one-line patterns rather than reading the full fp-ts documentation.

It documents the core imports:

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

plus a table of one-line patterns: wrapping a nullable value with O.fromNullable(value), supplying a default with O.getOrElse(() => default), transforming a present value with O.map(fn), chaining optionals with O.flatMap(fn), wrapping try/catch with E.tryCatch(() => risky(), toError), wrapping an async call with TE.tryCatch(() => fetch(url), toError), and composing functions with pipe(value, fn1, fn2, fn3). It also shows pattern-matching both an Option and an Either with O.match(...) and E.match(...), each branching on the missing/present or error/success case.

Specific to the fp-ts TypeScript library and its Option, Either, Task, TaskEither, and Array modules.

TypeScript developers using fp-ts who need a fast lookup for which type fits a given async, error, or missing-value combination, or a quick reminder of the one-line helper functions, instead of re-deriving the decision from the fp-ts docs each time.