Home

• Option
• TaskEither

FAQ

What does the suffix W mean? (example chainW)

The W suffix (short for Widening) signals that the environment types and the error types will be merged.

import * as E from 'fp-ts/Either'
import { pipe } from 'fp-ts/function'

const e1: E.Either<string, number> = E.right(1)
const e2: E.Either<number, number> = E.right(2)

export const result1 = pipe(
  // @ts-expect-error
  e1,
  E.chain(() => e2)
)

// merged error types -----v-------------v
// const result2: E.Either<string | number, number>
export const result2 = pipe(
  e1, // no error
  E.chainW(() => e2)
)

import * as R from 'fp-ts/Reader'

const r1: R.Reader<{ a: string }, number> = R.of(1)
const r2: R.Reader<{ b: number }, number> = R.of(2)

// merged environment types v----------------------------v
// const result3: R.Reader<{ a: string; } & { b: number; }, number>
export const result3 = pipe(
  r1,
  R.chainW(() => r2)
)

What does the suffix K mean? (example chainEitherK)

The K suffix (short for Kleisli) signals that the function will be automatically lifted

import * as TE from 'fp-ts/TaskEither'
import * as E from 'fp-ts/Either'
import { pipe } from 'fp-ts/function'

declare const getEither: (n: number) => E.Either<string, boolean>

const te: TE.TaskEither<string, number> = TE.right(1)

export const result1 = pipe(
  te,
  TE.chain((n) => TE.fromEither(getEither(n)))
)

// chainEitherK(f) = chain + f + fromEither
export const result2 = pipe(te, TE.chainEitherK(getEither))

Note that K and W suffixes can be combined, example chainEitherKW

import * as TE from 'fp-ts/TaskEither'
import * as E from 'fp-ts/Either'
import { pipe } from 'fp-ts/function'

declare const getEither: (n: number) => E.Either<number, boolean>

const te: TE.TaskEither<string, number> = TE.right(1)

export const result1 = pipe(
  // @ts-expect-error
  te,
  TE.chainEitherK(getEither)
)

// const result2: TE.TaskEither<string | number, boolean>
export const result2 = pipe(te, TE.chainEitherKW(getEither))