Function reducerFromState

Example

import { createAction, reducerFromState } from '@pacote/flux-actions'

const person = createAction<{ name: string }>('PERSON')
const dog = createAction<{ name: string }>('DOG')
const car = createAction<{ brand: string }>('CAR')

const reducer = reducerFromState({ now: 'None', then: '' })
  .on([person, dog], (state, action) => ({
    now: action.payload.name,
    then: state.now,
  }))
  .on(car, (state, action) => ({
    now: action.payload.brand,
    then: state.now,
  }))

const s2 = reducer(undefined, person({ name: 'Marty McFly' }))
// { now: 'Marty McFly', then: 'None' }
const s3 = reducer(s2, dog({ name: 'Einstein' }))
// { now: 'Einstein', then: 'Marty McFly' }
const s4 = reducer(s3, car({ brand: 'DeLorean' }))
// { now: 'DeLorean', then: 'Einstein' }
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Reducing actions with errors wrapped in Either can store the error in state while handling a successful payload:

import { type Either, tryCatch } from 'fp-ts/lib/Either'

type State = { year: number; error?: Error }
const changeYear = createAction<Either<Error, number>>('CHANGE_YEAR')
const reducer = reducerFromState<State>({ year: 1985 }).on(
  changeYear,
  (state, { payload }) =>
    payload.fold(
      (error) => ({ ...state, error }),
      (year) => ({ year, error: undefined }),
    ),
)

reducer(undefined, changeYear(tryCatch(() => 2025, (error) => error as Error)))
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