Update Function Types

Provides a specification and basic type classes for update function. Essentially there are two kinds of update functions —

1. ReducerFunction: takes in action and state and returns a new state.
2. CommandFunction: takes in action and state and return a new action.

Index

• Types
  • ReducerFunction
  • CommandFunction
• Library Functions
  • concatR
  • concatC
  • matchR
  • matchC
  • zeroR
  • zeroC

Types

Reducer Function

1. Takes two arguments viz. Value and State.
2. Always return a State.
3. Does not mutate the original objects provided.

export type ReducerFunction<Value, State> = {
  (value: Value, state: State): State
}

Command Function

1. Takes two arguments only viz. Value and State.
2. Always returns an Action. It can consider returning Nil if no meaningful value is intended.
3. Does not mutate the original objects provided.

export type CommandFunction<Value, State, Output> = {
  (value: Value, state: State): Action<Output>
}

Library Functions

concatR

Takes in multiple ReducerFunction(s) as arguments and returns a new ReducerFunction as a result.

1. Consider the ReducerFunction(s) R0 R1 then two statements will be logically equivalent —
   1. concatR(R0, R1)(action, state)
   2. R1(action, R0(action, state))
2. Its associative — concatR(concatR(R0, R1), R2) === concatR(R0, concatR(R1, R2))
3. Its additive — concatR(R0, zeroR) === R0.

Usage

import {concatR} from 'update-function-type'

const R0 = (a, b) => a + b
const R1 = (a, b) => a * b

concatR(R0, R1)(10, 20) // 10 * (10 + 20) === 300

concatC

Takes in multiple CommandFunction(s) as arguments and returns a new CommandFunction as a result.

1. Consider the CommandFunction(s) C0 C1 then two statements will be logically equivalent —
   1. concatC(C0, C1)(action, state)
   2. List(C0(action, state), C1(action, state))
2. Its associative — concatC(concatC(C0, C1), C2) === concatC(C0, concatC(C1, C2))
3. Its additive — concatC(C0, zeroC) === C0

Usage

import {concatC} from 'update-function-type'

const C0 = (a, b) => action('+', a + b)
const C1 = (a, b) => action('*', a * b)

concatC(C0, C1)(10, 500) // List(action('+', 510), action('B', 5000))

matchR

1. Takes in specification object with keys as action types and values as ReducerFunction(s) and returns another ReducerFunction.
2. The returned function when called with an Action and State internally calls the function matching the action's type.
3. The matching reducer function is called with value of the original action and the state is passed as is.
4. In case no action matches the given specification the original state is returned.

Usage

import {matchR} from 'update-function-type'

const reducer = matchR({
  add: (a, b) => a + b,
  mul: (a, b) => a * b
})

reducer(action('add', 10), 1000) // returns 1010

matchC

1. Takes in specification object with keys as action types and values as CommandFunction(s) and returns another CommandFunction.
2. The returned function when called with an Action and State internally calls the function matching the action's type.
3. The matching reducer function is called with value of the original action and the state is passed as is.
4. In case no action matches the given specification Nil is returned.

Usage

import {matchC} from 'update-function-type'

const reducer = matchR({
  add: (a, b) => action('added', a + b),
  mul: (a, b) => action('multiplied', a * b)
})

reducer(action('add', 10), 1000) // returns action('added', 1010)

zeroR

1. Is a ReducerFunction.
2. Takes in an action and a state and returns the same state.

zeroC

1. Is a CommandFunction.
2. Takes in an action and a state and returns the same action.