Referential transparent lazy composition.

Abstract

Композиция функций это чистая функция, которая зависит только от аргументов. Таким образов композируя одни и те же функции в одинаковом порядке мы должны получить идентичную фукнцию .

Motivation

Частично вопрос идентичности композиций можно решить мемоизацией аргументов. Однако популярные реализации композиции функций не решают вопроса ассоциативной идентичности композиции.

кроме этого, рекурсивная реализация функциональной композиции создаёт оверхед при создании дополнительных элементов стека вызовов. При вызове композиции 5-ти и более функции это хорошо заметно.

Solution

Create monoid (or Semigroppoid & Category) in terms of fantasy-land for function composition. Laws:

import compose, {identity} from './src/lazyCompose'
import {add} from 'ramda'
const a = add(1)
const b = add(2)
const c = add(3)
compose(a, compose(b, c)) === compose(compose(a, b), c) // associativity

compose(a, identity) === a  //right identity
compose(identity, a) === a  //left identity

Use cases

1. Advantages of using with redux mapStateToProps and selectors. Because composition of same function returns the same function

2. Lens composition. You can create the same lenses by different composition ways. You can calculate lenses on a fly with result function equality. In this example I memoize lens constructor to return the same functions for same arguments

   import {lensProp, memoize} from 'ramda'
   import compose from './src/lazyCompose'
      
   const constantLens = memoize(lensProp)
   const lensA = constantLens('a')
   const lensB = constantLens('b')
   const lensC = constantLens('c')
   const lensAB = compose(lensB, lensA)
      
   console.log(
       compose(lensC, lensAB) === compose(lensC, lensB, lensA)
   )
          

3. Memoized react callbacks directly composable with dispatch. In the following example elements of the list will not rerender with out changing id

   // constant - returns memoized identity function for an argument 
   // just like React.useCallback
   import {compose, constant} from './src/lazyCompose'
      
   const List = ({dispatch, data}) =>
       data.map( id =>
           <Button
               key={id}
               onClick={compose(dispatch, makeAction, contsant(id)) 
           />
       )
   )
          
   const Button = React.memo( props => 
       <button {...props} />
   )
          
   const makeAction = payload => ({
       type: 'onClick',
       payload,
   })
      

4. Lazy composition of react components with no extra HOCs. lazy composition folds plain list of functions, with no additional closures

   import {memoize, mergeRight} from 'ramda'
   import {constant, compose} from './src/lazyCompose'
      
      
   const defaultProps = memoize(mergeRight)
      
   const withState = memoize( defaultState =>
       props => {
           const [state, setState] = React.useState(defaultState)
           return {...props, state, setState}
       }
   )
   
   const Component = ({value, label, ...props)) => 
       <label {...props}>{label} : {value}</label>    
          
   const withCounter = compose(  
       ({setState, state, ...props}) => ({
           ...props
           value: state,
           onClick: compose(setState,  constant(state + 1))
       }),
       withState(0),
   )   
   const Counter = compose(
       Component, 
       withCounter,
       defaultProps({label: 'Clicks'}),
   )
   

5. Lazy Monads, where map - is a binary lazy composition, with possibility of strict equality implementation