Functions are functors

functions-are-functors.ts

import * as React from 'react'
import { compose } from 'redux'
import { reader } from 'fp-ts/lib/Reader'


type StateContext<A> = [A, (a: A) => void]

const keepMax: (input: StateContext<number>) => StateContext<number> =
  ([state, setState]) => {
    const keepMaxSetState = React.useCallback((n: number) => {
      if(n > state) {
        setState(n)
      }
    }, [state])
    return [state, keepMaxSetState];
  }

// small brain
const [max, trackMax] = keepMax(React.useState(0))

// large brain
const [max2, trackMax2] = compose(keepMax, React.useState)(0)

// galaxy brain
const [max3, trackMax3] = reader.map(React.useState, keepMax)(0)