2017-02-04

actionsTypeCheckingExercise.js

import { createType } from './type'
import type {Data, Type, TypeChecker} from './type'

type IncT = Type<'Inc', number>
type DecT = Type<'Dec', number>
type ResetT = Type<'Reset', number>
type WhatT = Type<'What', number>
const IncAction: Data<IncT, number> = createType('Inc')
const DecAction: Data<DecT, number> = createType('Dec')
const ResetAction: Data<ResetT, number> = createType('Reset')
const WhatAction: Data<WhatT, number> = createType('What')

type ActionT = IncT | DecT | ResetT
const id: TypeChecker<ActionT> = x => x

const match = createMatch(id)
const cata = match({
  Inc: x => x+1,
  Dec: x => x-1,
  Reset: x => 0
})

console.log(cata(IncAction.of(10)))
console.log(cata(DecAction.of(10)))
console.log(cata(ResetAction.of(null)))
console.log(cata(WhatAction.of(100)))

howToParametrizeAFunctionWithATypeInFlow.js

type IncT = {
  '@@type': 'Inc',
  '@@value': number
}

const inc = {
  '@@type': 'Inc',
  '@@value': 1
}

const match = typeChecker => x => typeChecker(x)['@@type']

// These two are just to abstract away the fact that we are passing the value through 
// the identity function just so that flow is coerced into type checking it
// it could be written more tersely as:
// const incTypeChecker: (a: Inct) => IncT = x => x
type TypeCheckerT<A> = (x: A) => A
const typeChecker = x => x

const incTypeChecker: TypeCheckerT<IncT> = typeChecker

const b = match(incTypeChecker)(inc) // type checks!

const c = match(incTypeChecker)('not an Inc') // fails!

error is still weird:

const hardEqualitySetoidId: TypeChecker<HardEqualitySetoidT> = x => x
const setoid = (equals: Equals<any>) => (createType) => (name) => ((type) => ({
  ...type,
  of: x => ({
    ...type.of(x),
    equals: y => equals(x)(hardEqualitySetoidId(y))
  })
}))(createType(name))

type HardEqualitySetoidT = Setoid<'HardEqualitySetoid', any>

const hardEqualityEquals: Equals<any> = x => y => (z => x === z)(hardEqualitySetoidId(y))

const HardEqualitySetoid: Data<HardEqualitySetoidT, any> = setoid(hardEqualityEquals)(createType)('HardEqualitySetoid')

const hardEqual2 = HardEqualitySetoid.of(2)

console.log(hardEqual2.equals(2))

// hardEqualitySetoidId(2)