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Monad transformer example (OptionT), Flow and fp-ts, npm install fp-ts
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// @flow | |
import type { Option } from 'fp-ts/lib/Option' | |
import type { Lazy } from 'fp-ts/lib/function' | |
import type { Monad } from 'fp-ts/lib/Monad' | |
import * as optionT from 'fp-ts/lib/OptionT' | |
import { array } from 'fp-ts/lib/Array' | |
// If M<A> is a monad then M<Option<A>> is a monad | |
const optionTArray = optionT.getOptionT(array) | |
export type URIT = <U, L, A>(x: [U, L, A]) => ArrayOption<A> | |
export class ArrayOption<A> { | |
+value: Array<Option<A>>; | |
constructor(value: Array<Option<A>>) { | |
this.value = value | |
} | |
map<B>(f: (a: A) => B): ArrayOption<B> { | |
return new ArrayOption(optionTArray.map(f, this.value)) | |
} | |
ap<B>(fab: ArrayOption<(a: A) => B>): ArrayOption<B> { | |
return new ArrayOption(optionTArray.ap(fab.value, this.value)) | |
} | |
chain<B>(f: (a: A) => ArrayOption<B>): ArrayOption<B> { | |
return new ArrayOption(optionTArray.chain(a => f(a).value, this.value)) | |
} | |
getOrElseValue(a: A): Array<A> { | |
return optionT.getOrElseValue(array)(a)(this.value) | |
} | |
fold<R>(none: Lazy<R>, some: (a: A) => R): Array<R> { | |
return optionT.fold(array)(none, some, this.value) | |
} | |
} | |
const map = <A, B>(f: (a: A) => B, fa: ArrayOption<A>): ArrayOption<B> => fa.map(f) | |
const of = <A>(a: A): ArrayOption<A> => new ArrayOption(optionT.some(array)(a)) | |
const ap = <A, B>(fab: ArrayOption<(a: A) => B>, fa: ArrayOption<A>): ArrayOption<B> => fa.ap(fab) | |
const chain = <A, B>(f: (a: A) => ArrayOption<B>, fa: ArrayOption<A>): ArrayOption<B> => fa.chain(f) | |
export const some = of | |
export const none: ArrayOption<any> = new ArrayOption(optionT.none(array)()) | |
export const fromOption = <A>(oa: Option<A>): ArrayOption<A> => new ArrayOption(optionT.fromOption(array)(oa)) | |
export const liftF = <A>(ma: Array<A>): ArrayOption<A> => new ArrayOption(optionT.liftF(array)(ma)) | |
export interface Instances | |
extends Monad<URIT> {} | |
export const arrayOption: Instances = { | |
map, | |
of, | |
ap, | |
chain | |
} | |
console.log(liftF([1, 2, 3]).map(n => n * 2)) | |
/* | |
[some(2), some(4), some(6)] | |
*/ |
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