Chapter 09: Monadic Onions
book
ts
// safeProp :: Key -> {Key: a} -> Maybe a
const safeProp = curry((x, obj) => Maybe.of(obj[x]));
// safeHead :: [a] -> Maybe a
const safeHead = safeProp(0);
// firstAddressStreet :: User -> Maybe Street
const firstAddressStreet = compose(
join,
map(safeProp('street')),
join,
map(safeHead),
safeProp('addresses'),
);
// firstAddressStreet :: User -> Maybe Street
const firstAddressStreetChained = compose(
chain(safeProp('street')),
chain(safeHead),
safeProp('addresses'),
);
firstAddressStreet({
addresses: [{ street: { name: 'Mulburry', number: 8402 }, postcode: 'WC2N' }],
});
// Maybe({name: 'Mulburry', number: 8402})
import { head } from "fp-ts/Array";
import { flow } from "fp-ts/function";
import * as O from "fp-ts/Option";
import { safeProp } from '../utils';
type Street = { name: string; number: number };
type AddressBook = {
addresses: {
street: Street;
postcode: string;
}[];
};
// safeHead :: [a] -> Maybe a
const safeHead = head;
// firstAddressStreet :: User -> Maybe Street
export const firstAddressStreet: (addressBook: AddressBook) => O.Option<Street> = flow(
safeProp("addresses"),
O.map(safeHead),
O.flatten,
O.map(safeProp("street")),
O.flatten,
);
// firstAddressStreet :: User -> Maybe Street
export const firstAddressStreetChained: (addressBook: AddressBook) => O.Option<Street> = flow(
safeProp("addresses"),
O.chain(safeHead),
O.chain(safeProp("street")),
);
firstAddressStreet({
addresses: [{ street: { name: "Mulburry", number: 8402 }, postcode: "WC2N" }],
});
// Option({name: 'Mulburry', number: 8402})
Last modified 2yr ago