Function: map()
Call Signature
map<
T,U>(mapFn): (maybe) =>Maybe<U>
Map over a Maybe instance: apply the function to the wrapped value if the instance is Just, and return Nothing if the instance is Nothing.
map works a lot like Array.prototype.map: Maybe and Array are both containers for other things. If you have no items in an array of numbers named foo and call foo.map(x => x + 1), you'll still just have an array with nothing in it. But if you have any items in the array ([2, 3]), and you call foo.map(x => x + 1) on it, you'll get a new array with each of those items inside the array "container" transformed ([3, 4]).
That's exactly what's happening with map. If the container is empty – the Nothing variant – you just get back an empty container. If the container has something in it – the Just variant – you get back a container with the item inside transformed.
(So... why not just use an array? The biggest reason is that an array can be any length. With a Maybe, we're capturing the idea of "something or nothing" rather than "0 to n" items. And this lets us implement a whole set of other interfaces, like those in this module.)
Examples
const length = (s: string) => s.length;
const justAString = Maybe.just('string');
const justTheStringLength = map(length, justAString);
console.log(justTheStringLength.toString()); // Just(6)
const notAString = Maybe.nothing<string>();
const notAStringLength = map(length, notAString);
console.log(notAStringLength.toString()); // "Nothing"Type Parameters
T
T extends object
The type of the wrapped value.
U
U extends object
The type of the wrapped value of the returned Maybe.
Parameters
mapFn
(t) => U
The function to apply the value to if Maybe is Just.
Returns
A function accepting a Maybe<T>, which will produce Maybe<U> after applying mapFn.
(
maybe):Maybe<U>
Parameters
maybe
Maybe<T>
Returns
Maybe<U>
Call Signature
map<
T,U>(mapFn,maybe):Maybe<U>
Map over a Maybe instance: apply the function to the wrapped value if the instance is Just, and return Nothing if the instance is Nothing.
map works a lot like Array.prototype.map: Maybe and Array are both containers for other things. If you have no items in an array of numbers named foo and call foo.map(x => x + 1), you'll still just have an array with nothing in it. But if you have any items in the array ([2, 3]), and you call foo.map(x => x + 1) on it, you'll get a new array with each of those items inside the array "container" transformed ([3, 4]).
That's exactly what's happening with map. If the container is empty – the Nothing variant – you just get back an empty container. If the container has something in it – the Just variant – you get back a container with the item inside transformed.
(So... why not just use an array? The biggest reason is that an array can be any length. With a Maybe, we're capturing the idea of "something or nothing" rather than "0 to n" items. And this lets us implement a whole set of other interfaces, like those in this module.)
Examples
const length = (s: string) => s.length;
const justAString = Maybe.just('string');
const justTheStringLength = map(length, justAString);
console.log(justTheStringLength.toString()); // Just(6)
const notAString = Maybe.nothing<string>();
const notAStringLength = map(length, notAString);
console.log(notAStringLength.toString()); // "Nothing"Type Parameters
T
T extends object
The type of the wrapped value.
U
U extends object
The type of the wrapped value of the returned Maybe.
Parameters
mapFn
(t) => U
The function to apply the value to if Maybe is Just.
maybe
Maybe<T>
The Maybe instance to map over.
Returns
Maybe<U>
A new Maybe with the result of applying mapFn to the value in a Just, or Nothing if maybe is Nothing.