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#2001 was a nice addition, but now there is a lot of issue reported like this:
const someList: List<Item>;
if (someList.size === 1) {
const item: Item = someList.first();
}This is now reported as an error because .first() might return undefined.
It would be nice to implement NotEmpty collections in that case.
Here is a quick implementation in my codebase:
import { Collection, List, Set } from 'immutable';
export interface NotEmptyCollection<K, V> extends Collection<K, V> {
first(): V;
last(): V;
}
export interface NotEmptyList<T> extends List<T> {
first(): T;
last(): T;
}
export interface NotEmptySet<T> extends Set<T> {
first(): T;
last(): T;
}
export function collectionHasItem<T>(list: List<T>): list is NotEmptyList<T>;
export function collectionHasItem<T>(list: Set<T>): list is NotEmptySet<T>;
export function collectionHasItem<K, T>(
list: Collection<K, T>
): list is NotEmptyCollection<K, T> {
return list.count() > 0;
}
export function collectionHasExactlyOneItem<T>(
list: List<T>
): list is NotEmptyList<T>;
export function collectionHasExactlyOneItem<T>(
list: Set<T>
): list is NotEmptySet<T>;
export function collectionHasExactlyOneItem<K, T>(
list: Collection<K, T>
): list is NotEmptyCollection<K, T> {
return list.count() === 1;
}It would be great for groupBy function too:
someList
.groupBy((i) => i.category)
.map((l: List<Item>) => l.first()) // reported as error, but it can't beMetadata
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