filter over Map

For ramda/ramda#3507

Author: davetapley

test/filter.test.ts

@@ -90,3 +90,8 @@ expectType<(number | undefined)[]>(filter(x => x != null, [] as (number | undefi
 // filter(() => narrow, dist)
 // no need for type annotations when using full signature
 expectType<Record<string, string>>(filter(isNotNil, {} as Dict));
+
+
+// filter(() => narrow)(dist)
+expectType<Map<string, string>>(filter(isNotNil, new Map() as Map<string, string | undefined>));
+expectType<Map<string, number>>(filter(gt5, new Map() as Map<string, number>));

types/filter.d.ts

@@ -10,11 +10,16 @@ export function filter<A, P extends A>(
   <B extends A>(list: readonly B[]): P[];
 };
 
-// filter(() => boolean)
+// filter(() => boolean, list)
 export function filter<T>(
   pred: (value: T) => boolean,
 ): <P extends T, C extends readonly P[] | Record<string, P>>(collection: C) => C;
 
+// filter(() => narrow, map)
+export function filter<K, T, P extends T>(pred: (val: T) => val is P, map: readonly Map<K, T>): Map<K, P>;
+// filter(() => boolean, map)
+export function filter<K, T>(pred: (val: T) => boolean, map: readonly Map<K, T>): Map<K, T>;
+
 // filter(() => narrow, list) - readonly T[] falls into Record<string T> for some reason, so list needs to come first
 export function filter<T, P extends T>(pred: (val: T) => val is P, list: readonly T[]): P[];
 // filter(() => narrow, dist)