From 3a70457266024a1fa873a618fe55330951a09e0e Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Lazar=20Ljubenovi=C4=87?= <lazar.ljubenovic.1995@gmail.com>
Date: Sat, 6 Jun 2020 18:47:14 +0200
Subject: [PATCH] fix: improve typings for the filter operator (#20)

Co-authored-by: Felix Becker <felix.b@outlook.com>
---
 src/filter.ts | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/src/filter.ts b/src/filter.ts
index 40b3228..cede035 100644
--- a/src/filter.ts
+++ b/src/filter.ts
@@ -1,12 +1,15 @@
-export class FilterIterator<T> implements Iterator<T> {
-    constructor(private source: Iterator<T>, private predicate: (element: T) => boolean) {}
+export class FilterIterator<T, V extends T = T> implements Iterator<T> {
+    constructor(
+        private source: Iterator<T>,
+        private predicate: ((element: T) => element is V) | ((element: T) => boolean)
+    ) {}
 
-    next(): IteratorResult<T> {
+    next(): IteratorResult<V> {
         let result: IteratorResult<T>
         // Skip elements until predicate returns true
         do {
             result = this.source.next()
         } while (!result.done && !this.predicate(result.value))
-        return result
+        return result as IteratorResult<V>
     }
 }