Added mapInto function

README.md

@@ -219,6 +219,22 @@ $doubles = $collection->map(function ($value) {
 });
 ```
 
+### Map Into
+The `mapInto()` method allows you to take any collection and in one call create
+a new collection of a given type, run a callable across the data, and have it
+immediately collected into the new collection.
+
+Here's an example where we have created new collections that are restricted to
+allowing only integers to be collected in the first, and strings in the second:
+```php
+$ints = new IntegerCollection([1, 2, 3, 4, 5]);
+$strings = $ints->mapInto(function (int $item): string {
+    return (string)($item*10);
+}, StringCollection::class);
+```
+We now have a `StringCollection` which contains a string representation of
+every value from the original `$ints` collection, multiplied by 10.
+
 ### Each
 Sometimes we need to iterate over values, but we don't want to make any
 changes. That is where `each()` is different to `map()`. Otherwise, the

src/Collection.php

@@ -146,21 +146,45 @@ private function checkType($value, ?string $type)
     /**
      * Copy entries into a new collection
      *
-     * @param string $collection The collection type to copy into
+     * @param string $type The collection type to copy into
      * @return self
      * @throws TypeError
      */
-    public function into(string $collection): Collection
+    public function into(string $type): Collection
     {
-        if (!class_exists($collection)) {
-            throw new TypeError(sprintf('Unknown class name "%s"', $collection));
+        return self::newCollectionOfType($type, $this->getArrayCopy());
+    }
+
+    /**
+     * Map collection into a new collection of a given type
+     *
+     * @param callable $callable
+     * @param string $type
+     * @return Collection
+     */
+    public function mapInto(callable $callable, string $type): self
+    {
+        return self::newCollectionOfType($type, array_map($callable, $this->getArrayCopy()));
+    }
+
+    /**
+     * Get a new collection of a given type
+     *
+     * @param string $type The collection type that you want an instance of
+     * @param array $items The items that you want to collect immediately (defaults to nothing)
+     * @return self
+     */
+    public static function newCollectionOfType(string $type, $items = []): Collection
+    {
+        if (!class_exists($type)) {
+            throw new TypeError(sprintf('Unknown class name "%s"', $type));
         }
-        if (!is_subclass_of($collection, Collection::class) &&
-            Collection::class !== $collection
+        if (!is_subclass_of($type, Collection::class) &&
+            Collection::class !== $type
         ) {
-            throw new TypeError(sprintf('Class "%s" is not a Collection type', $collection));
+            throw new TypeError(sprintf('Class "%s" is not a Collection type', $type));
         }
-        return new $collection($this->getArrayCopy());
+        return new $type($items);
     }
 
     /**

tests/CollectionTest.php

@@ -538,6 +538,46 @@ public function failToCollectIntoANonExitingType()
         $collection->into('my arbitrary type');
     }
 
+    /**
+     * @test
+     */
+    public function mapInto()
+    {
+        $stringCollection = new class extends Collection
+        {
+            protected $valueType = 'string';
+        };
+
+        $intCollection = new class([1,2,3,4,5]) extends Collection
+        {
+            protected $valueType = 'integer';
+        };
+
+        $finalCollection = $intCollection->mapInto(function (int $int): string {
+            return (string) ($int * 10);
+        }, get_class($stringCollection));
+
+        $finalCollection->each(function ($value, $key) use ($intCollection) {
+            $this->assertSame((string) ($intCollection[$key] * 10), $value);
+        });
+    }
+
+    /**
+     * @test
+     * @expectedException \TypeError
+     */
+    public function mapIntoThrows()
+    {
+        $intCollection = new class([1,2,3,4,5]) extends Collection
+        {
+            protected $valueType = 'integer';
+        };
+
+        $intCollection->mapInto(function (int $int): string {
+            return (string) ($int * 10);
+        }, get_class($intCollection));
+    }
+
     /**
      * @dataProvider collectibles
      * @param $data