No need to pass key in transformer.

src/Etl.php

@@ -230,7 +230,7 @@ public function execute($input): void
 
             // Transform data
             try {
-                $value = $transformer->transform($key, $value);
+                $value = $transformer->transform($value);
                 $this->dispatch(self::ON_TRANSFORM, $key, $value, $this);
             } catch (\Throwable $exception) {
                 $exception = new TransformException($exception, $key, $value);

src/Loader/LoaderInterface.php

@@ -8,7 +8,6 @@ interface LoaderInterface
     /**
      * Load or pre-load elements.
      *
-     * @param $key
      * @param $value
      */
     public function load($key, $value): void;

src/Transformer/CallableTransformer.php

@@ -20,9 +20,9 @@ public function __construct(callable $callable)
     /**
      * @inheritDoc
      */
-    public function transform($key, $value)
+    public function transform($value)
     {
         $call = $this->callable;
-        return $call($key, $value);
+        return $call($value);
     }
 }

src/Transformer/NullTransformer.php

@@ -7,7 +7,7 @@ class NullTransformer implements TransformerInterface
     /**
      * @inheritDoc
      */
-    public function transform($key, $value)
+    public function transform($value)
     {
         return $value;
     }

src/Transformer/TransformerInterface.php

@@ -8,9 +8,8 @@ interface TransformerInterface
     /**
      * Transform $value.
      *
-     * @param $key
      * @param $value
      * @return mixed - The transformed value
      */
-    public function transform($key, $value);
+    public function transform($value);
 }

tests/src/EtlTest.php

@@ -4,7 +4,7 @@
 
 use BenTools\ETL\Etl;
 use BenTools\ETL\Loader\ArrayLoader;
-use BenTools\ETL\Transformer\CallableValueTransformer;
+use BenTools\ETL\Transformer\CallableTransformer;
 use PHPUnit\Framework\TestCase;
 
 class EtlTest extends TestCase
@@ -15,7 +15,7 @@ public function testEtl()
         $loader = new ArrayLoader();
         $etl = Etl::create()
             ->withLoader($loader)
-            ->withTransformer(new CallableValueTransformer('strtoupper'))
+            ->withTransformer(new CallableTransformer('strtoupper'))
         ;
         $data = [
             'foo',

tests/src/Transformer/CallableTransformerTest.php

@@ -10,17 +10,10 @@ class CallableTransformerTest extends TestCase
 
     public function testTransform()
     {
-
-        $items = ['123e4567-e89b-12d3-a456-426655440000' => 'CAPS ARE HELL'];
-        $transform = new CallableTransformer(function ($key, &$value) {
-            return strtolower($value);
-        });
-
-        foreach ($items as $key => $item) {
-            $item = $transform->transform($key, $item);
-            $this->assertSame('123e4567-e89b-12d3-a456-426655440000', $key);
-            $this->assertSame('caps are hell', $item);
-        }
+        $item = 'CAPS ARE HELL';
+        $transform = new CallableTransformer('strtolower');
+        $item = $transform->transform($item);
+        $this->assertSame('caps are hell', $item);
 
     }
 }