Fix: Chain transformers using generators in the middle (#25)

src/Transformer/ChainTransformer.php

@@ -5,7 +5,7 @@
 namespace BenTools\ETL\Transformer;
 
 use BenTools\ETL\EtlState;
-use BenTools\ETL\Internal\TransformResult;
+use Generator;
 
 final readonly class ChainTransformer implements TransformerInterface
 {
@@ -32,13 +32,23 @@ public function with(TransformerInterface|callable $transformer): self
         return new self(...[...$this->transformers, $transformer]);
     }
 
-    public function transform(mixed $item, EtlState $state): mixed
+    public function transform(mixed $item, EtlState $state): Generator
+    {
+        $item = $this->doTransform($item, $state);
+
+        if ($item instanceof Generator) {
+            yield from $item;
+        } else {
+            yield $item;
+        }
+    }
+
+    public function doTransform(mixed $item, EtlState $state): mixed
     {
-        $output = $item;
         foreach ($this->transformers as $transformer) {
-            $output = TransformResult::create($transformer->transform($output, $state))->value;
+            $item = $items = $transformer->transform($item, $state);
         }
 
-        return $output;
+        return $item;
     }
 }

tests/Unit/Transformer/ChainTransformerTest.php

@@ -23,6 +23,7 @@ function (string $item): Generator {
             yield strtoupper($item);
         },
     ))
+        ->with(fn (Generator $items): array => [...$items])
         ->with(function (array $items): array {
             $items[] = 'hey';
 
@@ -52,12 +53,13 @@ function (string $item): Generator {
                 yield $item;
                 yield strtoupper($item);
             },
+            fn (Generator $items): array => [...$items],
             function (array $items): array {
                 $items[] = 'hey';
 
                 return $items;
             },
-            fn (array $items): string => implode('-', $items)
+            fn (array $items) => yield implode('-', $items)
         );
 
     // When