✨ Improve exception message for unexpected null value - mention expected type

src/ResolveUtils.php

@@ -24,7 +24,7 @@ final class ResolveUtils
     public static function assertInnerReturnType(mixed $result, Type $type): void
     {
         if ($type instanceof NonNull && $result === null) {
-            throw TypeMismatchRuntimeException::unexpectedNullValue();
+            throw TypeMismatchRuntimeException::unexpectedNullValue($type);
         }
         if ($result === null) {
             return;
@@ -56,7 +56,7 @@ public static function assertInnerReturnType(mixed $result, Type $type): void
     public static function assertInnerInputType(mixed $input, Type $type): void
     {
         if ($type instanceof NonNull && $input === null) {
-            throw TypeMismatchRuntimeException::unexpectedNullValue();
+            throw TypeMismatchRuntimeException::unexpectedNullValue($type);
         }
         if ($input === null) {
             return;

src/TypeMismatchRuntimeException.php

@@ -4,16 +4,24 @@
 
 namespace TheCodingMachine\GraphQLite;
 
+use GraphQL\Type\Definition\Type;
+
 use function gettype;
 
 /**
  * An exception thrown when a resolver returns a value that is not compatible with the GraphQL type.
  */
 class TypeMismatchRuntimeException extends GraphQLRuntimeException
 {
-    public static function unexpectedNullValue(): self
+    public static function unexpectedNullValue(Type|null $expectedType = null): self
     {
-        return new self('Unexpected null value for non nullable field.');
+        $expectedMessageTail = '';
+        if ($expectedType !== null) {
+            $expectedMessageTail = ' Expected: "' . $expectedType->toString() . '"';
+            // ToDo: support for NULL $expectedType should be dropped in the next major
+        }
+
+        return new self('Unexpected null value for non nullable field.' . $expectedMessageTail);
     }
 
     public static function expectedIterable(mixed $result): self

tests/ResolveUtilsTest.php

@@ -12,6 +12,7 @@ class ResolveUtilsTest extends TestCase
     public function testAssertNull(): void
     {
         $this->expectException(TypeMismatchRuntimeException::class);
+        $this->expectExceptionMessage('Unexpected null value for non nullable field. Expected: "String!"');
         ResolveUtils::assertInnerReturnType(null, Type::nonNull(Type::string()));
     }
 
@@ -30,6 +31,7 @@ public function testAssertObjectOk(): void
     public function testAssertInputNull(): void
     {
         $this->expectException(TypeMismatchRuntimeException::class);
+        $this->expectExceptionMessage('Unexpected null value for non nullable field. Expected: "String!"');
         ResolveUtils::assertInnerInputType(null, Type::nonNull(Type::string()));
     }