[feature] allow invoke parameters to be optional

src/Callback.php

@@ -53,10 +53,16 @@ public function invoke(...$arguments)
                 continue;
             }
 
-            if (!\array_key_exists($key, $parameters)) {
+            if (!\array_key_exists($key, $parameters) && !$argument->isOptional()) {
                 throw new \ArgumentCountError(\sprintf('No argument %d for callable. Expected type: "%s".', $key + 1, $argument->type()));
             }
 
+            if ($argument->isOptional()) {
+                $arguments[$key] = null;
+
+                continue;
+            }
+
             try {
                 $arguments[$key] = $argument->resolve($parameters[$key]);
             } catch (UnresolveableArgument $e) {

src/Callback/Parameter.php

@@ -12,6 +12,9 @@
  */
 abstract class Parameter
 {
+    /** @var bool */
+    private $optional = false;
+
     final public static function union(self ...$parameters): self
     {
         return new UnionParameter(...$parameters);
@@ -32,7 +35,16 @@ final public static function factory(callable $factory): ValueFactory
         return new ValueFactory($factory);
     }
 
+    final public function optional(): self
+    {
+        $this->optional = true;
+
+        return $this;
+    }
+
     /**
+     * @internal
+     *
      * @return mixed
      *
      * @throws UnresolveableArgument
@@ -54,6 +66,14 @@ final public function resolve(\ReflectionParameter $parameter)
         return $value($type->getName());
     }
 
+    /**
+     * @internal
+     */
+    final public function isOptional(): bool
+    {
+        return $this->optional;
+    }
+
     abstract public function type(): string;
 
     abstract protected function valueFor(\ReflectionParameter $refParameter);

tests/CallbackTest.php

@@ -261,6 +261,18 @@ public function invoke_with_not_enough_required_arguments(): void
             )
         ;
     }
+
+    /**
+     * @test
+     */
+    public function can_mark_invoke_parameter_arguments_as_optional(): void
+    {
+        $actual = Callback::createFor(static function() { return 'ret'; })
+            ->invoke(Parameter::typed('string', 'foobar')->optional())
+        ;
+
+        $this->assertSame('ret', $actual);
+    }
 }
 
 class Object1