Prototype diff formatting for discussion

src/Formatter/Formatter.php

@@ -95,9 +95,11 @@ public function objectToString($obj)
      */
     protected function getTemplateVars(TemplateInterface $template)
     {
+        $differExists = (property_exists($this->match, "differ") && is_callable($this->match->differ));
         $vars = [
             'expected' => $this->match->getExpected(),
             'actual' => $this->match->getActual(),
+            'diff' => $differExists?call_user_func($this->match->differ, $this->match->getActual(), $this->match->getExpected()):"",
         ];
 
         if ($tplVars = $template->getTemplateVars()) {

src/Matcher/AbstractMatcher.php

@@ -31,8 +31,13 @@ public function match($actual = '')
     {
         $isMatch = $this->doMatch($actual);
         $isNegated = $this->isNegated();
+        $fulfillsExpectation = ($isMatch xor $isNegated);
 
-        return new Match($isMatch xor $isNegated, $this->expected, $actual, $isNegated);
+        $match = new Match($fulfillsExpectation, $this->expected, $actual, $isNegated);
+        if ((!$fulfillsExpectation) && method_exists($this, "differ")) {
+            $match->differ = [$this, "differ"];
+        }
+        return $match;
     }
 
     /**

src/Matcher/SameMatcher.php

@@ -11,6 +11,10 @@
  */
 class SameMatcher extends AbstractMatcher
 {
+    public function differ($actual, $expected) {
+        return "THE CALCULATED DIFF";
+    }
+
     /**
      * Match if the actual value is identical to the expected value using an ===
      * comparison.
@@ -31,7 +35,7 @@ public function doMatch($actual)
     public function getDefaultTemplate()
     {
         return new ArrayTemplate([
-            'default' => 'Expected {{actual}} to be identical to {{expected}}',
+            'default' => 'Expected {{actual}} to be identical to {{expected}}. Difference: {{diff}}',
             'negated' => 'Expected {{actual}} not to be identical to {{expected}}',
         ]);
     }