Add TransformableRenderable capability

src/Html/Renderables/Container.php

@@ -3,8 +3,9 @@
 namespace Erusev\Parsedown\Html\Renderables;
 
 use Erusev\Parsedown\Html\Renderable;
+use Erusev\Parsedown\Html\TransformableRenderable;
 
-final class Container implements Renderable
+final class Container implements TransformableRenderable
 {
     use CanonicalStateRenderable;
 
@@ -44,4 +45,22 @@ final class Container implements Renderable
             ''
         );
     }
+
+    /**
+     * @param \Closure(string):Renderable $Transform
+     * @return Renderable
+     */
+    public function transformingContent(\Closure $Transform): Renderable
+    {
+        return new Container(\array_map(
+            function (Renderable $R) use ($Transform): Renderable {
+                if (! $R instanceof TransformableRenderable) {
+                    return $R;
+                }
+
+                return $R->transformingContent($Transform);
+            },
+            $this->Contents
+        ));
+    }
 }

src/Html/Renderables/Element.php

@@ -5,8 +5,9 @@ namespace Erusev\Parsedown\Html\Renderables;
 use Erusev\Parsedown\Html\Renderable;
 use Erusev\Parsedown\Html\Sanitisation\CharacterFilter;
 use Erusev\Parsedown\Html\Sanitisation\Escaper;
+use Erusev\Parsedown\Html\TransformableRenderable;
 
-final class Element implements Renderable
+final class Element implements TransformableRenderable
 {
     use CanonicalStateRenderable;
 
@@ -191,4 +192,26 @@ final class Element implements Renderable
 
         return $html;
     }
+
+    /**
+     * @param \Closure(string):Renderable $Transform
+     * @return Renderable
+     */
+    public function transformingContent(\Closure $Transform): Renderable
+    {
+        if (! isset($this->Contents)) {
+            return $this;
+        }
+
+        return new self($this->name, $this->attributes, \array_map(
+            function (Renderable $R) use ($Transform): Renderable {
+                if (! $R instanceof TransformableRenderable) {
+                    return $R;
+                }
+
+                return $R->transformingContent($Transform);
+            },
+            $this->Contents
+        ));
+    }
 }

src/Html/Renderables/Text.php

@@ -4,8 +4,9 @@ namespace Erusev\Parsedown\Html\Renderables;
 
 use Erusev\Parsedown\Html\Renderable;
 use Erusev\Parsedown\Html\Sanitisation\Escaper;
+use Erusev\Parsedown\Html\TransformableRenderable;
 
-final class Text implements Renderable
+final class Text implements TransformableRenderable
 {
     use CanonicalStateRenderable;
 
@@ -31,4 +32,13 @@ final class Text implements Renderable
     {
         return Escaper::htmlElementValueEscapingDoubleQuotes($this->text);
     }
+
+    /**
+     * @param \Closure(string):Renderable $Transform
+     * @return Renderable
+     */
+    public function transformingContent(\Closure $Transform): Renderable
+    {
+        return $Transform($this->text);
+    }
 }

src/Html/TransformableRenderable.php

@@ -0,0 +1,25 @@
+<?php
+
+namespace Erusev\Parsedown\Html;
+
+interface TransformableRenderable extends Renderable
+{
+    /**
+     * Takes a closure $Transform which will provide a transformation of
+     * a "contained text" into Renderables.
+     *
+     * In order for TransformableRenderable to make sense, a Renderable must
+     * have:
+     *   1. Some concept of "contained text". $Transform can be applied
+     *      piece-wise if your container contains logically disjoint sections
+     *      of text.
+     *   2. A generic mechanism for containing other Renderables, or replacing
+     *      the current renderable with a container.
+     *
+     * It is acceptable to only partially transform "contained text".
+     *
+     * @param \Closure(string):Renderable $Transform
+     * @return Renderable
+     */
+    public function transformingContent(\Closure $Transform): Renderable;
+}