lib/markdown2: introduce markdown rendering interfaces

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>

diff --git a/lib/markdown2/markdown_rendering.nit b/lib/markdown2/markdown_rendering.nit
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+# This file is part of NIT ( http://www.nitlanguage.org ).
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# Markdown document rendering
+module markdown_rendering
+
+import markdown_ast
+
+# Common interface for all markdown renderer
+interface MdRenderer
+       super MdVisitor
+
+       # Render `node`
+       fun render(node: MdNode): String is abstract
+end
+
+# A renderer that output raw text
+class RawTextVisitor
+       super MdRenderer
+
+       # Text under construction
+       private var text: Buffer is noinit
+
+       redef fun render(node) do
+               text = new Buffer
+               enter_visit(node)
+               return text.to_s
+       end
+
+       # Append `string` to `text`
+       fun add(string: String) do text.append(string)
+
+       redef fun visit(node) do node.render_raw_text(self)
+end
+
+redef class MdNode
+
+       # Return `self` as raw text
+       fun raw_text: String do
+               var v = new RawTextVisitor
+               return v.render(self)
+       end
+
+       # Render `self` as raw text
+       fun render_raw_text(v: RawTextVisitor) do visit_all(v)
+end
+
+redef class MdCode
+       redef fun render_raw_text(v) do v.add literal
+end
+
+redef class MdLineBreak
+       redef fun render_raw_text(v) do v.add "\n"
+end
+
+redef class MdText
+       redef fun render_raw_text(v) do v.add literal
+end