doc: generate haskell-users-guide.xml from Markdown input via Pandoc

Editing Docbook is no fun, IMHO, so I'd rather store the Haskell
documentation in Markdown format and use Pandoc to convert that into
Docbook as part of the build process.

doc/default.nix

@@ -6,7 +6,7 @@ stdenv.mkDerivation {
 
   sources = sourceFilesBySuffices ./. [".xml"];
 
-  buildInputs = [ libxml2 libxslt ];
+  buildInputs = [ pandoc libxml2 libxslt ];
 
   xsltFlags = ''
     --param section.autolabel 1
@@ -19,6 +19,22 @@ stdenv.mkDerivation {
   '';
 
   buildCommand = ''
+    {
+      echo "<chapter xmlns=\"http://docbook.org/ns/docbook\""
+      echo "         xmlns:xlink=\"http://www.w3.org/1999/xlink\""
+      echo "         xml:id=\"users-guide-to-the-haskell-infrastructure\">"
+      echo ""
+      echo "<title>User's Guide to the Haskell Infrastructure</title>"
+      echo ""
+      pandoc ${./haskell-users-guide.md} -w docbook | \
+        sed -e 's|<ulink url=|<link xlink:href=|' \
+            -e 's|</ulink>|</link>|' \
+            -e 's|<sect. id=|<section xml:id=|' \
+            -e 's|</sect[0-9]>|</section>|'
+      echo ""
+      echo "</chapter>"
+    } >haskell-users-guide.xml
+
     ln -s "$sources/"*.xml .
 
     echo ${nixpkgsVersion} > .version