src/Tools/Code/code_haskell.ML

     fun write_module width (SOME destination) (module_name, content) =
           let
             val _ = File.check destination;
             val filepath = (Path.append destination o Path.ext "hs" o Path.explode o implode
               o separate "/" o Long_Name.explode) module_name;
-            val _ = Isabelle_System.mkdir (Path.dir filepath);
+            val _ = Isabelle_System.mkdirs (Path.dir filepath);
           in
             (File.write filepath o format [] width o Pretty.chunks2)
               [str "{-# LANGUAGE ScopedTypeVariables #-}", content]
           end
       | write_module width NONE (_, content) = writeln (format [] width content);