Added XML parser (useful for parsing PGIP / PGML).

--- a/src/Pure/General/xml.ML	Wed Nov 27 17:16:47 2002 +0100
+++ b/src/Pure/General/xml.ML	Wed Nov 27 17:17:53 2002 +0100
@@ -1,16 +1,22 @@
 (*  Title:      Pure/General/xml.ML
     ID:         $Id$
-    Author:     Markus Wenzel, LMU München
+    Author:     Markus Wenzel, LMU Muenchen
+                Stefan Berghofer, TU Muenchen
     License:    GPL (GNU GENERAL PUBLIC LICENSE)
 
-Basic support for XML output.
+Basic support for XML input and output.
 *)
 
 signature XML =
 sig
+  datatype tree =
+      Elem of string * (string * string) list * tree list
+    | Text of string
   val element: string -> (string * string) list -> string list -> string
   val text: string -> string
   val header: string
+  val string_of_tree: tree -> string
+  val tree_of_string: string -> tree
 end;
 
 structure XML: XML =
@@ -25,11 +31,22 @@
   | encode "\"" = """
   | encode c = c;
 
+fun decode "&lt;" = "<"
+  | decode "&gt;" = ">"
+  | decode "&amp;" = "&"
+  | decode "&apos;" = "'"
+  | decode "&quot;" = "\""
+  | decode c = c;
+
 val text = implode o map encode o explode;
 
 
 (* elements *)
 
+datatype tree =
+    Elem of string * (string * string) list * tree list
+  | Text of string;
+
 fun attribute (a, x) = a ^ " = " ^ quote (text x);
 
 fun element name atts cs =
@@ -38,6 +55,59 @@
     else enclose "<" ">" elem ^ implode cs ^ enclose "</" ">" name
   end;
 
+fun string_of_tree (Elem (name, atts, ts)) =
+      element name atts (map string_of_tree ts)
+  | string_of_tree (Text s) = s
+  
 val header = "<?xml version=\"1.0\"?>\n";
 
+
+(* parser *)
+
+datatype token_kind = Idt | Sym | Str | Chr;
+
+fun is_kind k (k', _) = k = k';
+
+val stopper = ((Sym, ""), fn (Sym, "") => true | _ => false);
+
+val scan_whspc = $$ " " || $$ "\n";
+
+val scan_symb = Scan.literal (Scan.make_lexicon
+  (map explode ["<", "</", ">", "/>", "\"", "=", "&"])) >> implode;
+
+val scan_special = $$ "&" ^^ Symbol.scan_id ^^ $$ ";" >> decode;
+
+val lexer = Scan.repeat (Scan.repeat scan_whspc |--
+  (   Scan.max (op <= o pairself (size o snd))
+        (Symbol.scan_id >> pair Idt)
+        (Scan.repeat1 (scan_special || Scan.unless (scan_whspc || scan_symb)
+           (Scan.one Symbol.not_eof)) >> (pair Chr o implode))
+   || $$ "\"" |-- Scan.repeat
+        (   scan_special
+         || Scan.unless ($$ "\"") (Scan.one Symbol.not_eof)) --|
+           $$ "\"" >> (pair Str o implode)
+   || scan_symb >> pair Sym) --| Scan.repeat scan_whspc);
+
+val parse_att = (Scan.one (is_kind Idt) >> snd) --| $$ (Sym, "=") --
+  (Scan.one (is_kind Str) >> snd);
+
+fun parse_tree xs =
+  ($$ (Sym, "<") |-- (Scan.one (is_kind Idt) >> snd) --
+    Scan.repeat parse_att :-- (fn (s, _) =>
+          $$ (Sym, "/>") >> K []
+       || $$ (Sym, ">") |-- Scan.repeat
+            (   Scan.one (not o is_kind Sym) >> (Text o snd)
+             || parse_tree) --|
+          $$ (Sym, "</") --| Scan.one (equal (Idt, s)) --| $$ (Sym, ">")) >>
+    (fn ((s, atts), ts) => Elem (s, atts, ts))) xs;
+
+fun tree_of_string s =
+  let val msg = "Malformed XML expression:\n" ^ s
+  in case Scan.finite Symbol.stopper lexer (explode s) of
+      (toks, []) => (case Scan.finite stopper
+        (Scan.error (!! (K msg) parse_tree)) toks of
+          (t, []) => t | _ => error msg)
+    | _ => error msg
+  end;
+
 end;