type classes for XML data representation;

--- a/src/Tools/Haskell/Haskell.thy	Thu Aug 05 18:53:53 2021 +0200
+++ b/src/Tools/Haskell/Haskell.thy	Thu Aug 05 20:24:42 2021 +0200
@@ -1533,7 +1533,7 @@
 import Isabelle.Name (Name)
 import qualified Isabelle.Properties as Properties
 import qualified Isabelle.Markup as Markup
-import qualified Isabelle.XML.Encode as Encode
+import Isabelle.XML.Classes
 import qualified Isabelle.XML as XML
 import qualified Isabelle.YXML as YXML
 
@@ -1556,13 +1556,7 @@
 markup_element :: T -> (Markup.T, XML.Body)
 markup_element (Completion props total names) =
   if not (null names) then
-    let
-      markup = Markup.properties props Markup.completion
-      body =
-        Encode.pair Encode.int
-          (Encode.list (Encode.pair Encode.string (Encode.pair Encode.string Encode.string)))
-          (total, names)
-    in (markup, body)
+    (Markup.properties props Markup.completion, encode (total, names))
   else (Markup.empty, [])
 
 markup_report :: [T] -> Name
@@ -2259,6 +2253,90 @@
     \([], a) -> App (pair term term a)]
 \<close>
 
+generate_file "Isabelle/XML/Classes.hs" = \<open>
+{- generated by Isabelle -}
+
+{-  Title:      Isabelle/XML/Classes.hs
+    Author:     Makarius
+    LICENSE:    BSD 3-clause (Isabelle)
+
+Type classes for XML data representation.
+-}
+
+{-# LANGUAGE TypeSynonymInstances #-}
+{-# LANGUAGE FlexibleInstances #-}
+
+module Isabelle.XML.Classes
+  (Encode_Atom(..), Decode_Atom(..), Encode (..), Decode (..))
+where
+
+import qualified Isabelle.XML as XML
+import qualified Isabelle.XML.Encode as Encode
+import qualified Isabelle.XML.Decode as Decode
+import qualified Isabelle.Term_XML.Encode as Encode
+import qualified Isabelle.Term_XML.Decode as Decode
+import qualified Isabelle.Properties as Properties
+import Isabelle.Bytes (Bytes)
+import Isabelle.Term (Typ, Term)
+
+
+class Encode_Atom a where encode_atom :: Encode.A a
+class Decode_Atom a where decode_atom :: Decode.A a
+
+instance Encode_Atom Int where encode_atom = Encode.int_atom
+instance Decode_Atom Int where decode_atom = Decode.int_atom
+
+instance Encode_Atom Bool where encode_atom = Encode.bool_atom
+instance Decode_Atom Bool where decode_atom = Decode.bool_atom
+
+instance Encode_Atom () where encode_atom = Encode.unit_atom
+instance Decode_Atom () where decode_atom = Decode.unit_atom
+
+
+class Encode a where encode :: Encode.T a
+class Decode a where decode :: Decode.T a
+
+instance Encode Bytes where encode = Encode.string
+instance Decode Bytes where decode = Decode.string
+
+instance Encode Int where encode = Encode.int
+instance Decode Int where decode = Decode.int
+
+instance Encode Bool where encode = Encode.bool
+instance Decode Bool where decode = Decode.bool
+
+instance Encode () where encode = Encode.unit
+instance Decode () where decode = Decode.unit
+
+instance (Encode a, Encode b) => Encode (a, b)
+  where encode = Encode.pair encode encode
+instance (Decode a, Decode b) => Decode (a, b)
+  where decode = Decode.pair decode decode
+
+instance (Encode a, Encode b, Encode c) => Encode (a, b, c)
+  where encode = Encode.triple encode encode encode
+instance (Decode a, Decode b, Decode c) => Decode (a, b, c)
+  where decode = Decode.triple decode decode decode
+
+instance Encode a => Encode [a] where encode = Encode.list encode
+instance Decode a => Decode [a] where decode = Decode.list decode
+
+instance Encode a => Encode (Maybe a) where encode = Encode.option encode
+instance Decode a => Decode (Maybe a) where decode = Decode.option decode
+
+instance Encode XML.Tree where encode = Encode.tree
+instance Decode XML.Tree where decode = Decode.tree
+
+instance Encode Properties.T where encode = Encode.properties
+instance Decode Properties.T where decode = Decode.properties
+
+instance Encode Typ where encode = Encode.typ
+instance Decode Typ where decode = Decode.typ
+
+instance Encode Term where encode = Encode.term
+instance Decode Term where decode = Decode.term
+\<close>
+
 generate_file "Isabelle/UUID.hs" = \<open>
 {-  Title:      Isabelle/UUID.hs
     Author:     Makarius