diff --git a/src/GF/API/GrammarToHaskell.hs b/src/GF/API/GrammarToHaskell.hs
index c3ca33247..c8728bada 100644
--- a/src/GF/API/GrammarToHaskell.hs
+++ b/src/GF/API/GrammarToHaskell.hs
@@ -14,7 +14,7 @@
 -- AR 11/11/1999 -- 7/12/2000 -- 18/5/2004
 -----------------------------------------------------------------------------
 
-module GF.API.GrammarToHaskell (grammar2haskell) where
+module GF.API.GrammarToHaskell (grammar2haskell, grammar2haskellGADT) where
 
 import qualified GF.Canon.GFC as GFC
 import GF.Grammar.Macros
@@ -30,6 +30,12 @@ grammar2haskell gr = foldr (++++) [] $
   haskPreamble ++ [datatypes gr', gfinstances gr', fginstances gr']
     where gr' = hSkeleton gr
 
+grammar2haskellGADT :: GFC.CanonGrammar -> String
+grammar2haskellGADT gr = foldr (++++) [] $  
+  ["{-# OPTIONS_GHC -fglasgow-exts #-}"] ++ 
+  haskPreamble ++ [datatypesGADT gr', gfinstances gr', fginstances gr']
+    where gr' = hSkeleton gr
+
 -- | by this you can prefix all identifiers with stg; the default is 'G'
 gId :: OIdent -> OIdent 
 gId i = 'G':i
@@ -96,6 +102,26 @@ hDatatype (cat,rules) =
         [gId f +++ foldr (+++) "" (map gId xx) | (f,xx) <- rules] ++++
  "  deriving Show"
 
+-- GADT version of data types
+datatypesGADT :: (String,HSkeleton) -> String
+datatypesGADT (_,skel) = 
+    unlines (concatMap hCatTypeGADT skel)
+    +++++
+    "data Tree :: * -> * where" ++++ unlines (concatMap (map ("  "++) . hDatatypeGADT) skel)
+
+hCatTypeGADT :: (OIdent, [(OIdent, [OIdent])]) -> [String]
+hCatTypeGADT (cat,rules)
+    = ["type"+++gId cat+++"="+++"Tree"+++gId cat++"_",
+       "data"+++gId cat++"_"]
+
+hDatatypeGADT :: (OIdent, [(OIdent, [OIdent])]) -> [String]
+hDatatypeGADT (cat, rules) 
+    | isListCat (cat,rules) = [gId cat+++"::"+++"["++gId (elemCat cat)++"]" +++ "->" +++ t]
+    | otherwise =
+        [ gId f +++ "::" +++ concatMap (\a -> gId a +++ "-> ") args ++ t | (f,args) <- rules ]
+  where t = "Tree" +++ gId cat ++ "_"
+
+
 ----hInstance m ("Cn",_) = "" --- seems to belong to an old applic. AR 18/5/2004
 hInstance m (cat,[]) = "" 
 hInstance m (cat,rules) 
@@ -104,8 +130,9 @@ hInstance m (cat,rules)
      " gf (" ++ gId cat +++ "[" ++ concat (intersperse "," baseVars) ++ "])" 
            +++ "=" +++ mkRHS ("Base"++ec) baseVars ++++
      " gf (" ++ gId cat +++ "(x:xs)) = " 
-           ++ mkRHS ("Cons"++ec) ["x",prParenth (gId cat+++"xs")] ++++
-     " gf (" ++ gId cat +++ "xs) = error (\"Bad " ++ cat ++ " value: \" ++ show xs)" 
+           ++ mkRHS ("Cons"++ec) ["x",prParenth (gId cat+++"xs")] 
+-- no show for GADTs
+--     ++++ " gf (" ++ gId cat +++ "xs) = error (\"Bad " ++ cat ++ " value: \" ++ show xs)" 
  | otherwise =
   "instance Gf" +++ gId cat +++ "where" ++
   (if length rules == 1 then "" else "\n") +++
diff --git a/src/GF/Shell/HelpFile.hs b/src/GF/Shell/HelpFile.hs
index a54646710..e577d5b46 100644
--- a/src/GF/Shell/HelpFile.hs
+++ b/src/GF/Shell/HelpFile.hs
@@ -644,6 +644,7 @@ txtHelpFile =
   "\n    -printer=plbnf          grammar for BNF Converter, with precedence levels" ++
   "\n   *-printer=happy          source file for Happy parser generator (use lbnf!)" ++
   "\n    -printer=haskell        abstract syntax in Haskell, with transl to/from GF" ++
+  "\n    -printer=haskell_gadt   abstract syntax GADT in Haskell, with transl to/from GF" ++
   "\n    -printer=morpho         full-form lexicon, long format" ++
   "\n   *-printer=latex          LaTeX file (for the tg command)" ++
   "\n    -printer=fullform       full-form lexicon, short format" ++
diff --git a/src/GF/UseGrammar/Custom.hs b/src/GF/UseGrammar/Custom.hs
index bbfb1e09e..132c832cd 100644
--- a/src/GF/UseGrammar/Custom.hs
+++ b/src/GF/UseGrammar/Custom.hs
@@ -270,6 +270,7 @@ customGrammarPrinter =
   ,(strCI "bnf",     \_ -> prBNF False)
   ,(strCI "absbnf",  \_ -> abstract2bnf . stateGrammarST)
   ,(strCI "haskell", \_ -> grammar2haskell . stateGrammarST)
+  ,(strCI "haskell_gadt", \_ -> grammar2haskellGADT . stateGrammarST)
   ,(strCI "transfer", \_ -> grammar2transfer . stateGrammarST)
   ,(strCI "morpho",  \_ -> prMorpho . stateMorpho)
   ,(strCI "fullform",\_ -> prFullForm . stateMorpho)
diff --git a/src/HelpFile b/src/HelpFile
index 0b18e34a8..999b26382 100644
--- a/src/HelpFile
+++ b/src/HelpFile
@@ -615,6 +615,7 @@ q, quit: q
     -printer=plbnf          grammar for BNF Converter, with precedence levels
    *-printer=happy          source file for Happy parser generator (use lbnf!)
     -printer=haskell        abstract syntax in Haskell, with transl to/from GF
+    -printer=haskell_gadt   abstract syntax GADT in Haskell, with transl to/from GF
     -printer=morpho         full-form lexicon, long format
    *-printer=latex          LaTeX file (for the tg command)
     -printer=fullform       full-form lexicon, short format