added composOp generation to haskell-gadt, and an example in examples/gadt-transfer

examples/gadt-transfer/VeryFoods.hs

@@ -0,0 +1,23 @@
+{-# OPTIONS_GHC -fglasgow-exts #-}
+module Main where
+
+import PGF
+import Foods
+
+-- example of using GADT: turn every occurrence of "boring" to "very boring"
+
+main = do
+  pgf <- readPGF "Foods.pgf"
+  interact (doVery pgf)
+
+doVery pgf s = case parseAllLang pgf (startCat pgf) s of 
+  (l,t:_):_ -> unlines $ return $ linearize pgf l $ gf $ veryC $ fg t
+
+veryC :: GComment -> GComment
+veryC = very
+
+very :: forall a. Foods.Tree a -> Foods.Tree a
+very t = case t of
+  GBoring -> GVery GBoring
+  _ -> composOp very t
+