To handle metavariables in GF, don't use `fg` and `gf`

Metavariables.hs

import PGF
import Debug.Trace (trace)
import Data.Foldable (asum)
import Test

getMeta :: Expr -> Int
getMeta expr = case go expr of
                Just i -> trace (unwords ["expression", showExpr [] expr, "contains the metavariable", show i, ", returning it"]) i
                Nothing -> trace (unwords ["expression", showExpr [] expr, "doesn't contain a metavariable, defaulting to 42"]) 42
  where
    go e =
        case unMeta e of
            Just i -> Just i
            Nothing ->
                case unApp e of
                    Just (f,args) -> asum $ fmap go args
                    Nothing -> Nothing


main :: IO ()
main = do
  test <- readPGF "Test.pgf"
  let testLan = head $ languages test
  let testCat = fromJust $ readType "foo"
  let s = "foo"

  let tree = head $ parse test testLan testCat s
  putStrLn ("parsing done of " ++ linearize test testLan tree)
  let changedTree = changeMetaVar tree
  putStrLn ("tree changed to " ++ linearize test testLan changedTree)

changeMetaVar :: Expr -> Expr
changeMetaVar foo = mkApp (mkCId "mkBar") [mkInt $ getMeta foo]