import Language.Haskell.Extension
import Text.PrettyPrint

main = putStrLn $ render moduleDoc

moduleDoc :: Doc
moduleDoc =
    vcat [text "-- this module was automatically generated. do not edit!",
          text "module Hint.Extension (Extension(..), knownExtensions)",
          text "",
          text "where",
          text "",
          text "-- | This represents language extensions beyond Haskell 98" <+>
          text "     that are supported by GHC (it was taken from" <+>
          text "     Cabal's @Language.Haskell.Extension@)",
          align "data Extension " $
              punctuateL (text "| ") . onFirst (text "= ") $ known ++ [unknown],
          nest 8 $ text "deriving (Eq, Show, Read)",
          text "",
          text "knownExtensions :: [Extension]",
          align "knownExtensions = [" (punctuate comma known ++ [text "]"])
          ]

known :: [Doc]
known = map (text . show) knownExtensions

unknown :: Doc
unknown = text "UnknownExtension String"

align :: String -> [Doc] -> Doc
align s []     = text s
align s (d:ds) = hang (text s <> d) (length s) (vcat ds)

-- punctuateL p [d1, ..., dn] = [d1, p <> d2, ..., p <> dn]
punctuateL :: Doc -> [Doc] -> [Doc]
punctuateL _ []     = []
punctuateL _ [d]    = [d]
punctuateL p (d:ds) = d : map (p <>) ds

onFirst :: Doc -> [Doc] -> [Doc]
onFirst _ [] = []
onFirst p (d:ds) = p <> d : ds