Haskell 간단 파싱 예제 (Text.Parsec)

parsec.hs

import Text.Parsec (parse, char, (<|>))
import Text.Parsec.Char (spaces)
import Text.Parsec.Language (haskell)
import Text.Parsec.String (Parser)
import Text.Parsec.Token (makeTokenParser, integer, parens, whiteSpace, identifier)

data Tree a = Leaf a | Branch (Tree a) a (Tree a) deriving (Show)

int :: Parser Int
int = integer haskell >>= return.fromInteger

tree :: Parser a -> Parser (Tree a)
tree node = leaf node <|> branch node

leaf :: Parser a -> Parser (Tree a)
leaf node = node >>= return.Leaf

branch :: Parser a -> Parser (Tree a)
branch node = whiteSpace haskell >> parens haskell (Branch <$> tree node <*> node <*> tree node)

main :: IO()
main = do
  print $ parse (branch int) "" "  ( 1 2 ( 3  4 5))"
  print $ parse (leaf int) "" "  23 "
  print $ parse (tree (identifier haskell)) "" "  (a abc (def root xxx)) "