app/Main.hs

   1 module Main where
   2 import Control.Monad
   3 import Text.ParserCombinators.Parsec hiding (spaces)
   4 import System.Environment
   5
   6 data LispVal = Atom String
   7              | List [LispVal]
   8              | DottedList [LispVal] LispVal
   9              | Number Integer
  10              | String String
  11              | Bool Bool
  12              deriving Show
  13
  14 symbol :: Parser Char
  15 symbol = oneOf "!#$%&|*+-/:<=>?@^_~"
  16
  17 spaces :: Parser ()
  18 spaces = skipMany space
  19
  20 parseString :: Parser LispVal
  21 parseString = do char '"'
  22                  x <- many innerChar
  23                  char '"'
  24                  return $ String x
  25               where innerChar = noneOf ['\\', '\"'] <|> escapeChar
  26                     escapeChar = do char '\\'
  27                                     c <- oneOf "\""
  28                                     return $ '\"'
  29
  30 parseAtom :: Parser LispVal
  31 parseAtom = do
  32     a <- letter <|> symbol
  33     b <- many (letter <|> digit <|> symbol)
  34     let atom = a:b
  35     return $ case atom of
  36                 "#t" -> Bool True
  37                 "#f" -> Bool False
  38                 _    -> Atom atom
  39
  40 parseNumber :: Parser LispVal
  41 parseNumber = do ds <- many1 digit
  42                  let a = read ds
  43                  return $ Number a
  44
  45 parseExpr :: Parser LispVal
  46 parseExpr = parseString
  47         <|> parseAtom
  48         <|> parseNumber
  49
  50 readExpr :: String -> String
  51 readExpr input = case parse parseExpr "lisp" input of
  52     Left err -> "No match: " ++ show err
  53     Right val -> "Found value: " ++ show val
  54
  55 main :: IO ()
  56 main = do
  57     args <- getLine
  58     putStrLn (readExpr args)
  59     main