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 "\""
  26
  27 parseAtom :: Parser LispVal
  28 parseAtom = do
  29     a <- letter <|> symbol
  30     b <- many (letter <|> digit <|> symbol)
  31     let atom = a:b
  32     return $ case atom of
  33                 "#t" -> Bool True
  34                 "#f" -> Bool False
  35                 _    -> Atom atom
  36
  37 parseNumber :: Parser LispVal
  38 parseNumber = do ds <- many1 digit
  39                  let a = read ds
  40                  return $ Number a
  41
  42 parseExpr :: Parser LispVal
  43 parseExpr = parseString
  44         <|> parseAtom
  45         <|> parseNumber
  46
  47 readExpr :: String -> String
  48 readExpr input = case parse parseExpr "lisp" input of
  49     Left err -> "No match: " ++ show err
  50     Right val -> "Found value: " ++ show val
  51
  52 main :: IO ()
  53 main = do
  54     args <- getLine
  55     putStrLn (readExpr args)
  56     main