２　構文解析　ぱらぱらめくる『48時間でSchemeを書こう』

構文解析
例題

import System.Environment
import Text.ParserCombinators.Parsec hiding (spaces)

symbol :: Parser Char
symbol = oneOf "!#$%&|*+-/:<=>?@^_~"

readExpr :: String -> String
readExpr input = case parse symbol "lisp" input of
    Left err -> "No match: " ++ show err
    Right val -> "Found value"
    
main :: IO ()
main = do args <- getArgs
          putStrLn (readExpr (args !! 0))

- だんだんアップグレード

import System.Environment
import Text.ParserCombinators.Parsec hiding (spaces)

symbol :: Parser Char
symbol = oneOf "!#$%&|*+-/:<=>?@^_~"

spaces :: Parser ()
spaces = skipMany1 space

readExpr :: String -> String
readExpr input = case parse (spaces >> symbol) "lisp" input of
    Left err -> "No match: " ++ show err
    Right val -> "Found value"
    
main :: IO ()
main = do args <- getArgs
          putStrLn (readExpr (args !! 0))

import System.Environment
import Text.ParserCombinators.Parsec hiding (spaces)
import Control.Monad

symbol :: Parser Char
symbol = oneOf "!#$%&|*+-/:<=>?@^_~"

spaces :: Parser ()
spaces = skipMany1 space

data LispVal = Atom String
             | List [LispVal]
             | DottedList [LispVal] LispVal
             | Number Integer
             | String String
             | Bool Bool

parseString :: Parser LispVal
parseString = do char '"'
                 x <- many (noneOf "\"")
                 char '"'
                 return $ String x

parseAtom :: Parser LispVal
parseAtom = do first <- letter <|> symbol
               rest <- many (letter <|> digit <|> symbol)
               let atom = first:rest
               return $ case atom of
                          "#t" -> Bool True
                          "#f" -> Bool False
                          _    -> Atom atom

parseNumber :: Parser LispVal
parseNumber = liftM (Number . read) $ many1 digit


parseExpr :: Parser LispVal
parseExpr = parseAtom
        <|> parseString
        <|> parseNumber

readExpr :: String -> String
readExpr input = case parse parseExpr "lisp" input of
    Left err -> "No match: " ++ show err
    Right _ -> "Found value"

main :: IO ()
main = do args <- getArgs
          putStrLn (readExpr (args !! 0))

あまり面白くない…。この段階式・課題乗り越え式で、データ型やクラスの定義をして、モジュールを作ってパッケージを作れるようになる、みたいな記事を探して、それをやる方が自分にとっては有意義そう。