| This example compiles fine without --make. What's the difference? Michael ================== {- Author: Jeff Newbern Maintainer: Jeff Newbern <jnewbern@nomaware.com> Time-stamp: <Wed Jul 2 18:11:36 2003> License: GPL -} {- DESCRIPTION Example 13 - Using the List monad Usage: Compile the code and execute the resulting program with various arguments. Each argument will produce a list of possible parses of the argument as a decimal number, a hexadecimal number or a word. Try: ./ex13 34 f3 bean ./ex13 food f00d ./ex13 beef 10 "n!" ./ex13 "why?" "(punctuation)" -} import Monad import System import Char -- we can parse three different types of terms data Parsed = Digit Integer | Hex Integer | Word String deriving Show -- attempts to add a character to the parsed representation of a hex digit parseHexDigit :: Parsed -> Char -> [Parsed] parseHexDigit (Hex n) c = if isHexDigit c then return (Hex ((n*16) + (toInteger (digitToInt c)))) else mzero parseHexDigit _ _ = mzero -- attempts to add a character to the parsed representation of a decimal digit parseDigit :: Parsed -> Char -> [Parsed] parseDigit (Digit n) c = if isDigit c then return (Digit ((n*10) + (toInteger (digitToInt c)))) else mzero parseDigit _ _ = mzero -- attempts to add a character to the parsed representation of a word parseWord :: Parsed -> Char -> [Parsed] parseWord (Word s) c = if isAlpha c then return (Word (s ++ [c])) else mzero parseWord _ _ = mzero -- tries to parse the digit as a hex value, a decimal value and a word -- the result is a list of possible parses parse :: Parsed -> Char -> [Parsed] parse p c = (parseHexDigit p c) `mplus` (parseDigit p c) `mplus` (parseWord p c) -- parse an entire String and return a list of the possible parsed values parseArg :: String -> [Parsed] parseArg s = do init <- (return (Hex 0)) `mplus` (return (Digit 0)) `mplus` (return (Word "")) foldM parse init s -- show the original string and all possible parses for the string showResult :: String -> IO () showResult s = do putStr s putStr ": " print (parseArg s) -- prints possible parsed values for command-line arguments main :: IO () main = do args <- getArgs mapM_ showResult args -- END OF FILE --- On Fri, 5/15/09, Lennart Augustsson <lennart.augustsson@gmail.com> wrote:
|