On Thu, Mar 18, 2010 at 10:37 AM, Stefan Klinger
Hello!
Nice, Parsec 3 comes with a monad transformer [1]. So I thought I could use IO as inner monad, and perform IO operations during parsing.
But I failed. Monad transformers still bend my mind. My problem: I don't see a function to actually lift the IO operation into the ParsecT. It should be something like
lift :: IO a -> ParsecT s u IO a
That operation, with that name, and (a generalization of) that type, is *the* method of the MonadTrans class. Essentially the presence of that operation is the definition of what it means to be a monad transformer.
The following is a toy example, I just could not make something smaller: Let's parse command line arguments (tokens are Strings), and execute them while parsing.
import Text.Parsec.Prim import Text.Parsec.Pos import Text.Parsec import System.Environment ( getArgs )
Command line interface parser (Clip) type: Inner monad IO, user state u, tokens are Strings, returns something typed a.
type Clip u a = ParsecT [String] u IO a
Source code position for command line arguments: The line is always 1, column n represents the n-th command line argument.
nextPos p _ _ = incSourceColumn p 1
Two primitive parsers, one for flags (with a dash) and one for other arguments:
clipFlag "x" accepts the command line flag "-x", and returns "x".
clipFlag :: String -> Clip u String clipFlag x = tokenPrim id nextPos (\y -> if '-':x == y then Just x else Nothing)
clipValue accepts any command line argument that does not tart with a dash '-'.
clipValue :: Clip u String clipValue = tokenPrim id nextPos test where test ('-':_) = Nothing test other = Just other
Now the test program:
Load files given on the command line, and sum up their word count, until -p is given. -p prints the current word count and sets the counter to zero. Further files may be processed then. At the end, show the sum of all word counts.
Example: foo has 12 words, bar has 34 words:
main foo -p bar -p foo bar -p Counted 12 words, reset counter. Counted 34 words, reset counter. Counted 46 words, reset counter. Grand total: 92
type CurrentCount = Int -- the user state used with Clip/ParsecT.
root implements the command line grammar (<filename>+ "-p")* and returns the sum of all word counts.
root :: Clip CurrentCount Int root = do ns <- many (many1 loadFile >> printSize) eof return $ sum ns
Interprets each non-flag on the command line as filename, loads it, counts its words, and adds the count to the state.
loadFile :: Clip CurrentCount () loadFile = do -- expect a filename filename <- clipValue
-- load the file: IO content <- lift $ readFile filename
-- add wordcount to state modifyState ((+) (length $ words content))
If -p shows up on the command line, print accumulated count, reset counter to cero and return count for grand-total calculation.
printSize :: Clip CurrentCount Int printSize = do -- expect flag -p clipFlag "p"
-- print current word count: IO n <- getState lift . putStrLn $ "Counted "++show n++" words, reset counter."
-- reset user state to zero, return word count for grand total putState 0 return n
main just runs the root parser on the command line arguments and checks the result.
main = do result <- getArgs >>= runParserT root 0 "command line" case result of Left err -> error $ show err Right n -> putStrLn $ "Grand total: "++show n
So where is the lift function? Does it exist? Here, I need your help.
lift :: IO a -> ParsecT s u IO a lift = undefined
Any comments are appreciated.
Thank you! Stefan
____________________ [1] http://hackage.haskell.org/packages/archive/parsec/3.0.0/doc/html/Text-Parse...
-- Stefan Klinger o/klettern /\/ bis zum send plaintext only - max size 32kB - no spam \ Abfallen http://stefan-klinger.de _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe