Hi there Martin,
since the nested 'do' makes sense, there is little you can do about it.
However, you can make the code more beautiful and restructure it a bit.
This is how I would have written it:
import Control.Applicative
import System.Environment
import System.IO
stats :: String -> String
stats =
unwords .
sequence [show . length . words,
show . length . lines,
show . length]
main :: IO ()
main = do
args <- getArgs
case args of
[fn] -> fmap stats (readFile fn) >>= putStrLn
_ -> hPutStrLn stderr "Usage: wc FNAME"
This improves the statistics code slightly, but uses some monadic
machinery you may not be familiar with. Another way to read the 'stats'
function is this:
stats :: String -> String
stats str =
unwords [show . length . words $ str,
show . length . lines $ str,
show . length $ str]
Finally you can improve the command line argument processing itself
simply by being more sensible about what makes a valid command line:
main =
getArgs >>=
mapM_ (fmap stats . readFile >=> putStrLn)
Instead of expecting exactly one command line argument you print the
counts for every argument. That means, if there are no arguments, you
print no counts. This makes more sense than the highhanded "I want
exactly one argument, otherwise I won't work" syntax, because now your
whole program forms a homomorphism (shell syntax):
`prog x` `prog y` = `prog x y`
This allows reasoning and optimization.
Greets,
Ertugrul
Martin Drautzburg
in the code snippet below, is there a way to factor out the second "do"?
import System (getArgs) main :: IO () main = do args <- getArgs case args of [fname] -> do fstr <- readFile fname let nWords = length . words $ fstr nLines = length . lines $ fstr nChars = length fstr putStrLn . unwords $ [ show nLines , show nWords , show nChars , fname] _ ->putStrLn "usage: wc fname"
-- Not to be or to be and (not to be or to be and (not to be or to be and (not to be or to be and ... that is the list monad.