Hello all, I never did much IO in haskell, but now I have to process some files. I catch myself adding a mapM here and an fmap there. While it is clear to me what these functions do, I wonder if there is a way to avoid the noise. Also I could not find a simple way to pair the lines of a file with its filename. I ended up writing a function "nameAndContent". Finally I am amazed by the many "concat"s I need. Maybe these things just lie in the nature of the problem ("process a number of files"). Otherwise any style suggestions would be much appreciated. import System.FilePath.Glob import Data.List filePattern="*.hs" fileDirectory = "." processFile :: (FilePath, [String]) -> [String] processFile (path, flines) = ["done"] main = do matchingFiles <- fmap (concat . fst) $ globDir [compile filePattern] fileDirectory flines <- mapM nameAndContent matchingFiles result <- mapM (return . processFile) flines mapM putStrLn $ concat result where nameAndContent :: FilePath -> IO (FilePath, [String]) nameAndContent fn = do content <- fmap lines $ readFile fn return (fn, content)
Hi Martin,
While fmap you might consider noise, mapM you can't. If you have a list, you
have to map(M) over it. What you can do is use map(M) once, and combine
functions that you're mapping over. That way there is only one pass over the list.
There is an operator (<$>) that is a synonym for fmap, so you can use that for
noise reduction.
If you wanted to shorten the code, here is my try.
main = do
fs <- concat . fst <$> globDir [compile filePattern] fileDirectory
result <- mapM (fmap (concat . processFile) . nameAndContent) fs
mapM_ putStrLn result
-- This one combines the two above
-- mapM_ (join . fmap (print . concat . processFile) . nameAndContent) fs
where
nameAndContent :: String -> IO (FilePath, [String])
nameAndContent fn = do
content <- lines <$> readFile fn
return $ (fn, content)
vlatko
-------- Original Message --------
Subject: [Haskell-beginners] Too much mapM, fmap and concat
From: martin
Hello all,
I never did much IO in haskell, but now I have to process some files. I catch myself adding a mapM here and an fmap there. While it is clear to me what these functions do, I wonder if there is a way to avoid the noise. Also I could not find a simple way to pair the lines of a file with its filename. I ended up writing a function "nameAndContent". Finally I am amazed by the many "concat"s I need.
Maybe these things just lie in the nature of the problem ("process a number of files"). Otherwise any style suggestions would be much appreciated.
import System.FilePath.Glob import Data.List
filePattern="*.hs" fileDirectory = "."
processFile :: (FilePath, [String]) -> [String] processFile (path, flines) = ["done"]
main = do matchingFiles <- fmap (concat . fst) $ globDir [compile filePattern] fileDirectory flines <- mapM nameAndContent matchingFiles result <- mapM (return . processFile) flines mapM putStrLn $ concat result where nameAndContent :: FilePath -> IO (FilePath, [String]) nameAndContent fn = do content <- fmap lines $ readFile fn return (fn, content)
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On 2014-08-13 18:21, martin wrote:
Maybe these things just lie in the nature of the problem ("process a number of files"). Otherwise any style suggestions would be much appreciated.
I think a lot of this is because you're hopping into/outof the IO monad all the time. The first thing which caught my eye is result <- mapM (return . processFile) flines ...which is just let result = map processFile flines Seeing that you then just concatenate those entries and print them, you can drop this definition altogether and merge it with the next line so that it says mapM_ putStrLn (concatMap processFile flines) The next thing I noticed isthat your nameAndContent function un-wraps the result of "fmap lines $ readFile fn" only to then wrap it again: nameAndContent fn = do content <- fmap lines $ readFile fn return (fn, content) Much like you fmap'ed "lines" on the IO [String] returned by 'readFile fn', you can also fmap "((,) fn)" on the result. If you also use the "<$>" alias for fmap, this becomes nameAndContent fn = ((,) fn) . lines <$> readFile fn -- Frerich Raabe - raabe@froglogic.com www.froglogic.com - Multi-Platform GUI Testing
participants (3)
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Frerich Raabe -
martin -
Vlatko Basic