System.IO.Utils

Entire File Handle Utilities

Opened Handle Data Copying

hCopy :: (HVIO a, HVIO b) => a -> b -> IO ()

Copies from one handle to another in raw mode (using hGetContents).

hCopyProgress

Arguments

:: (HVIO b, HVIO c, Integral a)

=> b

Input handle

-> c

Output handle

-> (Maybe a -> Integer -> Bool -> IO ())

Progress function -- the bool is always False unless this is the final call

-> Int

-> Maybe a

-> IO Integer

Copies from one handle to another in raw mode (using hGetContents). Takes a function to provide progress updates to the user.

hLineCopy :: (HVIO a, HVIO b) => a -> b -> IO ()

Copies from one handle to another in text mode (with lines). Like hBlockCopy, this implementation is nice:

hLineCopy hin hout = hLineInteract hin hout id

lineCopy :: IO ()

Copies from stdin to stdout using lines. An alias for hLineCopy over stdin and stdout.

Disk File Data Copying

copyFileLinesToFile :: FilePath -> FilePath -> IO ()

Copies one filename to another in text mode.

Please note that the Unix permission bits are set at a default; you may need to adjust them after the copy yourself.

This function is implemented using hLineCopy internally.

Line Processing Utilities

hPutStrLns :: HVIO a => a -> [String] -> IO ()

Given a list of strings, output a line containing each item, adding newlines as appropriate. The list is not expected to have newlines already.

hGetLines :: HVIO a => a -> IO [String]

Given a handle, returns a list of all the lines in that handle. Thanks to lazy evaluation, this list does not have to be read all at once.

Combined with hPutStrLns, this can make a powerful way to develop filters. See the lineInteract function for more on that concept.

Example:

main = do
       l <- hGetLines stdin
       hPutStrLns stdout $ filter (startswith "1") l

Lazy Interaction

Character-based

hInteract :: (HVIO a, HVIO b) => a -> b -> (String -> String) -> IO ()

This is similar to the built-in interact, but works on any handle, not just stdin and stdout.

In other words:

interact = hInteract stdin stdout

Line-based

hLineInteract :: (HVIO a, HVIO b) => a -> b -> ([String] -> [String]) -> IO ()

Line-based interaction over arbitrary handles. This is similar to wrapping hInteract with lines and unlines.

One could view this function like this:

hLineInteract finput foutput func =
    let newf = unlines . func . lines in
        hInteract finput foutput newf

Though the actual implementation is this for efficiency:

hLineInteract finput foutput func =
    do
    lines <- hGetLines finput
    hPutStrLns foutput (func lines)

lineInteract :: ([String] -> [String]) -> IO ()

Line-based interaction. This is similar to wrapping your interact functions with lines and unlines. This equality holds:

lineInteract = hLineInteract stdin stdout

Here's an example:

main = lineInteract (filter (startswith "1"))

This will act as a simple version of grep -- all lines that start with 1 will be displayed; all others will be ignored.

Misc. Lazy

lazyMapM :: (a -> IO b) -> [a] -> IO [b]

Applies a given function to every item in a list, and returns the new list. Unlike the system's mapM, items are evaluated lazily.

Optimizations

optimizeForBatch :: IO ()

Sets stdin and stdout to be block-buffered. This can save a huge amount of system resources since far fewer syscalls are made, and can make programs run much faster.

optimizeForInteraction :: IO ()

Sets stdin and stdout to be line-buffered. This saves resources on stdout, but not many on stdin, since it it still looking for newlines.