Haksell: hGetContents: Get the file data as string

hGetContents function takes a handle as argument and returns the list of characters corresponding to the unread portion of the channel or file managed by the handle. When you call hGetContents on a handle, it puts the handle into semi-closed state.

Any operation that fails when handle closed is also fail when handle is in semi closed state.

When semi-closed handle becomes closed?

 a. When you call hClose on semi-closed handle

 b. If you got I/O error while reading data from semi closed handle

 c. When all the contents of semi closed handle are read.

What happens when I close a semi-closed handle?

The result contains at least all the items of the stream that were evaluated prior to the handle becoming closed.

Suppose today.txt contains following data.

$ cat today.txt 
1, 2, 3, 4, 5, 6, 7, 8, 9, 10
11, 12, 13, 14, 15, 16, 17, 18, 19, 20
21, 22, 23, 24, 25, 26, 27, 28, 29, 30
31, 32, 33, 34, 35, 36, 37, 38, 39 40

FileUtil.hs

FileUtil.hs
import System.IO

main = 
    do
        putStrLn "Enter file name (Including full path) to read"
        fileName <- getLine
        fileHandle <- openFile fileName ReadMode

        contents <- hGetContents fileHandle
        isHandleClosed <- hIsClosed fileHandle

        putStrLn $ "Is Handle closed " ++ show isHandleClosed
        putStrLn contents

        isHandleClosed <- hIsClosed fileHandle
        putStrLn $ "Is Handle closed " ++ show isHandleClosed

$ runghc FileUtil.hs
Enter file name (Including full path) to read
today.txt
Is Handle closed False
1, 2, 3, 4, 5, 6, 7, 8, 9, 10
11, 12, 13, 14, 15, 16, 17, 18, 19, 20
21, 22, 23, 24, 25, 26, 27, 28, 29, 30
31, 32, 33, 34, 35, 36, 37, 38, 39 40

Is Handle closed True

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