Haskell: openFile: Read contents of a file

In this post, I am going to explain how to read contents of a file. First we need to open a file, Haskell provides two functions openFile, openBinaryFile to get a file handle. ‘openFile’ is used to work with normal text files, where as openBinaryFile is used to work with binary files.

Prelude> import System.IO
Prelude System.IO> :t openFile
openFile :: FilePath -> IOMode -> IO Handle
Prelude System.IO> 
Prelude System.IO> :t openBinaryFile
openBinaryFile :: FilePath -> IOMode -> IO Handle

Observe the type signature of openFile and openBinaryFile functions. They are taking FilePath, IOMode as arguments. FilePath is a synonym to String, it is defined like below.

type FilePath = String

openFile :: FilePath -> IOMode -> IO Handle

openBinaryFile :: FilePath -> IOMode -> IO Handle

IPMode specifies in which mode file should be opened. Following table summarizes the modes.

IOMode	Is Readable	Is Writable	Description
ReadMode	Y	N	Open file in read mode. File must exist already.
WriteMode	N	Y	Open file in write mode, override he existing file contents. If file is not existed, create new one.
AppendMode	N	Y	Write data to the end of file. File is created if it don’t exist.
ReadWriteMode	Y	Y	File can be readable and writable, Create file, if it don’t exist.

          

openFile method gives a file handle, You can use hGetLine method, which takes this file handle as argument and reads a line from the file.

Prelude System.IO> :t hGetLine

hGetLine :: Handle -> IO String

Once you read the file completely you need to close the file handle. ‘hClose’ function is used to close file handle.

Prelude System.IO> :t hClose

hClose :: Handle -> IO ()

Following is the complete working application, that reads file contents.

FileUtil.hs

import System.IO

getFileContents fileHandle = 
    do
     isEofFile <- hIsEOF fileHandle

     if isEofFile
        then return ()
        else
            do
                 info <- hGetLine fileHandle
                 putStrLn info
                 getFileContents fileHandle

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

$ runghc FileUtil.hs
Enter file name (Including full path) to read
/Users/harikrishna_gurram/quotes.txt

"If you love a flower, don’t pick it up. Because if you pick it up it dies and it ceases to be what you love. So if you love a flower, let it be. Love is not about possession. Love is about appreciation."

"Experience life in all possible ways -- good-bad, bitter-sweet, dark-light, summer-winter. Experience all the dualities. Don't be afraid of experience, because the more experience you have, the more mature you become."

When you are using openFile function, you main get errors like isAlreadyInUseError, isDoesNotExistError, isPermissionError. Following table summarizes the errors.

Error	Description
isAlreadyInUseError	If the file is already open and cannot be reopened
isDoesNotExistError	If the file does not exist
isPermissionError	If the user does not have permission to open the file.

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