Data.Text.IO

Efficient locale-sensitive support for text I/O.

Skip past the synopsis for some important notes on performance and portability across different versions of GHC.

Performance

The functions in this module obey the runtime system's locale, character set encoding, and line ending conversion settings.

If you know in advance that you will be working with data that has a specific encoding (e.g. UTF-8), and your application is highly performance sensitive, you may find that it is faster to perform I/O with bytestrings and to encode and decode yourself than to use the functions in this module.

Whether this will hold depends on the version of GHC you are using, the platform you are working on, the data you are working with, and the encodings you are using, so be sure to test for yourself.

Locale support

Note: The behaviour of functions in this module depends on the version of GHC you are using.

Beginning with GHC 6.12, text I/O is performed using the system or handle's current locale and line ending conventions.

Under GHC 6.10 and earlier, the system I/O libraries do not support locale-sensitive I/O or line ending conversion. On these versions of GHC, functions in this library all use UTF-8. What does this mean in practice?

• All data that is read will be decoded as UTF-8.
• Before data is written, it is first encoded as UTF-8.
• On both reading and writing, the platform's native newline conversion is performed.

If you must use a non-UTF-8 locale on an older version of GHC, you will have to perform the transcoding yourself, e.g. as follows:

import qualified Data.ByteString as B
import Data.Text (Text)
import Data.Text.Encoding (encodeUtf16)

putStr_Utf16LE :: Text -> IO ()
putStr_Utf16LE t = B.putStr (encodeUtf16LE t)

readFile :: FilePath -> IO Text

The readFile function reads a file and returns the contents of the file as a string. The entire file is read strictly, as with getContents.

writeFile :: FilePath -> Text -> IO ()

Write a string to a file. The file is truncated to zero length before writing begins.

appendFile :: FilePath -> Text -> IO ()

Write a string the end of a file.

Operations on handles

hGetContents :: Handle -> IO Text

Read the remaining contents of a Handle as a string. The Handle is closed once the contents have been read, or if an exception is thrown.

Internally, this function reads a chunk at a time from the lower-level buffering abstraction, and concatenates the chunks into a single string once the entire file has been read.

As a result, it requires approximately twice as much memory as its result to construct its result. For files more than a half of available RAM in size, this may result in memory exhaustion.

hGetChunk :: Handle -> IO Text

Experimental. Read a single chunk of strict text from a Handle. The size of the chunk depends on the amount of input currently buffered.

This function blocks only if there is no data available, and EOF has not yet been reached. Once EOF is reached, this function returns an empty string instead of throwing an exception.

hGetLine :: Handle -> IO Text

Read a single line from a handle.

hPutStr :: Handle -> Text -> IO ()

Write a string to a handle.

hPutStrLn :: Handle -> Text -> IO ()

Write a string to a handle, followed by a newline.

Special cases for standard input and output

interact :: (Text -> Text) -> IO ()

The interact function takes a function of type Text -> Text as its argument. The entire input from the standard input device is passed to this function as its argument, and the resulting string is output on the standard output device.

getContents :: IO Text

Read all user input on stdin as a single string.

getLine :: IO Text

Read a single line of user input from stdin.

putStr :: Text -> IO ()

Write a string to stdout.

putStrLn :: Text -> IO ()

Write a string to stdout, followed by a newline.