Chapter 7
Basic Input/Output

The I/O system in Haskell is purely functional, yet has all of the expressive power found in conventional programming languages. To achieve this, Haskell uses a monad to integrate I/O operations into a purely functional context.

The I/O monad used by Haskell mediates between the values natural to a functional language and the actions that characterize I/O operations and imperative programming in general. The order of evaluation of expressions in Haskell is constrained only by data dependencies; an implementation has a great deal of freedom in choosing this order. Actions, however, must be ordered in a well-defined manner for program execution – and I/O in particular – to be meaningful. Haskell’s I/O monad provides the user with a way to specify the sequential chaining of actions, and an implementation is obliged to preserve this order.

The term monad comes from a branch of mathematics known as category theory. From the perspective of a Haskell programmer, however, it is best to think of a monad as an abstract datatype. In the case of the I/O monad, the abstract values are the actions mentioned above. Some operations are primitive actions, corresponding to conventional I/O operations. Special operations (methods in the class Monad, see Section 6.3.6) sequentially compose actions, corresponding to sequencing operators (such as the semicolon) in imperative languages.

7.1 Standard I/O Functions

Although Haskell provides fairly sophisticated I/O facilities, as defined in the IO library, it is possible to write many Haskell programs using only the few simple functions that are exported from the Prelude, and which are described in this section.

All I/O functions defined here are character oriented. The treatment of the newline character will vary on different systems. For example, two characters of input, return and linefeed, may read as a single newline character. These functions cannot be used portably for binary I/O.

In the following, recall that String is a synonym for [Char] (Section 6.1.2).

Output Functions These functions write to the standard output device (this is normally the user’s terminal).

  putChar  :: Char -> IO ()  
  putStr   :: String -> IO ()  
  putStrLn :: String -> IO ()  -- adds a newline  
  print    :: Show a => a -> IO ()

The print function outputs a value of any printable type to the standard output device. Printable types are those that are instances of class Show; print converts values to strings for output using the show operation and adds a newline.

For example, a program to print the first 20 integers and their powers of 2 could be written as:

main = print ([(n, 2^n) | n <- [0..19]])

Input Functions These functions read input from the standard input device (normally the user’s terminal).

  getChar     :: IO Char  
  getLine     :: IO String  
  getContents :: IO String  
  interact    :: (String -> String) -> IO ()  
  readIO      :: Read a => String -> IO a  
  readLn      :: Read a => IO a

The getChar operation raises an exception (Section 7.3) on end-of-file; a predicate isEOFError that identifies this exception is defined in the IO library. The getLine operation raises an exception under the same circumstances as hGetLine, defined the IO library.

The getContents operation returns all user input as a single string, which is read lazily as it is needed. The interact function takes a function of type String->String 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.

Typically, the read operation from class Read is used to convert the string to a value. The readIO function is similar to read except that it signals parse failure to the I/O monad instead of terminating the program. The readLn function combines getLine and readIO.

The following program simply removes all non-ASCII characters from its standard input and echoes the result on its standard output. (The isAscii function is defined in a library.)

main = interact (filter isAscii)

Files These functions operate on files of characters. Files are named by strings using some implementation-specific method to resolve strings as file names.

The writeFile and appendFile functions write or append the string, their second argument, to the file, their first argument. The readFile function reads a file and returns the contents of the file as a string. The file is read lazily, on demand, as with getContents.

  type FilePath =  String  
  writeFile  :: FilePath -> String -> IO ()  
  appendFile :: FilePath -> String -> IO ()  
  readFile   :: FilePath           -> IO String

Note that writeFile and appendFile write a literal string to a file. To write a value of any printable type, as with print, use the show function to convert the value to a string first.

main = appendFile "squares" (show [(x,x⋆x) | x <- [0,0.1..2]])

7.2 Sequencing I/O Operations

The type constructor IO is an instance of the Monad class. The two monadic binding functions, methods in the Monad class, are used to compose a series of I/O operations. The >> function is used where the result of the first operation is uninteresting, for example when it is (). The >>= operation passes the result of the first operation as an argument to the second operation.

  (>>=) :: IO a -> (a -> IO b) -> IO b  
  (>>)  :: IO a -> IO b        -> IO b

For example,

main = readFile "input-file"                       >>= \ s ->  
       writeFile "output-file" (filter isAscii s)  >>  
       putStr "Filtering successful\n"

is similar to the previous example using interact, but takes its input from "input-file" and writes its output to "output-file". A message is printed on the standard output before the program completes.

The do notation allows programming in a more imperative syntactic style. A slightly more elaborate version of the previous example would be:

main = do  
        putStr "Input file: "  
        ifile <- getLine  
        putStr "Output file: "  
        ofile <- getLine  
        s <- readFile ifile  
        writeFile ofile (filter isAscii s)  
        putStr "Filtering successful\n"

The return function is used to define the result of an I/O operation. For example, getLine is defined in terms of getChar, using return to define the result:

getLine :: IO String  
getLine = do c <- getChar  
             if c == '\n' then return ""  
                          else do s <- getLine  
                                  return (c:s)

7.3 Exception Handling in the I/O Monad

The I/O monad includes a simple exception handling system. Any I/O operation may raise an exception instead of returning a result.

Exceptions in the I/O monad are represented by values of type IOError. This is an abstract type: its constructors are hidden from the user. The IO library defines functions that construct and examine IOError values. The only Prelude function that creates an IOError value is userError. User error values include a string describing the error.

  userError :: String -> IOError

Exceptions are raised and caught using the following functions:

  ioError :: IOError -> IO a  
  catch   :: IO a    -> (IOError -> IO a) -> IO a 

The ioError function raises an exception; the catch function establishes a handler that receives any exception raised in the action protected by catch. An exception is caught by the most recent handler established by catch. These handlers are not selective: all exceptions are caught. Exception propagation must be explicitly provided in a handler by re-raising any unwanted exceptions. For example, in

f = catch g (\e -> if IO.isEOFError e then return [] else ioError e)

the function f returns [] when an end-of-file exception occurs in g; otherwise, the exception is propagated to the next outer handler. The isEOFError function is part of IO library.

When an exception propagates outside the main program, the Haskell system prints the associated IOError value and exits the program.

The fail method of the IO instance of the Monad class (Section 6.3.6) raises a userError, thus:

  instance Monad IO where  
    ...bindings for return, (>>=), (>>)  
    fail s = ioError (userError s)

The exceptions raised by the I/O functions in the Prelude are defined in Chapter 42.