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.
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).
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]])
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)
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]])
The do notation allows programming in a more imperative syntactic
style. A slightly more elaborate version of the previous example
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
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
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 21.