IO

Getting the ‘a’ “out of” ‘IO a’

A common question is “I have a value of IO a, but I want to do something to that a value: how do I get access to it?” How can one operate on data that comes from the outside world (for example, incrementing a number typed by the user)?

The point is that if you use a pure function on data obtained impurely, then the result is still impure. It depends on what the user did! A value of type IO a stands for a “side-effecting computation resulting in a value of type a” which can only be run by (a) composing it into main and (b) compiling and executing your program. For that reason, there is no way within pure Haskell world to “get the a out”.

Instead, we want to build a new computation, a new IO value, which makes use of the a value at runtime. This is another way of composing IO values and so again we can use do-notation:

-- assuming
myComputation :: IO Int

getMessage :: Int -> String
getMessage int = "My computation resulted in: " ++ show int

newComputation :: IO ()
newComputation = do
  int <- myComputation       -- we "bind" the result of myComputation to a name, 'int'
  putStrLn $ getMessage int   -- 'int' holds a value of type Int

Here we’re using a pure function (getMessage) to turn an Int into a String, but we’re using do notation to make it be applied to the result of an IO computation myComputation when (after) that computation runs. The result is a bigger IO computation, newComputation. This technique of using pure functions in an impure context is called lifting.

IO defines your program’s main action

To make a Haskell program executable you must provide a file with a main function of type IO ()

main :: IO ()
main = putStrLn "Hello world!"

When Haskell is compiled it examines the IO data here and turns it into a executable. When we run this program it will print Hello world!.

If you have values of type IO a other than main they won’t do anything.

other :: IO ()
other = putStrLn "I won't get printed"

main :: IO ()
main = putStrLn "Hello world!"

Compiling this program and running it will have the same effect as the last example. The code in other is ignored.

In order to make the code in other have runtime effects you have to compose it into main. No IO values not eventually composed into main will have any runtime effect. To compose two IO values sequentially you can use do-notation:

other :: IO ()
other = putStrLn "I will get printed... but only at the point where I'm composed into main"

main :: IO ()
main = do
  putStrLn "Hello world!"
  other

When you compile and run this program it outputs

Hello world!
I will get printed... but only at the point where I'm composed into main

Note that the order of operations is described by how other was composed into main and not the definition order.

Checking for end-of-file conditions

A bit counter-intuitive to the way most other languages’ standard I/O libraries do it, Haskell’s isEOF does not require you to perform a read operation before checking for an EOF condition; the runtime will do it for you.

import System.IO( isEOF )


eofTest :: Int -> IO ()
eofTest line = do
    end <- isEOF
    if end then
        putStrLn $ "End-of-file reached at line " ++ show line ++ "."
    else do
        getLine
        eofTest $ line + 1


main :: IO ()
main =
    eofTest 1

Input:

Line #1.
Line #2.
Line #3.

Output:

End-of-file reached at line 4.

Reading all contents of standard input into a string

main = do
    input <- getContents
    putStr input

Input:

This is an example sentence.
And this one is, too!

Output:

This is an example sentence.
And this one is, too!

Note: This program will actually print parts of the output before all of the input has been fully read in. This means that, if, for example, you use getContents over a 50MiB file, Haskell’s lazy evaluation and garbage collector will ensure that only the parts of the file that are currently needed (read: indispensable for further execution) will be loaded into memory. Thus, the 50MiB file won’t be loaded into memory at once.

Role and Purpose of IO

Haskell is a pure language, meaning that expressions cannot have side effects. A side effect is anything that the expression or function does other than produce a value, for example, modify a global counter or print to standard output.

In Haskell, side-effectful computations (specifically, those which can have an effect on the real world) are modelled using IO. Strictly speaking, IO is a type constructor, taking a type and producing a type. For example, IO Int is the type of an I/O computation producing an Int value. The IO type is abstract, and the interface provided for IO ensures that certain illegal values (that is, functions with non-sensical types) cannot exist, by ensuring that all built-in functions which perform IO have a return type enclosed in IO.

When a Haskell program is run, the computation represented by the Haskell value named main, whose type can be IO x for any type x, is executed.

Manipulating IO values

There are many functions in the standard library providing typical IO actions that a general purpose programming language should perform, such as reading and writing to file handles. General IO actions are created and combined primarily with two functions:

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

This function (typically called bind) takes an IO action and a function which returns an IO action, and produces the IO action which is the result of applying the function to the value produced by the first IO action.

return :: a -> IO a

This function takes any value (i.e., a pure value) and returns the IO computation which does no IO and produces the given value. In other words, it is a no-op I/O action.

There are additional general functions which are often used, but all can be written in terms of the two above. For example, (>>) :: IO a -> IO b -> IO b is similar to (>>=) but the result of the first action is ignored.

A simple program greeting the user using these functions:

main :: IO ()
 main =
   putStrLn "What is your name?" >>
   getLine >>= \name ->
   putStrLn ("Hello " ++ name ++ "!")

This program also uses putStrLn :: String -> IO () and getLine :: IO String.

Note: the types of certain functions above are actually more general than those types given (namely >>=, >> and return).

IO semantics

The IO type in Haskell has very similar semantics to that of imperative programming languages. For example, when one writes s1 ; s2 in an imperative language to indicate executing statement s1, then statement s2, one can write s1 >> s2 to model the same thing in Haskell.

However, the semantics of IO diverge slightly of what would be expected coming from an imperative background. The return function does not interrupt control flow - it has no effect on the program if another IO action is run in sequence. For example, return () >> putStrLn "boom" correctly prints “boom” to standard output.

The formal semantics of IO can given in terms of simple equalities involving the functions in the previous section:

return x >>= f ≡ f x, ∀ f x
 y >>= return ≡ return y, ∀ y
 (m >>= f) >>= g ≡ m >>= (\x -> (f x >>= g)), ∀ m f g

These laws are typically referred to as left identity, right identity, and composition, respectively. They can be stated more naturally in terms of the function

(>=>) :: (a -> IO b) -> (b -> IO c) -> a -> IO c
 (f >=> g) x = (f x) >>= g

as follows:

return >=> f ≡ f, ∀ f
 f >=> return ≡ f, ∀ f
 (f >=> g) >=> h ≡ f >=> (g >=> h), ∀ f g h

Lazy IO

Functions performing I/O computations are typically strict, meaning that all preceding actions in a sequence of actions must be completed before the next action is begun. Typically this is useful and expected behaviour - putStrLn "X" >> putStrLn "Y" should print “XY”. However, certain library functions perform I/O lazily, meaning that the I/O actions required to produce the value are only performed when the value is actually consumed. Examples of such functions are getContents and readFile. Lazy I/O can drastically reduce the performance of a Haskell program, so when using library functions, care should be taken to note which functions are lazy.

IO and do notation

Haskell provides a simpler method of combining different IO values into larger IO values. This special syntax is known as do notation* and is simply syntactic sugar for usages of the >>=, >> and return functions.

The program in the previous section can be written in two different ways using do notation, the first being layout-sensitive and the second being layout insensitive:

main = do
   putStrLn "What is your name?"
   name <- getLine
   putStrLn ("Hello " ++ name ++ "!")


 main = do {
   putStrLn "What is your name?" ;
   name <- getLine ;
   putStrLn ("Hello " ++ name ++ "!")
   }

All three programs are exactly equivalent.

*Note that do notation is also applicable to a broader class of type constructors called monads.

Reading a line from standard input

main = do
    line <- getLine
    putStrLn line

Input:

This is an example.

Output:

This is an example.

Reading words from an entire file

In Haskell, it often makes sense not to bother with file handles at all, but simply read or write an entire file straight from disk to memory^†, and do all the partitioning/processing of the text with the pure string data structure. This avoids mixing IO and program logic, which can greatly help avoiding bugs.

-- | The interesting part of the program, which actually processes data
--   but doesn't do any IO!
reverseWords :: String -> [String]
reverseWords = reverse . words

-- | A simple wrapper that only fetches the data from disk, lets
--   'reverseWords' do its job, and puts the result to stdout.
main :: IO ()
main = do
   content <- readFile "loremipsum.txt"
   mapM_ putStrLn $ reverseWords content

If loremipsum.txt contains

Lorem ipsum dolor sit amet,
consectetur adipiscing elit

then the program will output

elit
adipiscing
consectetur
amet,
sit
dolor
ipsum
Lorem

Here, mapM_ went through the list of all words in the file, and printed each of them to a separate line with putStrLn.

^†_{If you think this is wasteful on memory, you have a point. Actually, Haskell’s laziness can often avoid that the entire file needs to reside in memory simultaneously… but beware, this kind of lazy IO causes its own set of problems. For performance-critical applications, it often makes sense to enforce the entire file to be read at once, strictly; you can do this with the Data.Text version of readFile.}

Writing to stdout

Per the Haskell 2010 Language Specification the following are standard IO functions available in Prelude, so no imports are required to use them.

putChar :: Char -> IO () - writes a char to stdout

Prelude> putChar 'a'
aPrelude>  -- Note, no new line

putStr :: String -> IO () - writes a String to stdout

Prelude> putStr "This is a string!"
This is a string!Prelude>  -- Note, no new line

putStrLn :: String -> IO () - writes a String to stdout and adds a new line

Prelude> putStrLn "Hi there, this is another String!"
Hi there, this is another String!

print :: Show a => a -> IO () - writes a an instance of Show to stdout

Prelude> print "hi"
"hi"
Prelude> print 1
1
Prelude> print 'a'
'a'
Prelude> print (Just 'a')  -- Maybe is an instance of the `Show` type class
Just 'a'
Prelude> print Nothing
Nothing

Recall that you can instantiate Show for your own types using deriving:

-- In ex.hs
data Person = Person { name :: String } deriving Show
main = print (Person "Alex")  -- Person is an instance of `Show`, thanks to `deriving`

Loading & running in GHCi:

Prelude> :load ex.hs
[1 of 1] Compiling ex             ( ex.hs, interpreted )
Ok, modules loaded: ex.
*Main> main  -- from ex.hs
Person {name = "Alex"}
*Main>

Parsing and constructing an object from standard input

readFloat :: IO Float
readFloat =
    fmap read getLine


main :: IO ()
main = do
    putStr "Type the first number: "
    first <- readFloat

    putStr "Type the second number: "
    second <- readFloat

    putStrLn $ show first ++ " + " ++ show second ++ " = " ++ show ( first + second )

Input:

Type the first number: 9.5
Type the second number: -2.02

Output:

9.5 + -2.02 = 7.48

Reading from file handles

Like in several other parts of the I/O library, functions that implicitly use a standard stream have a counterpart in System.IO that performs the same job, but with an extra parameter at the left, of type Handle, that represents the stream being handled. For instance, getLine :: IO String has a counterpart hGetLine :: Handle -> IO String.

import System.IO( Handle, FilePath, IOMode( ReadMode ),
                  openFile, hGetLine, hPutStr, hClose, hIsEOF, stderr )

import Control.Monad( when )


dumpFile :: Handle -> FilePath -> Integer -> IO ()

dumpFile handle filename lineNumber = do      -- show file contents line by line
    end <- hIsEOF handle
    when ( not end ) $ do
        line <- hGetLine handle
        putStrLn $ filename ++ ":" ++ show lineNumber ++ ": " ++ line
        dumpFile handle filename $ lineNumber + 1


main :: IO ()

main = do
    hPutStr stderr "Type a filename: "
    filename <- getLine
    handle <- openFile filename ReadMode
    dumpFile handle filename 1
    hClose handle

Contents of file example.txt:

This is an example.
Hello, world!
This is another example.

Input:

Type a filename: example.txt

Output:

example.txt:1: This is an example.
example.txt:2: Hello, world!
example.txt:3: This is another example

Reading from stdin

As-per the Haskell 2010 Language Specification, the following are standard IO functions available in Prelude, so no imports are required to use them.

getChar :: IO Char - read a Char from stdin

-- MyChar.hs
main = do
  myChar <- getChar
  print myChar

-- In your shell

runhaskell MyChar.hs
a -- you enter a and press enter
'a'  -- the program prints 'a'

getLine :: IO String - read a String from stdin, sans new line character

Prelude> getLine
Hello there!  -- user enters some text and presses enter
"Hello there!"

read :: Read a => String -> a - convert a String to a value

Prelude> read "1" :: Int
1
Prelude> read "1" :: Float
1.0
Prelude> read "True" :: Bool
True

Other, less common functions are:

• getContents :: IO String - returns all user input as a single string, which is read lazily as it is needed
• interact :: (String -> String) -> IO () - 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