From HaskellWiki

Semantics of IO: A Free Approach

The following is inspired by Luke Palmer's post. This only describes one possible semantics of IO a ; your actually implementation may vary.

The idea is to define IO as

data IO a = Done a | PutChar Char ( IO a ) | GetChar ( Char -> IO a )

For simplicity this an example of IO that only gives semantics for teletype IO.

Think of IO a as a tree whose leaves are Done a that holds the result of the program. PutChar is a node that has one child tree and the node holds one character of data. GetChar is a node that has many children; it has one child for every Char , but GetChar holds no data itself.

This tree contains all the information needed to execute teletype interactions. One interprets (or executes) an IO a by tracing a route from root of the tree to a leaf.

If a PutChar node is encountered, the character data contained at that node is output to the terminal and then its subtree is executed. It is at this point that Haskell code evaluated in order to determine what character should be displayed before continuing. If a GetChar node is encountered, a character is read from the terminal (blocking if necessary) and the subtree corresponding to the character received is executed. If Done is encountered the program ends. Done holds the result of the computation, but in the case of main :: IO () the data is of type () and thus contains no information and is ignored.

This execution is not done anywhere in a haskell program, rather it is done by the run-time system.

The monadic operations are defined as follows:

return :: a -> IO a return x = Done x ( >>= ) :: IO a -> ( a -> IO b ) -> IO b Done x >>= f = f x PutChar c x >>= f = PutChar c ( x >>= f ) GetChar g >>= f = GetChar ( \ c -> g c >>= f )

As you can see return is just another name for Done . The bind operation takes a tree x and a function f and replaces the Done nodes (the leaves) of x by a new tree produce by applying f to the data held in the Done nodes.

The primitive IO commands are defined using these constructors.

putChar :: Char -> IO () putChar x = PutChar x ( Done () ) getChar :: IO Char getChar = GetChar ( \ c -> Done c )

The function putChar builds a small IO () tree that contains one PutChar node holding the character data followed by Done .

The function getChar builds a short IO Char tree that begins with a GetChar that holds one Done node holding every character.

Other teletype commands can be defined in terms of these primitives

putStr :: String -> IO () putStr = mapM_ putChar

More generally speaking, IO a will represent the desired interaction with the operating system. For every system call there will be a corresponding constructor in IOTree of the form

| SysCallName p1 p2 ... pn ( r -> IO a )