IO Semantics

Note:

• For simplicity, the examples here only gives semantics for teletype I/O.
• These are only some of the various ways to describe the semantics of IO a; your actual implementation may vary.

A free approach

(Inspired by Luke Palmer's post.)

The idea is to define IO as

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

Think of IO a as a tree:

• 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 character, but GetChar holds no data itself.
• Done a (a leaf) is a node that holds the result of the program.

This tree contains all the information needed to execute basic 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 is 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 a Done node is encountered, the program ends.

Done holds the result of the computation, but in the case of Main.main :: IO () the data is of type () and thus ignored as it contains no information.

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 produced by applying f to the data held in the Done nodes.

The primitive I/O 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 node that holds one Done node for every character.

Other 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 I/O-tree constructor of the form:

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

where:

• p1 ... pn are the parameters for the system call,
• and r is the result of the system call.

(Thus PutChar and GetChar will not occur as constructors for I/O trees if they don't correspond to system calls).

Further reading

• The Haskell 2010 Report

ed. Simon Marlow Marlow, 2010.

• A Functional Specification of Effects

Wouter Swierstra. Ph.D. thesis, University of Nottingham. (2009).

• Beauty in the Beast: A Functional Semantics for the Awkward Squad

Wouter Swierstra, Thorsten Altenkirch. In: Proceedings of the ACM SIGPLAN Workshop on Haskell, Haskell ’07, ACM, New York, NY, USA, pages 25–36 (2007).

• A Semantic Framework for Deterministic Functional Input/Output

Malcolm Dowse. PhD dissertation, University of Dublin, Trinity College (2006).

• Semantics of fixIO

Levent Erkök, John Launchbury, Andrew Moran. In Fixed Points in Computer Science Workshop, FICS'01 (2001).

• Tackling the Awkward Squad: monadic input/output, concurrency, exceptions, and foreign-language calls in Haskell

Simon Peyton Jones. In "Engineering theories of software construction", ed. Tony Hoare, Manfred Broy, Ralf Steinbruggen, IOS Press, ISBN 1 58603 1724, 2001, pages 47-96.

• Relating operational and denotational semantics for input/output effects

Roy L. Crole, Andrew D. Gordon. Mathematical Structures in Computer Science 9(2): 125-158 (1999).

• How to Declare an Imperative

Philip Wadler. ACM Computing Surveys, 29(3): 240-263, September 1997.

• A Sound Metalogical Semantics for Input/Output Effects

Andrew Gordon. In International Workshop on Computer Science Logic, January 1995. Springer Berlin Heidelberg.

• An Operational Semantics for I/O in a Lazy Functional Language

Andrew Gordon. In FPCA '93: Conference on Functional Programming Languages and Computer Architecture, Copenhagen, June 1993. ACM Press.

• Functional Programming and Input/Output

Andrew Gordon. Cambridge University Press. Revision of 1992 PhD dissertation.

• PFL+: A Kernel Scheme for Functional I/O

Andrew Gordon. Computer Laboratory Technical Report Number 160, University of Cambridge (1989).

• Interactive Functional Programs: A Method and a Formal Semantics

Simon Thompson. Technical Report 48, Computing Laboratory, University of Kent, Canterbury, UK, November 1987.