[Haskell-cafe] I/O without monads, using an event loop

[Edsko de Vries]

On Fri, May 30, 2008 at 03:09:37PM +0100, Robin Green wrote:
> I have been thinking about to what extent you could cleanly do I/O
> without explicit use of the I/O monad, and without uniqueness types
> (which are the main alternative to monads in pure functional
> programming, and are used in the Concurrent Clean programming language).
> 
> Suppose you have a main event handler function, like this:
> 
> eventMain :: (Event, SystemState AppState) -> (Command, SystemState
> AppState)
> 
> This function could be called over and over in an event loop, until an
> EndProgram command was received, and the event loop would itself do all
> the actual I/O (the SystemStates are only in-memory representations of
> some part of the system state, plus the application's own state). Things
> like disk I/O could be done with commands which generate events when
> complete. Interprocess communication could be done in the same way.
> 
> Then eventMain, and everything called by it, would be
> referentially-transparent, and yet non-monadic. You could of course
> build higher-level stuff on top of that.
> 
> On the other hand, it's quite stateful, because anything you need to
> remember between events need to be recorded, either in the SystemState
> or externally (e.g. in a file). I suppose this is the most important
> disadvantage?
> 
> Is there any published work or code using this approach, or something
> like it, in a pure functional language? I'm primarily interested in
> embedded system and desktop UIs, rather than say web-based systems,
> although both would be interesting.

Yeah, check the History of Haskell paper, in particular Section 7.

Edsko