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.
Given the above, without uniqueness typing, and because there is clearly
no monad,
I could write
breakMain
:: (Event,Event,SystemState AppState)
-> ((Command,SystemState AppState),(Command,SystemState AppState))
breakMain (e1,e2,sys) = ( eventMain (e1,sys) , eventMain (e2,sys) )
Now what happens? Do we get two copies of SystemState ?
Simpler still, I can write (sys,eventMain e sys) -- what happens here?
I have references to both before- and after- state.
It is this probelm with copying the un-copiable (external I/O state),
that requires pure functional languages to outlaw such programs,
either via uniqueness typing, or via a monad interface that completely
hides any direct reference to the underlying state and which
then sequences the application of state transformers.
--
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Andrew Butterfield Tel: +353-1-896-2517 Fax: +353-1-677-2204
Foundations and Methods Research Group Director.
School of Computer Science and Statistics,
Room F.13, O'Reilly Institute, Trinity College, University of Dublin
http://www.cs.tcd.ie/Andrew.Butterfield/
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