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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