Graham Klyne wrote:
Keean,
As far as I can tell, both your solutions to the "one-shot" problem require that:
(a) the expression to be one-shotted is in the IO monad. That seems reasonable, since why else does one care (semantically speaking)?
(b) they depend on the host operating system platform (semaphores, process id, environment variables) rather than pure Haskell language features.
Wouldn't it be easier to simply define "once" as a common Haskell library function?
#g --
Erm, it is a library function (I provided the NamedSem library as an attachment)... Are you suggesting it would be nice to be able to do this without talking to the OS? Remember a process is an operating system level identity... The boundries of a process are controlled by the OS not the language - therefore I think it is entirely appropriate to use it for this. Also see my proposal for NamedMVars, This would move the concept of this solution into a purely Haskell space, and would not need to communicate with the OS. The NamedMVar library could be implemented with a bit of C and Haskell in a library module - without changes to the language spec and the compiler. I guess I was relly looking for comments on the general technique to detemine if it is worth my while writing this (NamedMVar) library... Keean.