Just for the culture (because Heinrich's solution is undoubtly simpler) you
can also achive this using continuations.
This is one of the purposes of the Cont monad, used jointly with a State
monad, you can store the continuations and resume them later.
2011/6/10 Heinrich Apfelmus
Alexander V Vershilov wrote:
I'm writing a small tcp server with that can handle connections and answer by rules writen in a small script that can be interpreted by server. For this purpose I've written an interpreter that has type
ErrorT MyError (StateT ScriptState IO)
so I can call "native" IO function in that script, and define new one. I can run this script with runState (runErrorT (...)) oldState.
But there is one problem: in script i should be able to call functions that will stop script interpretation and wait for some server event. To continue interpretation. Can smb give an advice what is the best way to do it?
Basically, you want to stop the interpreter and resume it at some later point. You need to implement a custom monad for that, which can easily be done with the help of my operational package.
http://projects.haskell.org/operational/
In particular, the examples
WebSessionState.lhs TicTacToe.hs PoorMansConcurrency.hs
show how to suspend and resume the control flow. Feel free to request additional examples.
Best regards, Heinrich Apfelmus
-- http://apfelmus.nfshost.com
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