On 2009 Mar 25, at 0:24, Александър Л. Димитров wrote:
The gtk2hs docs suggest I 'keep around' the connectId, but herein lies my problem. The loadFile function can be called from several spots in the application (and I'd like to keep it that way.) Even if I wrapped loadFile in a State monad, those entry points to load file (command line, menu, button, possibly more) have no way of knowing the state of connectId, lest I make it global (*shudders*) using unsafePerformIO and a Maybe IORef.
You can make it "global" without using unsafePerformIO: Set up a ReaderT MyState IO and run the program inside it, after initializing your MyState with appropriate IORefs. You need to lift gtk2hs functions, and callbacks have to be wrapped to push them back up into the ReaderT: curState <- ask lift $ widget `on` buttonPressEvent $ runReaderT curState . myCallback The reason you want a ReaderT instead of a StateT is that any state not accessed via an IORef can't be propagated between the mainline code and callbacks, so you want something that is forced to be read- only after initialization except via an IORef stored within it. Aside: I've suggested at times that gtk2hs use class MonadIO instead of type IO, which would make explicit lifting (and I think "dropping") unnecessary. I never made a formal enhancement suggestion though; I should do so. -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery@kf8nh.com system administrator [openafs,heimdal,too many hats] allbery@ece.cmu.edu electrical and computer engineering, carnegie mellon university KF8NH