Dean Herington wrote: > At 7:31 PM +0100 2/24/06, minh thu wrote: >> Hi all, >> >> 1/ >> I'd like to know how can I implement an interactive program (an >> editor) in haskell where some "things" have to be updated. >> The "things" can be text in a word processor, or a pixel array in a 2d >> graphics editor, and so on. >> Have I to pass the state (the "things") explicitely in the arguments >> of the main loop (a recursive function) ? >> Sorry if it's stupid... > > No, it's not a stupid question at all... > > Essentially, the answer is "yes", the state needs to be passed around > (neglecting hackery to simulate global variables that is better > avoided). However, this can be made convenient by using a monad. You > define a monad which is based on IO and carries your state (something > like StateT YourState IO a; see > http://www.haskell.org/ghc/docs/latest/html/libraries/mtl/Control-Monad-State.html). >
If the things need to be more explicitly mutable you can *) Create a bunch of IORefs (or MVar for thread safety) *) Put them into some data structure (your own or perhaps tuples) *) Use a ReaderT from Control.Monad.Reader, ReaderT YourData IO a Then you can *) lookup the IORef you need with using "asks" *) read/write/update that mutable data _______________________________________________ Haskell mailing list Haskell@haskell.org http://www.haskell.org/mailman/listinfo/haskell
