On Tue, Dec 20, 2011 at 11:08 AM, Michael Serra
Hello Haskellers, I'm implementing a simple tree-manipulating (for sports tournaments) application prototype, with SDL for graphics and simple user interaction. For reference, I've posted the code on hpaste. My question is about code organization: everything was simple and elegant until I started writing the program's display/event loop. Every function in this section has to be passed the same parameters - the application window to draw on, the font to display text with, the tree representing the current application state, etc. The font is an especially egregious example of the problem, because it's only used by one function but to get there it must be threaded through all of them (looking at the hpaste, you will see I don't want to call openFont on every invocation of drawTexts; what's needed is to call it once in main and have the resulting value available to drawTxt. So my question: how can I mitigate the ugliness of this state-threading? I understand this is one purpose for monads; am I supposed to implement a monad transformer for this?
I think it would be great if I could define a type AppState as a tuple of the various things I need to thread, and specify some kind of automatic as-pattern, so that every function taking this AppState parameter would implicitly have its components bound to certain preset names. I've never seen anyone do this however. What is the right solution?
I've done that before in web-apps. It is conventional enough. To ease the threading-through of paramters you can use a Reader monad (perhaps ReaderT AppState IO). I don't know how well this fits in with the SDL libraries, though. Antoine
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