Krasimir Angelov wrote:
[...] Unfortunately proper implementation requires functional dependencies.
Hmmm, this is really a bit unfortunate, because I wanted to stay within the realms of Haskell 98 for HOpenGL. GHC and Hugs support this extension, but NHC98 doesn't even have MPTC, IIRC. But we'll always have #ifdef... :-] Another issue is the quality of the error messages, but after a little experimentation I found them quite bearable.
The attached module contains definitions for get and set functions for IO, ST, Reader, ReaderT, State, StateT, RWS and RWST monads. The idea is that each monad can maintains some kind of state and the corresponding Attr type gives access to some part of the state.
Cool! Only a few minor points: * It's a bit strange that a class with a "get" method is called "Readable". "HasGetter" or "Gettable" would look more consistent to me, but I'm not a native speaker. * A list version of (=:) is missing (called "set" in GIO/HOpenGL). But even its type wouldn't be clear to me given this general version of (=:). Hmmm... * Some people have already mentioned that "=:" may be a bit misleading, so a different operator may be appropriate. But not ":=" at the price of even more non-standard typing!
[...] I propose to standardize this concept and add Attr module to the GHC base package.
That was my intention. We should probably move this thread to the libraries list and continue discussion there (e.g. naming, where should the classes and instances reside in the module hierarchy, etc.). Krasimir: Do you want to propose your Attr module there (without the GIO/HOpenGL stuff, of course)? It's your baby, after all... Cheers, S.