[darcs-devel] darcs monad

[Florent Becker]

Le 26/08/2012 21:39, Ganesh Sittampalam a écrit :
> Hi,
> 
> I've been going on for a while about the need to replace the IO monad in
> Darcs with something custom. There's some initial discussion of this in
> http://bugs.darcs.net/issue1895
> 
> I hope to get started on this next, after rebase has been merged. I
> think it's best to write the code around a typeclass, albeit that
> initially at least there would only be one implementation. In the long
> run, this should for example make things easier for clients of darcslib
> that are already running in a custom monad.
> 
Don't we want a monad transformer over MonadIO for that?

instance MonadIO m => DarcsMonad (DarcsT m) where […]

--
Florent

> Roughly speaking my current initial plan is to define a class in
> Darcs.Repository.Monad:
> 
> class DarcsMonad m where
>     darcsIO :: IO a -> m a
> 
> Then most darcs code with a return type of IO will be changed to use
> DarcsMonad - e.g. Int -> IO Char would become DarcsMonad m => Int -> m Char.
> 
> As this monad would live at the Repository level, it wouldn't go down
> into the Patch code. Very little of that lives in IO in the first place.
> 
> Any actual uses of IO would be lifted with darcsIO. Of course I could
> just use MonadIO, but the long-term goal is to actually get rid of
> darcsIO and instead abstract all the IO that darcs might want to do into
> the class.
> 
> Getting this basic skeleton is likely to be the most disruptive. Once
> that's done, I would probably focus on moving working directory handling
> into the monad to allow customisation - we want clients of libdarcs to
> have a thread-safe option using absolute paths.
> 
> Of course I haven't actually tried this yet, so it may all work out very
> differently. Let me know if you have any comments etc.
> 
> Cheers,
> 
> Ganesh