On Thu, Oct 14, 2010 at 11:13 PM, Antoine Latter <aslat...@gmail.com> wrote: > On Thu, Oct 14, 2010 at 1:28 PM, Michael Snoyman <mich...@snoyman.com> wrote: >> >> I thought a bit about this, and I believe the only extra primitive we >> need is one of bracket, bracket_ or finally. I also noticed the >> exception-transformers package[1], which seems to be a good >> replacement for MonadCatchIO. I contacted the author about adding a >> MonadBracket typeclass, and he said he'll try to get to it. I'm >> planning on making that my replacement for MonadCatchIO (assuming it >> turns out correctly), and if so I'd recommend others do the same. >> > > Ah! > > I didn't notice at first that the Exception type class was simply > re-exported from base. At first glance, that package looked like > entirely too much for what I wanted.
For a while now I've had the idea of "inverting" monads: flipping them inside out so that we can automatically apply any of the special IO-specific functions to them. I always got caught when implementing Reader and State, but last night I had a break-through and finally got it to work. For the moment, the code is up in my neither repo on github[1], and it has a fairly complete test suite[2]. It's a pretty complicated implementation, since it needs to be generic enough to address a wide variety of monads, but I'm fairly certain that the theoretical foundations are sound. I intend to write a full blog post to explain the idea. With this, I have been able to implement catch, finally, bracket, alloca, allocaBytes and withForeignPtr simply by calling the standard IO-specific functions. I'm excited about this approach: I think it has the possibility to liberate us from a lot of tedious, error-prone code. Michael [1] http://github.com/snoyberg/neither/blob/master/Control/Monad/Invert.hs [2] http://github.com/snoyberg/neither/blob/master/runtests.hs _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
