Re: [Haskell-cafe] Re: MonadCatchIO, finally and the error monad

On Thu, Oct 14, 2010 at 11:13 PM, Antoine Latter wrote:

On Thu, Oct 14, 2010 at 1:28 PM, Michael Snoyman 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