[Haskell-cafe] Future of MonadCatchIO

[Edward Z. Yang]

While block and unblock have been removed from base, they are still implementable
in modern GHC.  So another possible future is to deprecate MonadCatchIO
(which should have been done a while ago, honestly!), but manually redefine
the functions so that old code keeps working.

Edward

Excerpts from Arie Peterson's message of Sun Mar 03 07:40:06 -0800 2013:
> Hi all,
> 
> 
> The function 'block' and 'unblock' (from Control.Exception) have been 
> deprecated for some time, and are apparantly now being removed (in favour of 
> 'mask').
> 
> Generalisations of these functions are (part of) the interface of 
> MonadCatchIO-transformers (the 'MonadCatchIO' class has methods 'block' and 
> 'unblock'). So, the interface would have to change to keep up with base.
> 
> I'm inclined to deprecate MonadCatchIO-transformers itself, in favour of 
> monad-control.
> 
> I suspect that most clients do not use 'block' or 'unblock' directly, but use 
> only derived functions, like 'bracket'. (I have partly confirmed this, by 
> inspecting some reverse dependencies on hackage.) This allow an easy 
> transition to monad-control: in many cases, only imports will need to be 
> changed. In the minority of cases where 'block' and 'unblock' are used and/or 
> instances of MonadCatchIO are defined, code will need to be updated.
> 
> There is a difference in functionality between MonadCatchIO and monad-control. 
> In the former, 'bracket' will not perform the final action if the main action 
> is an ErrorT that throws an error (in contrast with exceptions in the 
> underlying IO monad). In monad-control, 'bracket' will perform the final action 
> in this case. (See this discussion for background:
> <http://www.haskell.org/pipermail/haskell-cafe/2010-October/084890.html>.)
> 
> Probably, in most use cases the behaviour of monad-control is preferred. This 
> seems to be the case also for snap, which uses MonadCatchIO-transformers, but 
> defines its own variant of 'bracket' to get the right behaviour.
> 
> 
> Would anyone have a problem with a deprecation of MonadCatchIO-transformers, 
> and a failure to update it to work with a base without 'block' and 'unblock'?
> 
> 
> Regards,
> 
> Arie
>