[Haskell-cafe] Monad-control rant

Edward Z. Yang ezyang at MIT.EDU
Tue Jan 10 17:12:59 CET 2012

Excerpts from Mikhail Vorozhtsov's message of Tue Jan 10 09:54:38 -0500 2012:
> On 01/10/2012 12:17 AM, Edward Z. Yang wrote:
> > Hello Mikhail,
> Hi.
> >
> > (Apologies for reviving a two month old thread). Have you put some thought into
> > whether or not these extra classes generalize in a way that is not /quite/ as
> > general as MonadBaseControl (so as to give you the power you need) but still
> > allow you to implement the functionality you are looking for? I'm not sure but
> > it seems something along the lines of unwind-protect ala Scheme might be
> > sufficient.
> I'm not sure I'm following you. The problem with MonadBaseControl is 
> that it is /not/ general enough.

Sorry, I mispoke.  The sense you are using it is "the more general a type class
is, the more instances you can write for it." I think the design goal I'm going
for here is, "a single signature which covers MonadAbort/Recover/Finally in a
way that unifies them."  Which is not more general, except in the sense that it
"contains" more type classes (certainly not general in the mathematical sense.)

> It assumes that you can eject/inject 
> all the stacked effects as a value of some data type. Which works fine 
> for the standard transformers because they are /implemented/ this way. 
> But not for monads that are implemented in operational style, as 
> interpreters, because the interpreter state cannot be internalized. This 
> particular implementation bias causes additional issues when the lifted 
> operation is not fully suited for ejecting/injecting. For example the 
> `Control.Exception.finally` (or unwind-protect), where we can neither 
> inject (at least properly) the effects into nor eject them from the 
> finalizer. That's why I think that the whole "lift operations from the 
> bottom" approach is wrong (the original goal was to lift 
> `Control.Exception`). The right way would be to capture the control 
> semantics of IO as a set of type classes[1] and then implement the 
> general versions of the operations you want to lift. That's what I tried 
> to do with the monad-abord-fd package.

I think this is generally a useful goal, since it helps define the semantics
of IO more sharply.  However, the exceptions mechanism is actually fairly
well specified, as far as semantics go, see "A Semantics for Imprecise
Exceptions" and "Asynchronous Exceptions in Haskell."  So I'm not sure if
monad-abort-fd achieves the goal of expressing these interfaces, in
typeclass form, as well as allowing users to interoperate cleanly with
existing language support for these facilities.

> [1] Which turn out to be quite general: MonadAbort/Recover/Finally are 
> just a twist of MonadZero/MonadPlus

Now that's interesting! Is this an equivalence, e.g. MonadZero/MonadPlus
imply MonadAbort/Recover/Finally and vice-versa, or do you need to make
some slight modifications?  It seems that you somehow need support for
multiple zeros of the monad, as well as a way of looking at them.

> MonadMask is expectedly more specific, but permits a nice no-op
> implementation.

(See my earlier comments about asynchronous exceptions.)


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