[Haskell-cafe] Re: [Haskell] Top Level <-

Judah Jacobson judah.jacobson at gmail.com
Tue Aug 26 04:14:34 EDT 2008

On Tue, Aug 26, 2008 at 12:07 AM, Adrian Hey <ahey at iee.org> wrote:
> But from a top level aThing <- someACIO point of view, if we're going to
> say that it doesn't matter if someACIO is executed before main is
> entered (possibly even at compile time) or on demand, then we clearly
> don't want to observe any difference between the latter case and the
> former (if aThing becomes garbage without ever being demanded).
> Maybe it would be safest to just say anything with a finaliser can't be
> created at the top level. We can always define an appropriate top level
> "get" IO action using runOnce or whatever.

I've been wondering: is there any benefit to having top-level ACIO'd
<- instead of just using runOnce (or perhaps "oneshot") as the
primitive for everything?  For example:

oneshot uniqueRef :: IO (MVar Integer)
uniqueRef =< newMVar 0

It was also suggested in that wiki page:

Those proposals eliminate the need for creating an ACIO monad and
enforcing its axioms, since one-shot actions are executed in-line with
other I/O actions (rather than at some nebulous "before the program is
run" time).

So, in the context of top-level initializers, does ACIO offer
something beyond what oneshot provides on its own?  If not, I prefer
the latter since it seems like a much simpler solution.


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