[Haskell-cafe] Execution Contexts

[Ian.Stark at ed.ac.uk]

On Sat, 27 Nov 2004, Benjamin Franksen wrote:
> An execution context is a mutable finite map from types to (monomorphic)
> values. Each IO action implicitly carries exactly one such map and by default
> passes it on to the actions that follow.

Execution contexts sound a good description of them.  Building on your
recoding of this, if you have top-level declarations of newMVar / newIORef
then how much of this can you do by just keeping a dictionary in a global
variable?  This should certainly save some of the StateT plumbing; and
such declarations are safe, becuase they are affine central (see
http://groups.google.com/groups?selm=fa.doh68b9.96sgjd%40ifi.uio.no )

> A function is provided to (implicitly) create a new mapping and run a
> given IO action with the new mapping as its execution context, instead
> of the default one.

Update the global MVar, do the IO, then reset it?

> I am almost sure that even the trick of indexing the dictionary via
> types (and thus the dependency on Data.Typeable and ghc extensions) can
> be avoided with a little more effort.

Another global MVar to issue a sequence of unique index keys?

Ian

--
Ian Stark                               http://www.ed.ac.uk/~stark
LFCS, School of Informatics, The University of Edinburgh, Scotland