Passing an environment around
Jeffrey R. Lewis
Wed, 08 Nov 2000 08:43:22 -0800
Fergus Henderson wrote:
> On 27-Oct-2000, Josť Romildo Malaquias <firstname.lastname@example.org> wrote:
> > On Fri, Oct 27, 2000 at 09:07:24AM -0700, Jeffrey R. Lewis wrote:
> > > Yes, as implemented using the dictionary
> > > translation, implicit parameterization can lead to loss of sharing, exactly in
> > > the same way that overloading (and HOF in general) can lead to loss of sharing.
> > >
> > > However, I can imagine that a compiler might chose to implement implicit
> > > parameters more like dynamic variables in lisp. Each implicit param essentially
> > > becomes a global variable, implemented as a stack of values - the top of the
> > > stack is the value currently in scope. This would avoid the sharing problem
> > > nicely.
> > I suppose your implementation of implicit parameterization in GHC and Hugs
> > uses the dictionary translation, right?
> I believe so.
Sorry - that wasn't clear from my reply - yes, implicit parameters in GHC are compiled
using the dictionary translation.
> > Would an alternative implementation
> > based on a stack of values be viable
> > and even done?
> Unlikely ;-)
That's a shame ;-)
> An alternative is to store the values of the implicit parameters in
> thread-local storage rather than global storage. But this is more
> complicated. It may also be less efficient on some targets (depending
> on how efficiently thread-local storage is implemented).
I don't know the costs associated w/ thread local storage - how is it likely to compare
w/ the dictionary passing implementation?