Passing an environment around

[Jeffrey R. Lewis]

Fergus Henderson wrote:

> On 27-Oct-2000, José Romildo Malaquias <romildo@urano.iceb.ufop.br> 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
>
> Yes.
>
> > 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?

--Jeff