Re: Proposal for a new (ice-9 history)

[Mikael Djurfeldt <mikael@djurfeldt.com>]

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On Tue, Oct 30, 2018 at 1:26 AM Mark H Weaver <mhw@netris.org> wrote:

> Mikael Djurfeldt <mikael@djurfeldt.com> writes:
>
> > The interface of (value-history) would instead have a lazy-binder
> > which provides a syntax transformer for every $... actually being
> > used. The $... identifier would expand into a list-ref into the value
> > history.
>
> A few more suggestions:
>
> If I write (define (foo x) (+ $$0 x)) at the repl, then I expect 'foo'
> to continue to refer to the same entry in the value history, even after
> the value history is later extended.
>

Well, this could be interpreted in two ways. What I expect is that $$0
always refers to the last entry of the value history, even if it has been
extended, such that $$0 will evaluate to new values as new values are
pushed onto value history.

This is also the effect we get if $$0 expands to (list-ref value-history 0).

>
> I'm also a bit concerned about the efficiency implications of expanding
> these variable references into 'list-ref' calls when the history grows
> large.  If I write a loop that evaluates $$0 a million times, I'd prefer
> to avoid a million 'list-ref' calls.
>

Maybe this is a Microsoft-style argument, but do we really expect users to
use value history in that way? If so, I guess value-history could be stored
in a dynamically enlarged vector.


> To address these concerns, I'd like to suggest a slightly different
> approach:
>
> * $0, $1, ... would continue to be ordinary variable bindings in
>   (value-history), as they are now.
>
> * The 'count' in 'save-value-history' would be made into a top-level
>   variable in (ice-9 history).
>

(This (count) is what I had in mind for $<N>: $<N> -> (list-ref
value-history (- count <N>)) )


> * $$0, $$1, $$2, ... would be handled by a lazy-binder, providing a
>   syntax transformer that looks at the value of 'count' at macro
>   expansion time, and expands into the appropriate variable
>   reference $N.
>

> For example, if $5 is the most recent value, $$0 would expand into $5
> instead of (list-ref ...).  This would eliminate my concerns over
> efficiency.
>
> What do you think?
>

This would then have the problem that $$0 would get a more complex meaning:
It would mean "the most recent result at the time of macro expansion"
rather than "the most recent result".

If efficiency really is a concern, I would expect that vector references
would be rather efficient after compilation.

Best regards,
Mikael

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