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