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

[Mark H Weaver <mhw@netris.org>]

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.

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.

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).

* $$0, $$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?

      Mark