Re: Installing cond* in core

[Stefan Monnier <monnier@iro.umontreal.ca>]

> > AFAICT the implementation of `match*` is not designed to handle the task
> > of matching against several patterns to select which branch to use.
> I do not follow you.  Could you please describe the issue more concretely?
> Pehraps wiht an example  Then I could judge whether this is grave.

When matching EXP against patterns PAT1..PATn, Pcase is careful to share
the common part of those patterns in the resulting code, so that the
resulting code will not test (consp EXP) more than once, for example.

Try (cond* ((match* `(a1 . ,b1) EXP) ...) ((match* `(a2 . ,b2) EXP) ...))
and see how, before testing `a2`, it re-tests whether EXP is `consp` and
it re-extracts its `car`, even though it already just did those very
same operations.

Of course, the same happens with my PoC `pcase*` addition to `cond*`.
But not if you do (pcase EXP (`(a1 . ,b1) ...) (`(a2 . ,b2) ...)).


        Stefan