Re: Pure (side-effect-free) calls into c/c++?

[Christopher Lam <christopher.lck@gmail.com>]

I can add a contribution! The good thing about memoize is it's simple to
create. You forgot a catch however: if the memoized return-val is #f then
your memoizer https://hg.sr.ht/~bjoli/misc/browse/default/memoize.scm will
not recognise that #f is a valid cached return-val and will call the lambda
again. (FWIW I shudder think what a *fast* memoizer would do).

Here's how I did mine:

(define (memoize f)
  (let ((h (make-hash-table)))
    (lambda args
      (cond
       ((hash-ref h args) => car)
       (else (let ((res (apply f args)))
               (hash-set! h args (list res))
               res))))))

(define-syntax-rule (lambda/macro args body ...)
  (memoize (lambda args body ...)))

(define-syntax-rule (define/macro (f . args) body ...)
  (define f lambda/macro args body ...))



On Sat, 11 Jan 2020 at 17:40, Linus Björnstam <linus.internet@fastmail.se>
wrote:

> I have a macro called lambda/memo and define/memo for these situations:
> https://hg.sr.ht/~bjoli/misc/browse/default/memoize.scm
>
> If the function gets called with a gazillion different arguments the
> memoizatiin hash gets large, and there are no mechanisms to stop that from
> happening. It also lacks a fast path for single argument functions.
>
> You can disregard the repo license. Use that function is you like, if you
> like to.
>
>
> --
>   Linus Björnstam
>
> On Fri, 10 Jan 2020, at 23:36, Linas Vepstas wrote:
> > So, I've got lots of C code wrapped up in guile, and I'd like to declare
> > many of these functions to be pure functions, side-effect-free, thus
> > hopefully garnering some optimizations.  Is this possible? How would I do
> > it? A cursory google-search reveals no clues.
> >
> > To recap, I've got functions f and g that call into c++, but are pure
> (i.e.
> > always return the same value for the same arguments).   I've got
> > user-written code that looks like this:
> >
> > (define (foo x)
> >     (g  (f 42) (f x) (f 43))
> >
> > and from what I can tell, `f` is getting called three times whenever the
> > user calls `foo`. I could tell the user to re-write their code to cache,
> > manually: viz:
> >
> > (define c42 (f 42))
> > (define c43 (f 43))
> > (define (foo x) (g c42 (f x) c43))
> >
> > but asking the users to do this is .. cumbersome.  And barely worth it:
> `f`
> > takes under maybe 10 microseconds to run; so most simple-minded caching
> > stunts don't pay off. But since `foo` is called millions/billions of
> times,
> > I'm motivated to find something spiffy.
> >
> > Ideas? suggestions?
> >
> > -- Linas
> > --
> > cassette tapes - analog TV - film cameras - you
> >
>
>