call with value

call with value

[Stefan Israelsson Tampe]

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I have this (on guile 3.0.0),

(define (f x) (call-with-values (lambda () x) (case-lambda ((x) ...) (x
...)))))

and the code does not simplify to (define (f x) x), why?

It would be great to have guile optimize this as in python we have

x=1,0
=> x=(1,0)

And the great thing with python on guile is that we get the same behavior
with (values 1 0) as a r.h.s. so that we get good interoperability between
guile and python.

This is implemented as

(call-with-values (lambda () r.h.s.)
   (case-lambda
      ((x) x)
      (x   x)))

This has nice semantics but is expensive. Many times we can prove that
r.h.s. has a single value return and we would like to be speed optimal for
this case. I do not think that the python compiler should do this analysis
or what do you think?

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Re: call with value

[William ML Leslie]

On Mon, 13 Apr 2020 at 02:59, Stefan Israelsson Tampe
<stefan.itampe@gmail.com> wrote:
>
> I have this (on guile 3.0.0),
>
> (define (f x) (call-with-values (lambda () x) (case-lambda ((x) ...) (x ...)))))
>
> and the code does not simplify to (define (f x) x), why?
>
> It would be great to have guile optimize this as in python we have
>
> x=1,0
> => x=(1,0)
>
> And the great thing with python on guile is that we get the same behavior with (values 1 0) as a r.h.s. so that we get good interoperability between guile and python.
>

Hi Stis,

I'm not sure if I mentioned this to you earlier, but I'm worried that
this won't be a great user experience.  When a python function returns
a tuple, whether "implicitly" as part of the return statement or not,
the invoker still gets one tuple, and is free to unpack it.  If I were
to call a function that returned a 1-value tuple and instead it got
unpacked for me, I would be very confused.

Similarly, if a python function `f` returns a tuple and I invoke
`g(f(x))`, I expect g to receive one tuple; not a number of arguments.
I'd have to say `g(*f(x))` if I wanted it to receive the items as
argument values.

-- 
William Leslie

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