Let me precise my concerns. I know I can do
(begin (initialization-code) (let ((vals (call-with-values (lambda () (bar)) (lambda vals vals)))) (finalization) (apply values vals))) this is the hack I'm using now in my code. But my question is: does it incur performance penalties (it will be a macro, so used also in many other context where bar returns only a single value) Is there no better way to have multiple values interact in a composable way with macros??? Max > On 18. Mar 2020, at 10:55, Massimiliano Gubinelli <m.gubine...@gmail.com> > wrote: > > Thanks Taylan, > >> On 18. Mar 2020, at 07:12, Taylan Kammer <taylan.kam...@gmail.com> wrote: >> >> On 18.03.2020 00:50, Massimiliano Gubinelli wrote: >>> (let ((a (values "a" "b" "c"))) a) >> >> The result of (values x y z) is not a kind of object that contains three >> values (like a list or vector). It's three separate values that would need >> to be put into three separate variables. But you're only naming one >> variable (a). So strictly speaking the code is "wrong". >> >> In Guile 1.8, multiple values were actually put into some special kind of >> object (which was not very efficient) so code like that still somehow worked >> even thought it's technically wrong. >> >> Starting from Guile 2.0, providing multiple values in a context where only >> one is expected causes the extra values to be ignored. >> >> > > I understand the point but then it comes to the problem how to handle this in > macros. For example if bar is a proceduce which returns multiple values and I > have a macro "my-macro" which wraps the call with some initialization and > finalization code and I write > > (my-macro (bar)) > > how to write this macro without knowing if bar is returning multiple values > or not? For example I would like the code above to expand into > > (begin > (initialization-code) > (let ((ret (bar))) > (finalization-code) > ret)) > > > But this does not work as shown by the example above. How to implement this > macro correctly? > > best > Max > > >> Happy to answer further questions about this. Multiple-values can be a >> tricky concept to grasp because most other programming languages don't have >> it. >> >> - Taylan
