On 22 Sep 2013, at 22:18, Scott McKay <swmc...@gmail.com> wrote: > It's sad that this conses…
True. I think I will use the single-value version for my own purposes. If this were to become part of a utilities library, it should probably best be split into a single-value ASSURE and a MULTIPLE-VALUE-ASSURE for multiple values, so you only pay for the overhead when you actually need it. Pascal > On Sun, Sep 22, 2013 at 3:44 PM, Pascal Costanza <p...@p-cos.net> wrote: > > On 22 Sep 2013, at 20:45, Steve Haflich <shafl...@gmail.com> wrote: > >> No, it is generally possible to write portable setf expanders that work with >> multiple-value places provided the platform conforms to the ANS >> requirements. I have some extreme examples on another machine I may share to >> the list later. >> >> I think to be portable transparent assure would need to collect and return >> multiple values, with the hope that the compiler would eliminate that stuff >> if the place were a single-value form like a variable or known function. > > OK, it seems you mean something like this: > > (defmacro assure (type form &environment env) > (multiple-value-bind > (vars vals store-vars writer reader) > (get-setf-expansion ,form env) > (declare (ignore store-vars writer)) > `(let* ,(mapcar 'list vars vals) > (check-type ,reader ,type) > ,reader))) > > However, this is not good enough. Something as simple as (assure integer (+ x > y)) already doesn't work. The requirement that form is a generalized > reference is an artifact that comes from check-type, but that shouldn't leak > through. > > I came up with another version. I believe this should work: > > (defmacro assure (type form) > (let ((values (copy-symbol 'values))) > `(let ((,values (multiple-value-list ,form))) > (declare (dynamic-extent values)) > (etypecase (values-list ,values) > (,type (values-list ,values)))))) > > …except that this also doesn't work in some Common Lisp implementations for > multiple values, but I think it should and those implementations need to be > fixed. > > > Pascal > > -- > Pascal Costanza > The views expressed in this email are my own, and not those of my employer. > > > > -- Pascal Costanza The views expressed in this email are my own, and not those of my employer.
