On 22 Sep 2013, at 22:18, Scott McKay swmckay@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 pc@p-cos.net wrote:
On 22 Sep 2013, at 20:45, Steve Haflich shaflich@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.