It's sad that this conses…

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.