assert / check-type / assure...

[Pascal Costanza]

On 22 Sep 2013, at 20:45, Steve Haflich <shaflich at 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.



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