assert / check-type / assure...

[Scott McKay]

It's sad that this conses...


On Sun, Sep 22, 2013 at 3:44 PM, Pascal Costanza <pc at p-cos.net> wrote:

>
> 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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