Getting the error from __traits(compiles, ...)

[Bill Baxter]

On Fri, Nov 13, 2009 at 12:09 AM, Don <nospam at nospam.com> wrote:
> Bill Baxter wrote:
>>
>> We can pretty much already use __traits(compiles,{...}) to implement
>> static interface checking.
>>
>> template checkInterface(T) {
>>    enum bool checkInterface =
>>    __traits(compiles,
>>    {
>>       T x;
>>       // some code exercising various aspects of the interface
>>    });
>> }
>>
>> The main problem with this is that a static "implements interface"
>> check is done like so:
>>
>>   static assert(checkInterface!(myType));
>>
>> And all it tells you is a single yes or no.  It would help in
>> debugging if you could somehow get the reason for the failure.
>>
>> So how to do it?  All that comes to mind is something like the evil
>> Errno from C.  A global that gets set by the last failure.
>> Let's call it __errmsg, but it could be a pragma, or a __traits thing.
>>  If you had that then you could write this:
>>
>>   assertImplements!(checkInterface!(myType));
>>
>> With:
>> template assertImplements(bool v)
>> {
>>       static if (!v) {
>>            pragma(msg, __errmsg);
>>            static assert(false);
>>       }
>> }
>>
>> There are lots of ways you could provide such a concept-checker, but
>> they all require the basic ability to get the reason for the
>> __traits(compiles) failure.
>>
>> Any other thoughts about how to get the failure info?   This is
>> probably the main complaint against __traits(compiles), that there's
>> no way to find out what went wrong if the code doesn't compile.  Often
>> it can just be a typo.  I know I've spent plenty of time looking at
>> static if(__traits(compiles, ...)) checks that weren't working only to
>> discover I switched an x for a y somewhere.  Or passed the wrong
>> number of arguments.
>>
>> --bb
>
> I think you might be asking for:
>
> static try {
>   xxx;
> }
> catch( CompilerError[] errors){
>   pragma(msg, "Failure in Frobozz!");
>   pragma(msg, errors[0].msg);
> }

Heh heh.  Well I proposed something along those lines before, and it
didn't catch on, so I decided to avoid mentioning it this time.  :-)
http://digitalmars.com/d/archives/digitalmars/D/static_try_catch_construct_would_be_helpful_66794.html

But yeh, actually now that you mention it that handles both problems I
brought up recently.

1) how to run code only if it compiles and avoid repeating yourself.
(the original reason I proposed it)

2) how to get and report errors related to failure to compile some
code. (this one I hadn't thought of back then)

But yeh, it looks like that could solve both problems.

--bb