Why cannot scopes be used in template mixins?

[Austin Hastings]

On 10/19/2010 6:18 AM, Justin Johansson wrote:
> For example:
>
> private string THIS_MODULE_NAME = "abc.def";
>
> mixin template MyCorporationStandardUnitTest()
> {
> scope(success) {
> writeln( THIS_MODULE_NAME ~ " unittest passed");
> }
>
> scope(failure) {
> writeln( THIS_MODULE_NAME ~ " unittest failed");
> }
> }
>
>
> I think the D spec does not say this is allowed, but why not?
>
> Thanks in advance for answers,
>
> Justin

(Note: I'm only using PL/D 2.0, so my answers are specific to that
version on the DMD compiler.)

Mixin templates may only contain declarations - not executable 
statements. For this, you want a string mixin. There are some things you 
could try.

1. I have some GPL code that does what you're trying to do, in a 
different way.

Have a look at 
http://github.com/aghast/Leda/blob/master/source/leda/test/unit.d . It 
outputs TAP, instead of success/failure messages, but IMO that's a 
better solution because there are plenty of TAP harnesses around.

2. You could have a look at "scope classes" in the docs:

http://www.digitalmars.com/d/2.0/class.html#auto  (I know, "auto" makes 
no sense, but it's in the html source..)

This indicates that a class is intended to be used RAII-style, such that 
the destructor is called.

To use this, you'd probably have to catch Assertion failures, or put a 
hook in the unit test failure handler (see the core.* doc pages in your 
local install, since the net links aren't up on the dmd site).

3. You could use a string mixin.

In order to get the module name added, you could generate the string in 
a function. I think there's a std* template that does compile-time 
formatting, but naturally googling on the obvious keywords doesn't work. 
So you'd so something like:

    mixin(  function_to_generate_the_scope_statements( MODULE_NAME ) );

at the top of your code.

(Note: mixing( "string" ) is different from mixin-templates, and *can* 
have statements and expressions, not just declarations.)


Good luck,

=Austin