template debuging

[John Colvin]

On Thursday, 18 July 2013 at 11:16:26 UTC, JS wrote:
> Anyone successful at debugging templates and ctfe?
>
> Also, I am using pragma a lot because it seems to be the most 
> direct way to get ctfe state information.
>
> I would like to be able to toggle when to show pragma so I can 
> conditionally enable it during compilation.
>
> e.g., suppose I have a template T that I has pragma debug 
> messaging and I want to enable it only for a call to T:
>
> ___DEBUG___ = true;
> T!()
> ___DEBUG___ = false;
>
>
> (Not sure if this will produce the correct result depending on 
> how the compiler analyzes the code)
>
> Right now I have to use an immutable bool for debug which is a 
> global toggle... I don't see any thing around it because I 
> can't change an immutable at compile time(or can I?)... which, 
> technically should be valid because an immutable is a 
> declaration about the state of a variable at run time, so I 
> should be able to change it at compile time without changing 
> that fact.
>
> e.g.,
>
> immutable bool x = true;
>
> x $= false; // $= means static assignment, a compile time 
> assignment.. not sure if x should be true or false for the 
> whole program at run time(probably true, site of definition).
>
> Or
>
> ctfe bool x = true; defines a ctfe variable at compile time. x 
> doesn't exist a run time.

I had another go:

mixin template DbgMsg(alias T)
{
     mixin("
         mixin template Msg(msgs ...)
         {
             version(" ~ T.stringof ~ "Dbg)
             {
                 mixin(MsgString!(msgs)());
             }
         }"
     );
}

string MsgString(msgs...)()
{
     static if(msgs.length == 0)
     {
         return "pragma(msg,\"\");";
     }
     else
     {
         string s = "pragma(msg, " ~ msgs[0].stringof;

         static if(msgs.length > 1)
         {
             foreach(m; msgs[1 .. $])
             {
                 s ~= ", " ~ m.stringof;
             }
         }
         return s ~ ");";
     }
}

unittest
{
     struct A
     {
         mixin DbgMsg!A;
         mixin Msg!("hello world from ", typeof(this));
         mixin Msg!();
         mixin Msg!("one arg");
     }
}

When compiled with -version=ADbg you get the messages printed, 
otherwise not.
There are probably some improvements to be made, but it works.