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digitalmars.D.learn - Avoiding __traits(getAttributes, ...) on alias

reply "Stefan Frijters" <sfrijters gmail.com> writes:
I've been playing with UDAs a bit and I wanted to find all 
variables with a particular attribute in various modules. I 
thought I had it cracked, until I added a module that contains an 
alias declaration, which makes it choke when trying to execute 
__traits(getAttributes, ...). A small example is shown below. Is 
there any conditional I can insert between the two foreach lines 
to make it detect such an alias declaration, and move on to the 
next derived member? Or should getAttributes handle this by just 
returning no attributes?

import std.traits;

 ("testattr") int foo;
alias char[256] MyChar;
 ("testattr") int bar;

void main() {
   foreach(e ; __traits(derivedMembers, mixin(__MODULE__)))  {
     foreach( t; __traits(getAttributes, mixin(e)) ){
       pragma(msg, t);
     }
   }
   // testattr
   // test.d(9): Error: first argument is not a symbol
   // test.d(9): Error: invalid foreach aggregate false
   // testattr
}

Any hints would be appreciated!

Kind regards,

Stefan Frijters
May 09 2014
parent reply "John Colvin" <john.loughran.colvin gmail.com> writes:
On Friday, 9 May 2014 at 11:53:59 UTC, Stefan Frijters wrote:

 I've been playing with UDAs a bit and I wanted to find all 
 variables with a particular attribute in various modules. I 
 thought I had it cracked, until I added a module that contains 
 an alias declaration, which makes it choke when trying to 
 execute __traits(getAttributes, ...). A small example is shown 
 below. Is there any conditional I can insert between the two 
 foreach lines to make it detect such an alias declaration, and 
 move on to the next derived member? Or should getAttributes 
 handle this by just returning no attributes?

 import std.traits;

  ("testattr") int foo;
 alias char[256] MyChar;
  ("testattr") int bar;

 void main() {
   foreach(e ; __traits(derivedMembers, mixin(__MODULE__)))  {
     foreach( t; __traits(getAttributes, mixin(e)) ){
       pragma(msg, t);
     }
   }
   // testattr
   // test.d(9): Error: first argument is not a symbol
   // test.d(9): Error: invalid foreach aggregate false
   // testattr
 }

 Any hints would be appreciated!

 Kind regards,

 Stefan Frijters


You could always do a static if with __traits(compiles, 
__traits(getAttributes, mixin(e))
May 09 2014
parent reply "Stefan Frijters" <sfrijters gmail.com> writes:
On Friday, 9 May 2014 at 12:19:12 UTC, John Colvin wrote:

 On Friday, 9 May 2014 at 11:53:59 UTC, Stefan Frijters wrote:

 I've been playing with UDAs a bit and I wanted to find all 
 variables with a particular attribute in various modules. I 
 thought I had it cracked, until I added a module that contains 
 an alias declaration, which makes it choke when trying to 
 execute __traits(getAttributes, ...). A small example is shown 
 below. Is there any conditional I can insert between the two 
 foreach lines to make it detect such an alias declaration, and 
 move on to the next derived member? Or should getAttributes 
 handle this by just returning no attributes?

 import std.traits;

  ("testattr") int foo;
 alias char[256] MyChar;
  ("testattr") int bar;

 void main() {
  foreach(e ; __traits(derivedMembers, mixin(__MODULE__)))  {
    foreach( t; __traits(getAttributes, mixin(e)) ){
      pragma(msg, t);
    }
  }
  // testattr
  // test.d(9): Error: first argument is not a symbol
  // test.d(9): Error: invalid foreach aggregate false
  // testattr
 }

 Any hints would be appreciated!

 Kind regards,

 Stefan Frijters


 You could always do a static if with __traits(compiles, 
 __traits(getAttributes, mixin(e))


Thank you for the fast reply; this solves my problem. I actually 
tried this before, but in my actual code instead of the example, 
where I'm deep into backticks and quotes and escaped quotes so I 
probably made a mistake there...
May 09 2014
parent reply "Vlad Levenfeld" <vlevenfeld gmail.com> writes:
I've recently found out that you can deal with the abundance of 
backticks and escapes in a couple of ways:
   q{ your code here... }
will resolve to a string but your editor can still highlight it 
as D code, making it more readable.
also, turning things like:
   mixin ("if ("~expr~")")
into:
   if (mixin(expr))
can improve readability but beware I've noticed that sometimes 
this is not equivalent to the previous version and I'm not sure 
how or why that happens.
In particular I notice that

   mixin("const bool value = "~expr~";)");
and
   const bool value = mixin(expr);

are not the same, for some reason.
May 09 2014
parent reply Philippe Sigaud via Digitalmars-d-learn writes:
Vlad Levenfeld:

 but beware I've noticed that sometimes this is not
 equivalent to the previous version and I'm not sure how or why that happens.
 In particular I notice that

   mixin("const bool value = "~expr~";)");
 and
   const bool value = mixin(expr);

 are not the same, for some reason.


What are the differences?
May 09 2014
parent reply "Vlad Levenfeld" <vlevenfeld gmail.com> writes:
I'm not really sure. Here is an example of the problem:

const bool has_attribute (T, string member, alias attribute) ()
{
   static if (1) // ok
     mixin(
       `foreach (type; __traits (getAttributes, T.`~member~`))`
         `static if (is (type == attribute))`
           `return true;`
     );
   else // doesn't compile, member is not accessible error
     foreach (type; __traits (getAttributes, mixin(`T.`~member)))
       static if (is (type == attribute))
         return true;
   return false;
}

Maybe its trying to use it inside of __traits that is causing it?
May 10 2014
next sibling parent reply "Vlad Levenfeld" <vlevenfeld gmail.com> writes:
because this works:

foreach (type; mixin(`__traits (getAttributes, T.`~member~`)`))
   static if (is (type == attribute))
     return true;
May 10 2014
parent "Vlad Levenfeld" <vlevenfeld gmail.com> writes:
I don't have any examples of the const bool thing not working, 
its just something I feel like I recall, though I could be 
mistaking it for the __traits example.
May 10 2014
prev sibling parent reply Philippe Sigaud via Digitalmars-d-learn writes:
   else // doesn't compile, member is not accessible error
     foreach (type; __traits (getAttributes, mixin(`T.`~member)))
       static if (is (type == attribute))
         return true;
   return false;



 Maybe its trying to use it inside of __traits that is causing it?


Maybe __traits is trying to extract attributes from the mixin
expression, before the mixin injection?
That looks like a bug to me, no?
May 10 2014
parent "Vlad Levenfeld" <vlevenfeld gmail.com> writes:
Agreed, new bug report submitted.
May 10 2014