• Tobias Pankrath (19/24) Nov 11 2011 I'm trying to learn template meta programming in D. To do this, I'm work...

Tobias Pankrath <tobias pankrath.net> writes:

I'm trying to learn template meta programming in D. To do this, I'm working
on a  meta state machine like Boost.MSM 
(http://www.boost.org/doc/libs/1_47_0/libs/msm/doc/HTML/index.html).

States are types and can have sub states. If they do, they need to have
an attribut 'SubStates', which should be a TypeTuble with the 
sub states.

The task at hand is: Given a State and a list of all States, create
a list of all parents of State including State itself.

This code (http://paste.pocoo.org/show/506154/) ought to do this
and it works sometimes. 

You can make to interesting observations of this code:

In its current form, 'dmd test.d -unittest' will print: "
 false

But if you uncomment the first pragma() in the unittest block,
it will print 
 () 
 true

If you change the body of ParentsDesc to 
 alias TypeTuple!() Result;

it will also print
 true

Although the static if in ParentsDesc is never true in this
example. 

What's going on?