What are AST Macros?

[Stefan Koch]

On Friday, 6 April 2018 at 21:45:45 UTC, Zach Tollen wrote:
>
> I think Walter's reason was that such macros would hide too 
> many idiosyncrasies in how they were programmed, such that a 
> lot of code which seems simple on the surface will actually 
> obfuscate complicated and arbitrary macro-programming patterns. 
> Thus, code that uses them will become much less maintainable, 
> because it is liable to do so many different and hidden things. 
> Also, the tasks for which AST-macros would typically be used 
> are already largely accommodated by templates and other 
> features. Thus, the real need for them isn't that high.

I think it's time to revisit this.

The reason being that templates are only well suited to very 
specific types of meta-programs which have a low degree of 
parameterization and a low degree of abstraction.

Using templates to introspect and manipulate types is like using 
a hammmer's flat back to remove a nail.
It _can_ be done but with an absurd amount of work.
You just have to remove all of the wall around the nail by 
pounding it until the wall has around the nail is disintegrated :)

This is not an exaggeration.

Templates used for introspection (or anything else really that's 
modestly complex) are equally hard to reason about for compilers 
and for programmers. I guess programmers have an advantage when 
it comes to _efficient_ pattern recognition.