Walter, I need a __trait please.

[ZombineDev via Digitalmars-d]

On Tuesday, 15 March 2016 at 08:02:34 UTC, Jaocb Carlborg wrote:
> On Tuesday, 15 March 2016 at 00:29:17 UTC, Stefan Koch wrote:
>> Hi,
>>
>> I found myself in need of __trait that might be useful to 
>> include.
>>
>> something that would give me the parameters of a the body of 
>> the lambda as string and the parmeters of a lambda as 
>> AliasSequence.
>> e.g
>> foreach (p;__traits(lambda, (x,y) => x < y)) {
>>  writeln(p);
>> }
>>
>> would output
>> x
>> y
>> x < y
>>
>> I am sure this is doable unfortunately I lack the insight into 
>> DMD to implement this in a reasonable amount of time, and code 
>> quality.
>>
>> Thanks in advance!
>>
>> Stefan
>
> To get the parameters as as strings, I think this [1] will 
> allow you to that. To get the body as a string, that sounds 
> almost as AST macros :)
>
> [1] https://github.com/D-Programming-Language/dmd/pull/5201
>
> --
> /Jacob Carlborg

Hi Jacob,

I've been thinking quite a bit about the next step in the 
evolution of D's metaprogramming, and I thought that AST macros 
are the best way forward, however since then I become convinced 
that macros are a distraction from a much more powerful language 
feature.

Firstly, macros are a kind of declarative preprocessor, in the 
case of AST macros they process ASTs. Surely operating at AST 
level is a significant improvement than plain text processing, 
however macros by themselves are separate language that requires 
a lot of additions to work. One of the biggest advantages of D is 
you don't need to use a different language for CTFE. Furthermore 
being purely declarative is very limiting, compared to how 
powerful imperative CTFE is.
Next, macros may add new syntax to the language or change the 
meaning of the existing syntax which would be a very disruptive 
change to the language (even if it's made in a backwards 
compatible way).

Instead I think that if we improve D's existing introspection 
capabilities and expose the compiler as a library at 
compile-time, we will have a much powerful system than any 
potential macro system, for a fraction of the complexity.

These videos are what changed my mind:
1. https://www.youtube.com/watch?v=OHZwYYW9koI
2. https://www.youtube.com/watch?v=59lKAlb6cRg
(Jonathan Blow's programming language for games)