DeRailed DSL (was Re: compile-time regex redux)

[Andrei Alexandrescu (See Website For Email)]

kris wrote:
> Andrei Alexandrescu (See Website For Email) wrote:
>> Tom S wrote:
>>
>>> When the compiler is used for the processing of a DSL, it simply 
>>> masks a simple step that an external tool would do. It's not much of 
>>> a problem to run an external script or program, that will read the 
>>> DSL and output D code, while perhaps also doing other stuff, 
>>> connecting with databases or making coffee.
>>
>>
>> This is a misrepresentation. Code generation with external tools has 
>> been done forever; it is never easy (unless all you need is a table of 
>> logarithms), and it always incurs a non-amortized cost of parsing the 
>> DSL _plus_ the host language. Look at lex and yacc, the prototypical 
>> examples. They aren't small or simple nor perfectly integrated with 
>> the host language. And their DSL is extremely well understood. That's 
>> why there's no proliferation of lex&yacc-like tools for other DSLs (I 
>> seem to recall there was an embedded SQL that got lost in the noise) 
>> simply because the code generator would basically have to rewrite a 
>> significant part of the compiler to do anything interesting.
> 
> Are you stating that D will address all these concerns? Without any 
> detrimental side-effects?

It will naturally address the concerns exactly by not relying on an 
external generator. This was my point all along.

>> I think there is a lot of apprehension and misunderstanding 
>> surrounding what metacode is able and supposed to do or simplify. 
>> Please, let's focus on understanding _before_ forming an opinion.
> 
> If that's the case, then perhaps it's due to a lack of solid & practical 
> examples for people to examine? There's been at least two requests 
> recently for an example of how this could help DeRailed in a truly 
> practical sense, yet both of those requests appear to have been ignored 
> thus far.
> 
> I suspect that such practical examples would help everyone understand 
> since, as you suggest, there appears to be "differences" in perspective? 
> Since Walter brough RoR up, and you apparently endorsed his point, 
> perhaps one of you might enlighten us via those relevant examples?
> 
> There's a request in the original post on "The DeRailed Challenge" for 
> just such an example ... don't feel overtly obliged; but it might do 
> something to offset the misunderstanding you believe is prevalent.

I saw the request. My problem is that I don't know much about DeRailed, 
and that I don't have time to invest in it. My current understanding is 
that DeRailed's approach is basically dynamic, a domain that 
metaprogramming can help somewhat, but not a lot. The simplest example 
is to define variant types (probably they are already there) using 
templates. Also possibly there are some code generation aspects (e.g. 
for various platforms), which I understand RoR does a lot, that could be 
solved using metacode.

On the other hand, there are many good examples coming from C++ (e.g. 
most of Boost and Loki) offering good experimental evidence that 
metacode can help a whole lot. I've tried to add a couple of ad-hoc 
examples in posts, but I understand they can't qualify because they 
don't directly and obviously help DeRailed.


Andrei