[ddragon]

Lisp macros (as opposed to macros in other languages besides C) are especially powerful not because you're changing the syntax, as Lisp doesn't have a syntax in the first place, you're always working directly on it's Abstract Syntax Tree. So stuff like conditional, loops and functions all have the same structure, which is why you could already write something that looks like a loop but it's a function.

The difference is how it's arguments are evaluated, for example if I wanted to make a function that implements a for loop of the form (myfor a from 1 to 10 do (print a)), if it's a function then every argument will be evaluated immediately, so it will try to find a variable called a, from, to, do and it will also try to evaluate (print a) immediately before looping. Macros allows for delayed evaluation, as myfor will receive all arguments (at compile-time) as symbols instead of values, which you can then rearrange in a form that can evaluate properly at runtime. You can also go further if you actually want to create syntax and use reader macros, which allows you to write your own parser and therefore escape writing directly on the AST (then you can even write a C syntax within Lisp).

And if your question is: do I actually need them? The answer is obviously no as many languages do not support it (and there are even alternatives for many use cases, like lazy evaluation). The advantage is that your language can have a very simple core and features that were not implemented (say a pattern match structure) can be added entirely within userspace (which is also good for testing new functionality before adding to the core language). Macros are also very efficient since they run at compilation (so if you use the macro a lot of time you only have to evaluate them once, unlike functions that will usually have to run it's logic every time it's called). And all of that means that for DSL, it's not just making a nice adaptation for your domain within the host language, but effectively writing an optimized language for your domain reusing the compiler of the host language without having to change it's source code.

[AnimalMuppet]

OK, I understand why you might want to do that. But there's nothing domain specific about that. I might want a decent looping construct in any domain. That's just trying to make a decent language, not a domain specific one.

Or is the idea that, as soon as I go beyond the Common Lisp standard, it's "domain specific", no matter how completely general my extensions are?

I've always interpreted DSL as creating a way to write programs in the language of the problem to be solved. A loop construct, no matter how useful, seems to fall short of that.

[ddragon]

The for loop example was just to illustrate the difference between function and macro, a complete DSL would be something like making prolog within a Lisp or other examples in racket [1], or for example a special configuration file for electric circuits within the language, or a SQL like interface for manipulating data (you can write LINQ using macros).

[1] https://en.wikipedia.org/wiki/Racket_features#Language_Exten...

It's not a direct comparison of course (nor it is a Lisp), but here is an example of linear optimization in a library that uses macros to make it a DSL closer to the description (in Julia @ before a name means it's macro, so it's easy to see) and one that uses methods:

https://nbviewer.jupyter.org/github/jump-dev/JuMPTutorials.j...

https://www.cvxpy.org/examples/basic/linear_program.html

If in the Julia example they if @variable was a function, then x >= 0 would have been evaluated immediately and it would fail since x was not defined (and if it was x >= 0 would return a boolean). To emulate that you'd probably have to pass a string "x >= 0", which the function would then have to parse (it would be a DSL as well, but one you're writing from scratch), the difference here is that you can just use the language parser directly and compile already with the result.