Need help learning defmacro
reddit / emacs

----------------------------------------------------------------------
ixlxixl · 2022-07-28 19:00

I have the following code:

    (defmacro print-name-from-language (language)
      (let ((name (symbol-name language)))
        `(message ,name)))

    (print-name-from-language 'c++)

and I'm getting this error:

     let: Wrong type argument: symbolp, 'c++

----------------------------------------------------------------------
TheBB · 2022-07-28 19:24

Don't quote c++ in the macro call.

Macro arguments aren't evaluated, so you're getting `(quote c++)`.

----------------------------------------------------------------------
Phil-Hudson · 2022-07-29 06:42

Having said that, and understanding that you may have given us an
over-simplified example, bear in mind that `message` uses the same
escapes as `format`, meaning the `%S` (uppercase S, not lowercase)
escape will do the `symbol-name` lookup for you.  You can thus replace
the whole body of this macro with `(message "%S" language)`.

This leads to a more important insight: don't create a Lisp macro
where a function will do.  At first it might seem like it makes no
difference, but trust me, it does.  I can go into it at length if you
like, but basically, functions are more *tractable*, as they are
first-class objects available at runtime.  Lisp macros are a fantastic
tool for automated programming, making Lisp "the programmable
programming language", but they should not be used for ordinary
computation.

----------------------------------------------------------------------
purcell · 2022-07-29 10:28

Read the corresponding chapters in Practical Common Lisp, available
free online.

----------------------------------------------------------------------
ieure · 2022-07-29 14:28

On Lisp also has a chapter on macros.

----------------------------------------------------------------------
ixlxixl · 2022-07-29 19:33

Very good points!

I think of macros as template programming in C++.

----------------------------------------------------------------------
ixlxixl · 2022-07-29 19:33

Thanks for sharing!

----------------------------------------------------------------------
Phil-Hudson · 2022-07-29 19:44

You're free now. Free, I tell you! You will never have to deal with
such an abomination as C++ templates again. Relax, take a deep breath
and let it go. Ahhh. That's better!

----------------------------------------------------------------------
Phil-Hudson · 2022-07-30 02:10

And straight after "On Lisp", read "Let Over Lambda", which is all
about Lisp macros (in Common Lisp):
https://letoverlambda.com/index.cl/toc