Re: macros and macroexpand

[Yuri Khan <yuri.v.khan@gmail.com>]

On Mon, 7 Aug 2023 at 18:04, Heime <heimeborgia@protonmail.com> wrote:

> I have made a macro and know that they are supposed to return
> expanded code for use.  Still I cannot understand the need to
> call "macroexpand".  Should't the macro already perform the
> expansion ?

You should be posting small examples of code that you’re trying,
otherwise, there is high chance people will either misunderstand you
or just disregard your questions as ill-posed.

----

When you define a macro, you indeed write the definition similarly to
a function that returns expanded code.

    (defmacro foo (&rest body)
      `(bar ,@body))

When you evaluate a form that references a macro, Elisp will (1)
expand the macro, and (2) evaluate the result of the expansion:

    (foo 'quux)
    ⇒ Debugger entered--Lisp error: (void-function bar)

On the other hand, calling ‘macroexpand’ on a data representation of
that form will just return the expansion result:

    (macroexpand '(foo 'quux))
    ⇒ (bar 'quux)

In this example, I did not bother to define ‘bar’, so Elisp assumes it
would be a function and complains at evaluation time. But I could
further define ‘bar’ as a macro:

    (defmacro bar (&rest body)
      `(baz ,@body))

In this case, evaluating the original form shows that Elisp expanded
both macros ‘foo’ and ‘bar’, and then tried to call the undefined
function ‘baz’:

    (foo 'quux)
    ⇒ Debugger entered--Lisp error: (void-function baz)

Meanwhile, ‘macroexpand’ still just expands a single level of macros:

    (macroexpand '(foo 'quux))
    ⇒ (bar 'quux)

and you can invoke it repeatedly until you get to the fixed point:

    (macroexpand (macroexpand '(foo 'quux)))
    ⇒ (baz 'quux)

    (macroexpand (macroexpand (macroexpand '(foo 'quux))))
    ⇒ (baz 'quux)