Re: macros and macroexpand

[Heime <heimeborgia@protonmail.com>]

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------- Original Message -------
On Monday, August 7th, 2023 at 11:46 PM, Yuri Khan <yuri.v.khan@gmail.com> wrote:


> 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)

Then macroexpand is useful for diagnostics to expand at one level only
at a time.  Thusly, if I just want to get the expanded code produced by 
a macro, I can just do pp-to-string upon the object made by a macro.

(defmacro adder (mopi mopj)
  `(+ ,(cl-second mopi) ,(cl-third mopj)))

(princ (pp-to-string '(adder (* 3 5) (* 5 7)) ))

I would not do

(princ (pp-to-string (macroexpand '(adder (* 3 5) (* 5 7))) ))