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Re: macros and macroexpand
From: |
Heime |
Subject: |
Re: macros and macroexpand |
Date: |
Mon, 07 Aug 2023 12:43:13 +0000 |
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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))) ))
- macros and macroexpand, Heime, 2023/08/07
- Re: macros and macroexpand, Yuri Khan, 2023/08/07
- Re: macros and macroexpand,
Heime <=
- Re: macros and macroexpand, Philip Kaludercic, 2023/08/07
- Re: macros and macroexpand, Heime, 2023/08/07
- Re: macros and macroexpand, Philip Kaludercic, 2023/08/07
- Re: macros and macroexpand, Heime, 2023/08/07
- Re: macros and macroexpand, Philip Kaludercic, 2023/08/08
- Re: macros and macroexpand, Emanuel Berg, 2023/08/08
- Re: macros and macroexpand, Heime, 2023/08/08
RE: [External] : macros and macroexpand, Drew Adams, 2023/08/07