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Re: problem with macro definitions


From: hector
Subject: Re: problem with macro definitions
Date: Sat, 29 Apr 2017 18:06:32 +0200
User-agent: Mutt/1.5.20 (2009-06-14)

Thank you for your reply.

On Sat, Apr 29, 2017 at 11:39:13AM +0200, Michael Heerdegen wrote:
> hector <address@hidden> writes:
> 
> > I was looking for the complementary of nth, that is, a function that
> > returns the index of an element. Since it can be done at compilation
> > time
> 
> Not if you want to use it on values that are known only at run-time.

I know. In this case when the list is constant it seemed to be right.

> > I thought it was a good candidate for a macro:
> 
> No, not really:
> 
> > (defmacro idx (list telt)
> >   `(let (found
> >      (idx 0))
> >      (dolist (elt ,list found)
> >        (when (eq elt ,telt)
> >      (setq found idx))
> >        (setq idx (1+ idx)))))
> 
> You loose nothing when you rewrite this as a function.  In this
> implementation, the index is calculated at run-time.

Let me rephrase your statement:

"You gain nothing when you write this as a macro.  In this
implementation, the index is calculated at run-time"

I'm aware of that now. This is not what I meant.
I'll try to rewrite it.

> Note that macros don't evaluate their arguments.  The argument 'blue you
> pass, a read syntax for (quote blue), is a list of two elements.  This
> doesn't appear as an element of `start-states' which consists of four
> symbols.

This is the key to my confusion.



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