Re: BUG PROBLEM-> recursive macro inplace substitution

[Eric Blake]

On 9/30/20 5:03 AM, hhmm wrote:
> please see this reference
> 
> https://stackoverflow.com/questions/64066166/how-to-rescan-m4-data-for-recursive-macro-inplace-substitution/64066869?noredirect=1#comment113299085_64066869

Better yet, ask your question directly here, instead of making every
reader chase a URL that may have a different lifetime than the mailing
list archive.

So doing that on your behalf:

> I have this very simple code.
> 
> define(`S',`some')
> define(`T',`thing')
> define(`something',`st_todo') 
> S`'T 

In the interim, this expands to:
some`'thing

and thus the scanner never gets to see "something" as a single macro name.

> OR 
> S()T() 

Partway through evaluation, this has expanded to:

someT()

but someT is not a macro name, and thus the scanner never gets to see
"something" as a single macro name.

> 
> the actual end result is
> 
> something 
> OR 
> something 

You probably meant "someT()" here, based on your examples above.

> 
> but expected recursive substitution result as
> 
> st_todo
> 
> how I can rescan the code to the input again ?

The existing answers on that topic appear to gracefully cover it: any
time you want a second macro to be expanded and that expansion
concatenated with the output of the first, for evaluating the entire
concatenation for further macros, you have to use a glue macro.

define(`concat',`$1$2')
concat(S,T)

evaluates to:

concat(`some',`thing')

which in turn evaluates to:

something

which is now a valid macro name, and finally expands to:

st_todo

If you need a macro that concatenates more than just $1 and $2, you can
write it.
https://www.gnu.org/software/m4/manual/m4-1.4.15/html_node/Shift.html#index-joining-arguments-130
documents a "join" macro shipped with the m4 documentation that can be
used with an empty separator to concatenate an arbitrary number of
arguments into a single-quoted string with no intervening `' or (); pass
that through one more macro call to perform macro expansion on the
entire string.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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