Mfcttrf

I'm trying to create a template (.sty file) that will typeset

[value of macro 1], [value of macro 2]

if this expression does not exceed the length of the line, and

[value of macro 1]
[value of macro 2]

otherwise in a centered environment. My first idea was using mbox to prevent linebreaks within the fields themselves, but that leaves me with the problem of figuring out how to make LaTeX ignore the comma if the line break occurs. I then tried to solve the problem using

newcommand@macroambox[value of macro1]
newcommand@macrobmbox[value of macro2]
newcommand@macrocmbox[value of macro1], [value of macro2]
discretionary@macroa@macrob@macroc

which kind of does what I want, but with a major caveat - if a linebreak occurs, the text isn't centered anymore, but now gets typeset as

| [value of macro1]|
|[value of macro 2] |

and I can't figure out how to insert the proper glue without causing LaTeX to raise an Improper discretionary list. error...

Ouch, I have figured out my mistake - I wasn't in a centered environment after all, but using hfils. Changing to centering solved the problem for me. Guess I should revisit the source code once more next time before writing up a question... however, I think I'm just gonna leave this here, since Google didn't come up with useful results for this use case, so maybe it'll be useful to someone else one day. My apologies!

You can typeset the comma as if it were glue (using xleaders); since glue disappears at line breaks, also the comma will.

documentclassarticle

newcommanddisappearingcomma%
 leavevmode
 sbox0,%
 xleaderscopy0hskipwd0relax
 space


newcommandmacroaValue of macro A
newcommandmacrobValue of macro B
newcommandmacroabmboxmacroadisappearingcommamboxmacrob

begindocument

beginminipage6cm
centering
macroab
endminipagevrule
beginminipage4cm
centering
macroab
endminipage

enddocument

As @PhelypeOleinik has already pointed out in a comment, the macro names @macro1, @macro2, and @macro3 are invalid -- the digits 0 thru 9 are not allowed in multiple-character TeX and LaTeX macro names. I therefore suggest you use @macroa, @macrob, and @macroc instead.

documentclassarticle
usepackage[utf8]inputenc % 
makeatletter
newcommand@macroambox[value of macro A]
newcommand@macrobmbox[value of macro B]
newcommand@macrocmbox[value of macro A], [value of macro B]
newcommandblurbdiscretionary@macroa@macrob@macroc
%% Or, far more succinctly, David Carlisle's excellent suggestion: 
%% newcommandblurb@macroadiscretionaryhbox, @macrob
%% With this approach, it's not necessary to define '@macroc'.
makeatother

setlengthparindent0pt
%setlengthtextwidth1mm % uncomment to get a very narrow measure

begindocument
blurb
enddocument