I think it is best not to compare the expandable/not expandable
distinction to concepts from other languages. The main issues relating
to expansion are really particular (some would say peculiar) to the
execution model of TeX.
TeX has two main modes of operation. All assignments and boxing
operations happen (in "the stomach" in The TeXBook terminology) as
non-expandable operations. Macro expansion happens before that but
unlike (say) the macro expansion of the C pre-processor, macro
expansion and non-expandable operations are necessarily intertwined.
It is probably worth noting that the question as posed is not well defined.
TeX tokens are either expandable or non-expandable but "fully expandable"
is a grey area full of traps into which the unwary may fall.
Any token defined by \def
(or \newcommand
etc) is by definition expandable.
A character token such as a
is by definition non-expandable.
\def
is a non-expandable token.
So if you define
\def\zza{}
\def\zzb{a}
\def\zzc{\def\zze{}}
\def\zzd{\ifmmode a \else b\fi}
then each of these is expandable, with expansion <nothing>
a
\def\zze{}
\ifmmode a \else b\fi
respectively.
However which of these is fully expandable ?
Clearly \zza
is. But if the definition of "fully expandable" means may be expanded repeatedly leaving no unexpandable tokens then the only fully expandable tokens will all expand to nothing.
So most preople would class \zzb
as fully expandable, even though it expands to a
which is not expandable.
So a better (or at least more accurate) term than "fully expandable" is "safe in an expansion-only context". Inside \edef
and \write
and when TeX is looking for a number or dimension, and a few other places, TeX only does expansion and does not do any assignment or other non-expandable operations.
\edef\yyb{\zzb}
is of course safe, it is the same as \def\yyb{a}
. So \yyb
is safe in an expansion-only context.
\edef\yyc{\zzc}
is not safe, it is the same as
\edef\yyc{\def\zze{}}
Now \def
doesn't expand but in an expansion-only context the token
just stays inert it does not make a definition so TeX then tries to
expand \zze
which typically is not yet defined so this leads to an
error, or if \zze
has a definition then this will be expanded which
is almost always unwanted behaviour. This is the basic cause of the
infamous "fragile command in a moving argument" errors in LaTeX.
So \zzc
is not safe in an expansion-only context. If it had been defined by the e-TeX construct
\protected\def\zzc{\def\zze{}}
Then in an expansion-only construct protected tokens are made non-expandable so
\edef\yyc{\zzc}
would then be safe, and the same as \def\yyc{\zzc}
So a protected
command is safe in an expansion-only context but since this safety
comes by making the token temporarily non-expandable it probably isn't
accurate to say it is "fully expandable".
\edef\yyd{\zzd}
is
\edef\yyd{\ifmmode a \else b\fi}
which is
\def\yyd{b}
or \def\yyd{a}
if the definition is happening inside $...$
(or an equation display). Similarly it will
expand to b
at the start of an array cell as the expansion will
happen while TeX is expanding looking for \omit
(\multicolumn
) and
so before it has inserted the $
to put the array cell in to math
mode. Again a protected definition to limit expansion is what is
required here.
So sometimes it is good to make things expandable as it keeps more
options open.
\def\testa#1#2#3{%
\ifnum#1=0
\def\next{#2}%
\else
\def\next{#3}%
\fi
\next}
\def\firstoftwo#1#2{#1}
\def\secondoftwo#1#2{#2}
\def\testb#1{%
\ifnum#1=0
\expandafter\firstoftwo
\else
\expandafter\secondoftwo
\fi}
both \testa{n}{yes}{no}
and \testb{n}{yes}{no}
will exectute yes
if n
is 0
and no
otherwise but \testb
works by expansion and
so is safe in an expansion-only context (if its arguments are
safe). The \testa
version relies on the internal non-expandable
operation of \def\next
. (Plain TeX and LaTeX2.09 use many tests
using \def\next
, LaTeX2e changed them to the expandable form where
possible.)
For a numeric test it is easy to use the expandable form, but if you
want to test if two "strings" are equal by far the easiest way is to go
\def\testc#1#2{%
\def\tempa{#1}\def\tempb{#2}%
\ifx\tempa\tempb
\expandafter\firstoftwo
\else
\expandafter\secondoftwo
\fi}
but now even though we have used the \expandafter\firstoftwo
construct, the test relies on two non-expandable definitions. If you
really need to test in an expandable way you can find some questions
on this site but any answer is typically full of special conditions
and cases where it doesn't work and relies on some kind of slow token
by token loop through the two arguments testing if they are equal. In
99% of the cases this complication is just not needed and the
non-expandable test is sufficient. If you are trying to define a
consistent set of tests (as in the ifthen
package \ifthenelse
for
example, then if you resign yourself to the fact that some tests are
necessarily non-expandable then you may choose to make them all
non-expandable so they behave in a consistent way.
So the answer is:
It all depends....
Best Answer
Personally, I prefer a suffix
and compare
for the user maybe
GetTheMacro
if you follow LaTeX2e conventions, which to be honest I am not very fond of...Personal preference I guess, and is doomed to disappear if and when I move onto LaTeX3, which has a standard convention.