Can one define a '\superexpandafter{n}' that would expand to 2^n-1 '\expandafter's?'

Question

This question was inspired by comment banter in this question. See title.

Answer 1

(Sorry, this gets more of an essay than an answer.)

TH claims that "There's not really a good way to do this that takes arguments." Well, I thought the same, but Philippe's answer to the original question of Yiannis proved me wrong. I've used his ingenious \csname-idea for a \multexpafter command that takes the number of expansions as an argument. New: I've implemented a much more efficient version that allows 0-100 expansions.

The crucial feature of the \multexpafter command is that it always needs 3 steps to be fully expanded. (I believe less steps to be impossible, but feel free to prove me wrong; see above :-)) This is in contrast with 2^n-1 consecutive \expandafters that need n expansion steps. I want the following TeX code to work (compare the "Hello world" example in my answer to Yiannis question):

\def\a#1.{#1}
\def\b#1:{#1.}
\def\c#1,{#1:}
\def\d{Hello world!,}
\multexpafter4\a
\multexpafter4\b
\multexpafter1\c
\d

Explanation, starting in the end: The \multexpafter1 should expand the \d after \c once. This should happen before the expansion of \b, so the \multexpafter4 in front of \b expands the \multexpafter1... 4 times. Three of these expansion steps are needed to fully expand the \multexpafter1, the last steps then expands the \c once. Same for the \multexpafter4 in front of \a: It expands the \multexpafter4\b... 4 times; the last of these steps expands the \b once. More generally, use

\multexpafter{<3+n>}\a
\multexpafter{<m>}\b\c

to first expand \c m times and then \b n times before expansion of \a. Again, 3 of the 3+n expansion steps are needed to fully expand \multexpafter{<m>}; then the \multexpafter{<m>} is gone, and so the next n expansion steps act on the \b.

---

Here's the new implementation of \multexpafter (to be compiled with tex or pdftex). It uses e-TeX's \numexpr (which to me feels a bit like cheating):

\catcode`@=11
\let\expandafter@i\expandafter
\count255=1
\loop\ifnum\count255<200
    \edef\temp{\the\count255}
    \advance\count255 by 1
    \expandafter\def\csname expandafter@\romannumeral\count255\expandafter\endcsname
    \expandafter{\expandafter\expandafter
        \csname expandafter@\romannumeral\temp\endcsname \expandafter}%
\repeat
\def\expandafter@{}
\def\expandafter@e{\errmessage{Argument of \noexpand\multexpafter must be between 0 and 100}}
\def\multexpafter#1{\csname expandafter@\ifnum#1>100 e\else
    \csname expandafter@\ifnum#1=0 ii\else\romannumeral\numexpr2*#1\fi\fi\endcsname
    \endcsname\csname expandafter@\romannumeral#1\endcsname}
\catcode`@=12

\def\a#1.{#1}
\def\b#1:{#1.}
\def\c#1,{#1:}
\def\d{Hello world!,}
\multexpafter4\a
\multexpafter4\b
\multexpafter1\c
\d
\bye

Only a short explanation: In the loop I construct macros \expandafter@ii to \expandafter@cc for expanding up to 200 times. The 3 expansion steps of \multexpafter{<n>} are as follows: The first step yields the replacement text of the macro. In the second step the \csname is executed, which triggers

\csname expandafter@\romannumeral<n>\endcsname

to be expanded 2n times, which in turn executes the desired n expansions. The result of the second step is just the control sequence \expandafter@, which in the third step expands to nothing.

---

For further illustration, here's how Philippe's example looks like with \multexpafter:

\def\a{a}
\def\b{b}
\def\c{\C}\def\C{c}
\def\d{d}
\def\e{\E}\def\E{\ee}\def\ee{e}
\multexpafter3\def
\multexpafter3\temp
\multexpafter4{%
\multexpafter4\a
\multexpafter5\b
\multexpafter4\c
\multexpafter3\d
\e
}
\show\temp

---

For completeness, here's my old inefficient implementation for 0-6 expansions:

\catcode`@=11
\def\expandafter@one{\expandafter}
\def\expandafter@two{\expandafter\expandafter\expandafter}
\def\expandafter@three{\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter}
\def\expandafter@four{\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter}
\def\expandafter@five{\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter}
\def\expandafter@six{\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter}
\def\do@nothing{}
\def\do@nothing@{\errmessage{Argument of \noexpand\multexpafter must be between 0 and 6}}
\def\m@e@a#1{\ifcase#1
\expandafter\do@nothing \or
\expandafter\expandafter@one \or
\expandafter\expandafter@two \or
\expandafter\expandafter@three \or
\expandafter\expandafter@four \or
\expandafter\expandafter@five \or
\expandafter\expandafter@six \else
\expandafter @\fi}
\def\multexpafter#1{\csname do@nothing\expandafter@six\m@e@a
    \expandafter@five#1\expandafter@four\endcsname\m@e@a#1}
\catcode`@=12

Answer 2

(I apologize for the length of this answer.)

There's not really a good way to do this that takes arguments. There are two reasons for this. The first is that it adds more tokens that have to be skipped over. For example, consider \sea{3}. If you need to skip over this, there's no simple way to get over those three tokens, at least not using another \sea. Even if you omit the braces, there's still one extra token.

The second reason has to do with implementing this. Consider

\let\ea\expandafter
\def\sea#1{%
        \ifcase#1
        \or \ea
        \or \ea\ea\ea
        \or \ea\ea\ea\ea\ea\ea\ea
        \fi
}

This is the obvious first choice, but that doesn't really work; consider \sea1. It'll expand to the \ifcase ... \fi and then if the \ifcase is expanded next, it'll expand to \ea\or \ea\ea\ea...\fi and then if the \ea is next, it'll skip the \or, start expanding a whole series of other \ea. To deal with this, you have to add yet more \eas in the definition to make it to the next \or or the \fi. It quickly becomes totally unworkable.

You can sort of use

\let\ea\expandafter
\def\seai{%
        \ea
}
\def\seaii{%
        \ea\ea\ea
}
\def\seaiii{%
        \ea\ea\ea\ea\ea\ea\ea
}
\def\seaiv{%
        \ea\ea\ea\ea\ea\ea\ea\ea\ea\ea\ea\ea\ea\ea\ea
}
\def\seav{%
        \ea\ea\ea\ea\ea\ea\ea\ea\ea\ea\ea\ea\ea\ea\ea\ea
        \ea\ea\ea\ea\ea\ea\ea\ea\ea\ea\ea\ea\ea\ea\ea
}

But it's not extremely easy to use or follow.

\def\a{A}
\def\b{B}
\def\c{C}
\def\d{D}

\tracingall
\seai\a\b

\seaii\a\seai\b\c

\seaiii\a\seai\b\c

\seaiv\a\seaiii\b\seai\c\d

\seav\a\seaiii\b\seai\c\d
\bye

The first line expands \b and then \a. The second line expands \c, \a, then \b. The third line expands \c, \b, \a. The third expands \d, \c, \a, then \b. The fourth expands \d, \c, \b, \a. You can verify this by reading the log.

This can be simplified somewhat by not using \seai and just using \ea because then it doesn't have to be expanded twice to have the effect.

Edit:
After having played with this a bit. A pattern emerges. Using appropriate definitions, you can use

\seavii\a\seav\b\seaiii\c\seai\d\e

to expand \e, \d, \c, \b, and finally \a. Using \seavi instead of \seavii swaps the order of \a and \b. Other expansion orders are possible by changing the values, but it's not immediately obvious (at least it isn't to me) what a particular order will do.

For example, guess what

\seavi\a\seaiv\b\seaii\c\seai\d\e

will do.

Edit 2:
It's possible to get TeX to create the various \seaX macros for us rather than the error-prone manual method above.

\def\seai{\expandafter}
\count255 1
\loop\ifnum\count255<20
        \expandafter\let\expandafter\temp\csname
        sea\romannumeral\count255\endcsname
        \advance\count255 1
        \expandafter\gdef\csname sea\romannumeral\count255\expandafter
        \expandafter\expandafter\endcsname\expandafter\expandafter
        \expandafter{\expandafter\temp\temp\expandafter}%
\repeat

Answer 3

[Again, not really answering the question but posing an alternative.]

Nowadays, rather than attempting to repeat \expandafter in unreadable (and unmaintainable) chains, I recommend using the expl3 programming environment to deal with complex expansion problems.

Here's an example:

\exp_args:Nooo \foo \a \b \c

Here, \foo is not expanded (the N in the exp_args command) and \a, \b, \c are all expanded once (the o letters). But this isn't exactly how expl3 is intended to be used; instead, one would define that foo takes, say, five arguments by first defining \foo:nnnnn and then creating a variant for which each argument would be expanded once before execution of the function:

\usepackage{expl3}
\ExplSyntaxOn
\cs_set:Nn \foo:nnnnn { ...#1...#3...#5... }
\cs_generate_variant:Nn \foo:nnnnn {ooooo}
...
   \foo:ooooo \a \b \c \d \e
...
\ExplSyntaxOff

As well as o (for ‘expand once’) there are other argument specifiers such as V (value of macro or register), c (create a csname), x (complete expansion), f (‘romannumeral’ expansion), and so on. Using this system allows programmers to bypass many of the problems involved with expansion in classical TeX programming.

Answer 4

EDIT2: the previous code was broken when given conditional text as an input. I think this should be better. Also, the explanations were confusing. Now, for all the explanations below I \let\ea\expandafter. But not in the code.

TOC: Explanations ("Two user commands", "How it works"). Code ("Implementation", "Tests").

Explanations

Two user commands

• For n>0, \MultiExpand{n}\macro gives the n-th expansion of \macro after two steps of expansion.

• For n>0, \MultiExpandAfter{10}\macroA\MultiExpandAfter{4}\macroB\macroC expands \macroC 4 times before expanding \macroB 8=10-2 times, and finally \macroA. It also requires two steps.

The first expansion of \MultiExpand yields a very useful sequence of tokens: expanding \unless\ifcsname\multiexpand\fi{n} once expands the following token n times. The same exists for expanding after.

These are especially useful when we want to expand several times a very specific token which is buried behind many others. Example: after \def\macroA{\macroY\macroZ}, the code

\MultiExpand{5}\expandafter\macroA\expandafter\macroB%
\unless\ifcsname\multiexpandafter\fi{4}\macroC\macroD

will expand \macroD 4 times, then will expand \macroA 4=5-1 additional times. Also, these tokens can be used to force expansion of an expandable macro, more or less like TH. was asking for: just prepend it the macro with the relevant few tokens.

Note: if you really need \MultiExpand{0}, just do \empty\empty.

How it works

\unless\ifcsname expands tokens fully until it reaches \endcsname. If the control sequence thus built exists, then it jumps to the matching \fi and eats it. All this happens in one step: try \ea\def\ea\foo\ea{\unless\ifcsname let\endcsname \fi}. Since \let exists, \foo becomes empty. Try removing \unless. Then the conditional becomes true, and the \fi remains: the full expansion would require two steps. We will always arrange for this conditional to be false, to get rid of the \fi. Since it is much easier to ensure that a command exists than not, we use \unless\ifcsname ifcsname\endcsname\fi.

Now, as I said before, \ifcsname expands tokens. The trick is to put the various tokens that we want to expand between the \ifcsname and the matching \endcsname. The simplest example is \unless\ifcsname\endifcsname:. In one step, it expands to the empty token list, and additionally expands its argument once. If you think about it, (almost) everything is just as if these three tokens were not there.

This is in fact how the construction ends. Let us quickly look at the definition of \multiexpand:n, whose argument k is the number of expansion left to do. If k=1, we end the \ifcsname as described in the paragraph above, throwing away two last lines in braces (I hide this fact at the end of the macro name \endifcsname:n). Otherwise, we keep the two lines in braces: they do one step of expansion (\numexpr#1-1\expandafter), and leave \multiexpand:n{k-1} on the stream. It will be expanded, since we are still inside the construction of the cs name for \ifcsname.

The code

Implementation

% I follow the LaTeX3 convention of finishing each macro name by its
% argument specification, e.g. ":nnnN" for three braced arguments and
% one single token. For this, I need "_" and ":" to be letters.
\catcode`\:=11\relax%
\catcode`\_=11\relax%
\def\use:n#1{#1}%
\def\endifcsname:{ifcsname\expandafter\endcsname\expandafter\fi}%
\def\endifcsname:n#1{\endifcsname:}%
\def\endifcsnameafter:n#1{\endifcsname:\expandafter}%
%
% The user commands are \MultiExpand and \MultiExpandAfter.
\def\MultiExpand{\unless\ifcsname\multiexpand\fi}%
\def\multiexpand\fi{\multiexpand:n}%
\def\multiexpand:n#1{%
  \ifnum#1<2 \expandafter \endifcsname:n%
  \else      \expandafter \use:n%
  \fi%
  {\expandafter \multiexpand:n \expandafter {%
      \number\numexpr#1-1\expandafter}}%
}% 
% Almost identical definitions for expanding after...
\def\MultiExpandAfter{\unless\ifcsname\multiexpandafter\fi}%
\def\multiexpandafter\fi{\multiexpandafter:n}%
\def\multiexpandafter:n#1{%
  \ifnum#1<2 \expandafter\endifcsnameafter:n%
  \else      \expandafter \use:n%
  \fi%
  {\expandafter \multiexpandafter:n \expandafter {%
      \number\numexpr#1-1\expandafter}\expandafter}%
}%
\catcode`\_=8\relax%
\catcode`\:=12\relax%

Tests

There is no limit on the number of expansions.

% ======= Tests =====
\long\gdef\expandonce#1{% redefines #1 as #1 expanded once.
  \long\edef#1{\unexpanded\expandafter\expandafter\expandafter{#1}}}

% Commands that expand to each other, to count how many times things 
% were expanded.
\def\0{\1}    \def\1{\2}    \def\2{\3}    \def\3{\4}    \def\4{\5}
\def\5{\6}    \def\6{\7}    \def\7{\8}    \def\8{\9}    \def\9{\0x}

% Test \MultiExpand, after two expansions, 
% we get the 2011th expansion of \0.
\def\:{\MultiExpand{2011}\0}    \show\:
\expandonce\:                \show\:
\expandonce\:                \show\:

% Test \MultiExpandAfter, two expansions to get the specified expansion.
% The results also show that the expansion happens in the expected order.
\def\:{%
  \MultiExpandAfter{27}\0% note that we get \5xx (25=27-2).
  \MultiExpandAfter{14}\0% note that we get \2x (12=14-2).
  \MultiExpandAfter{38}\0\0}% note that we get \8xxx... no "-2".
\show\: 
\expandonce\:                  \show\:
\expandonce\:                  \show\:

For comparison, the "Hello World!" example

\def\a#1.{#1}
\def\b#1:{#1.}
\def\c#1,{#1:}
\def\d{Hello world!,}
\MultiExpandAfter3\a
\MultiExpandAfter3\b
\MultiExpandAfter1\c
\d

now uses 32 \expandafters, slightly better than Hendrik's solution (however, we should count the overhead with all the \csname and friends). The other example on which Hendrik was optimizing, namely, expanding seven tokens once each in the reverse order, takes 74 \expandafter:

\def\a{a}    \def\b{b}    \def\c{c}    \def\d{d}
\def\e{e}    \def\f{f}    \def\g{g}

\begingroup\tracingall
\MultiExpandAfter3\a    \MultiExpandAfter3\b
\MultiExpandAfter3\c    \MultiExpandAfter3\d
\MultiExpandAfter3\e    \MultiExpandAfter1\f\g
\endgroup

Some experimentation tells me that the number of \expandafter is roughly 5 times the sum of the arguments of the various \MultiExpandAfter.

Answer 5

We first give a direct answer to the question. Later we give another, in our opinion much nicer, way to reverse the expansion of tokens.

The direct answer is given by the following code. In the first macro we make an expandable countdown counter which is used in the macro \expand, which expands to itself with a doubled argument (#2#2) with a decreased counter (\number\BSp@decnumber{#1})

\def\BSp@decnumber#1{%
    \ifcase#1\or0\or1\or2\or3\or4\or5\or6\or7\or8\or9\or10
             \or11\or12\or13\or14\or15\or16\or17\or18\fi}

\def\expandafterN#1{\romannumeral-`X\expand{#1}\expandafter}

\def\expand#1#2{\ifnum#1=\z@\putafterfi{ }\else
    \putafterfi{\expandafter\expand
              \expandafter{\number\BSp@decnumber{#1}}{#2#2}#2}\fi}

\def\putafterfi#1#2\fi{\fi#1}

In this definition we use a trick described in Forcing full expansion. An expansion of \expandafterN{n} gives after two expansion 2^n-1 \expandafter's (the answer to the question).

Now we define some macros to show the order of expansion (see also the answer by Hendrik Vogt of Jan 2, 2011)

\def\a#1.{#1}
\def\b#1:{#1.}
\def\c#1,{#1:}
\def\d{Hello world,}

\edef\aaa{\expandafterN{10}\a\expandafterN6\b\expandafterN2\c\expandafter\d\e}
\show\aaa

This gives in the output window

> \aaa=macro:
->Hello world.
l.153 \show \aaa

exactly what we want. Note that for every extra token that we want to expand in reversed order, we need to increase the counter by 4. This seems not to much, but essentially, at every step the number of \expandafter's is multiplied by 16.

For another way to reverse the order of expansion we use the same trick, but now directly. Note that for reversing the order of expansion of n tokens, we only need 4(n-1) \expandafter's and (n-1) \romannumeral's.

\edef\aaa{\romannumeral-`X\expandafter\expandafter\expandafter\space\expandafter\a
          \romannumeral-`X\expandafter\expandafter\expandafter\space\expandafter\b
          \romannumeral-`X\expandafter\expandafter\expandafter\space\expandafter\c
          \romannumeral-`X\expandafter\expandafter\expandafter\space\expandafter\d
          \e}%

which again gives the desired result. As inserting the tokens

 \romannumeral-`X\expandafter\expandafter\expandafter\space\expandafter 

may obscure the TeX code, we can at the cost of more (namely 2n-5)) \expandafter's define the macro

\def\RNexpandafter{%
   \romannumeral-`X\expandafter\expandafter\expandafter\expandafter
                   \expandafter\expandafter\expandafter\space
                   \expandafter\expandafter\expandafter}%

\edef\aaa{\RNexpandafter\a\RNexpandafterRN\b\RNexpandafter\c\expandafter\d\e}

which again gives the desired result.