我想定义两个宏 x 和 y，这样它们都有某种实现，但在我们在 y 上调用 x 的特定情况下，结果不同。例如，我必须如何更改以下定义
\newcommand{\x}[1]{Foo #1}
\newcommand{\y}[1]{Bar #1}
获取代码
\x{SomeExpression}
\y{SomeExpression}
\x{\y{SomeExpression}}
\x{\y{SomeExpression}\y{SomeExpression}}
产生以下输出：
Foo 某些表达式的结果
某些表达式的结果
你在 y 之后调用了 x
Foo Bar 某些表达式的结果Bar 某些表达式的结果
这里 SomeExpression 是一个表达式，其计算结果为字符串 ResultOfSomeExpression。我不在乎如果你做了这样的事情会发生什么，比如调用
\x{\y}
因为它不是在我的应用程序中预期使用的。
答案1
\documentclass{article}
\begin{document}
\makeatletter
\newcommand{\x}[1]{%
\expandafter\def\expandafter\zz\expandafter{\@firstoftwo#1..}%
\ifx\z\zz
You called x after y%
\else
Foo #1%
\fi}
\newcommand{\y}[1]{Bar #1}
\def\z{\y..}
\x{a}
\y{a}
\x{\y{a}}
\x{\y{a}\y{a}}
\x{SomeExpression}
\y{SomeExpression}
\x{\y{SomeExpression}}
\x{\y{SomeExpression}\y{SomeExpression}}
\end{document}
答案2
在 TeX 和 LaTeX 中，一切都与处理标记、扩展（可扩展）宏标记等有关。
您不需要正确地（用文字）指定一个算法来描述宏的期望行为\x以及\y处理其参数/其参数的标记时的情况，而只需指定四种行为情况：
案例 1：
\x{SomeExpression}
→Foo ResultOfSomeExpression情况 2：
\y{SomeExpression}
→Bar ResultOfSomeExpression案例 3：
\x{\y{SomeExpression}}
→You called x after y案例 4：
\x{\y{SomeExpression}\y{SomeExpression}}
→Foo Bar ResultOfSomeExpressionBar ResultOfSomeExpression
你确定在案例 4 中你确实希望得到这个结果
Foo Bar ResultOfSomeExpressionBar ResultOfSomeExpression
但你又不希望得到这个结果
Foo ResultOfBar ResultOfSomeExpressionBar ResultOfSomeExpression吗？
也许一些括号会更好地表明结合性？——类似于：
Foo ResultOf(Bar ResultOf(SomeExpression)Bar ResultOf(SomeExpression))？
是因为它可能。
下面我实现了一个宏\y和一个宏\x。
宏\y只是“抓取”它的参数（即删除分隔括号）并返回Bar ResultOf添加了前导短语的该参数：
Bar ResultOf<Argument>。
该宏\x抓取其参数（即删除分隔括号）并分叉三种情况：
情况 1：参数的模式为
⟨control-sequence-token \y⟩ + ⟨single undelimited parameters⟩
在这种情况下，您会得到以下短语：You called x after y。
情况 2：参数的模式为
⟨control-sequence-token \y⟩ + ⟨某些不是单个未限定参数的东西，也可能是空⟩
在这种情况下，您会得到短语Foo <Argument>。
情况 3：参数为任何其他模式，也可能为空。
在这种情况下，您会得到短语Foo ResultOf<Argument>。
请注意，我既没有使用任何 eTeX 扩展或类似的东西，也没有使用任何\if..\else..\fi-token，也没有进行临时分配。
不发生临时分配的情况使得宏在扩展上下文中也可以使用。
例如，如果您（例如就\@namedef）定义了一个名为的宏标记\You called x after y，则可以通过调用该宏\csname\x{\y{something}}\endcsname。
\documentclass[a4paper]{article}
%%=============================================================================
%% Paraphernalia:
%% \UD@firstoftwo, \UD@secondoftwo, \UD@Exchange,
%% \UD@CheckWhetherNull, \UD@CheckWhetherSingleUndelimitedArg
%%=============================================================================
\makeatletter
\newcommand\UD@firstoftwo[2]{#1}%
\newcommand\UD@secondoftwo[2]{#2}%
\newcommand\UD@Exchange[2]{#2#1}%
%%-----------------------------------------------------------------------------
%% Check whether argument is empty:
%%.............................................................................
%% \UD@CheckWhetherNull{<Argument which is to be checked>}%
%% {<Tokens to be delivered in case that argument
%% which is to be checked is empty>}%
%% {<Tokens to be delivered in case that argument
%% which is to be checked is not empty>}%
%%
%% The gist of this macro comes from Robert R. Schneck's \ifempty-macro:
%% <https://groups.google.com/forum/#!original/comp.text.tex/kuOEIQIrElc/lUg37FmhA74J>
%%
%% A concern in his posting is that the argument is hit with \string
%% after some expansions which in edge cases might result in unbalancing
%% surrounding \if..\fi-constructs if the macro is used inside of such
%% \if..\fi-constructs.
%%
%% That challenging concern sickened me. ;-)
%%
%% Therefore I decided to implerment a variant where this cannot happen
%% as expansion is forced by \romannumeral:
%%
%% After the first expansion-step, \string is not applied yet.
%% After the second expansion-step, any possibly disturbing remainders
%% are already removed due to \romannumeral-expansion.
%%
%% No eTeX- or whatsoever extensions. No \if.. .Only \romannumeral,
%% digit 0, space token for terminating \romannumeral-expansion,
%% \string, \expandafter, \UD@firstoftwo, \UD@secondoftwo, {, }.
%%
%% May 20, 2016
%%
%% Ulrich Diez (e-mail: [email protected])
%%
\newcommand\UD@CheckWhetherNull[1]{%
\romannumeral0\expandafter\UD@secondoftwo\string{\expandafter
\UD@secondoftwo\expandafter{\expandafter{\string#1}\expandafter
\UD@secondoftwo\string}\expandafter\UD@firstoftwo\expandafter{\expandafter
\UD@secondoftwo\string}\expandafter\expandafter\UD@firstoftwo{ }{}%
\UD@secondoftwo}{\expandafter\expandafter\UD@firstoftwo{ }{}\UD@firstoftwo}%
}%
%%-----------------------------------------------------------------------------
%% Check whether argument itself consists of exactly one undelimited
%% argument:
%%.............................................................................
\newcommand\UD@CheckWhetherSingleDelimitedArg[1]{%
\romannumeral0%
\UD@CheckWhetherNull{#1}{\UD@secondoftwo{}}{%
\expandafter\UD@CheckWhetherNull\expandafter{\UD@secondoftwo#1{}}%
{\expandafter\expandafter\UD@firstoftwo{ }{}\UD@firstoftwo}%
}%
{\expandafter\expandafter\UD@firstoftwo{ }{}\UD@secondoftwo}%
}%
%%=============================================================================
%% Internal macros of a mechanism checking whether a macro argument is of
%% pattern \y<single undelimited argument> -- the user-level-macro will be
%% \UDAtCheckWhetherLeadingyAndSingleUndelimited
%%-----------------------------------------------------------------------------
%% Check whether brace-balanced argument starts with a \y-token
%%.............................................................................
%% \UDAtCheckWhetherLeadingy{<Argument which is to be checked>}%
%% {<Tokens to be delivered in case <argument
%% which is to be checked>'s 1st token is a
%% \y-token>}%
%% {<Tokens to be delivered in case <argument
%% which is to be checked>'s 1st token is not
%% a \y-token>}%
\newcommand\UDAtCheckWhetherLeadingy[1]{%
\romannumeral0\UD@CheckWhetherNull{#1}%
{\expandafter\expandafter\UD@firstoftwo{ }{}\UD@secondoftwo}%
{\expandafter\UD@secondoftwo\string{\UD@CheckWhetherLeadingyB.#1\y}{}}%
}%
\newcommand\UD@CheckWhetherLeadingyB{}%
\long\def\UD@CheckWhetherLeadingyB#1\y{%
\expandafter\UD@CheckWhetherNull\expandafter{\UD@secondoftwo#1{}}%
{\UD@Exchange{\UD@firstoftwo}}{\UD@Exchange{\UD@secondoftwo}}%
{\UD@Exchange{ }{\expandafter\expandafter\expandafter\expandafter
\expandafter\expandafter\expandafter}\expandafter\expandafter
\expandafter}\expandafter\UD@secondoftwo\expandafter{\string}%
}%
%%-----------------------------------------------------------------------------
%% Check whether brace-balanced argument is of pattern
%% \y<single undelimited argument>
%%.............................................................................
\newcommand\UDAtCheckWhetherLeadingyAndSingleUndelimited[1]{%
\romannumeral0%
\UDAtCheckWhetherLeadingy{#1}{%
\expandafter\UD@CheckWhetherSingleDelimitedArg
\expandafter{\UD@firstoftwo{}#1}{%
\expandafter\expandafter\UD@firstoftwo{ }{}\UD@firstoftwo
}%
}{\UD@secondoftwo{}}%
{%
\expandafter\expandafter\UD@firstoftwo{ }{}\UD@secondoftwo
}%
}%
\makeatother
%% This delivers the texts that you specified.
\newcommand\x[1]{%
\UDAtCheckWhetherLeadingyAndSingleUndelimited{#1}{%
You called x after y%
}{%
\UDAtCheckWhetherLeadingy{#1}{Foo #1}{Foo ResultOf#1}%
}%
}%
\newcommand\y[1]{Bar ResultOf#1}%
%%\newcommand\x[1]{%
%% \UDAtCheckWhetherLeadingyAndSingleUndelimited{#1}{%
%% operation x with the result of applying operation y to #1 as operand%
%% }{%
%% operation x with operand (#1)%
%% }%
%%}%
%%
%%newcommand\y[1]{operation y with operand (#1)}%
\begin{document}
\x{SomeExpression}
\y{SomeExpression}
\x{\y{SomeExpression}}
\x{\y{SomeExpression}\y{SomeExpression}}
\makeatletter
\@namedef{You called x after y}{Calling x after y is nice!}
\@namedef{Foo ResultOfSomethingWeird}{Calling x on something weird is nice too!}
\@namedef{Bar ResultOfSomethingWeird}{Calling y on something weird is nice as well!}
\@namedef{Foo Bar ResultOfSomethingWeirdBar ResultOfSomethingWeird}{Calling x on the square of result of calling y on something weird is extraordinarily nice!}
\makeatother
\csname\x{\y{something weird}}\endcsname
\csname\x{SomethingWeird}\endcsname
\csname\y{SomethingWeird}\endcsname
\csname\x{\y{SomethingWeird}\y{SomethingWeird}}\endcsname
\end{document}