Non-letters in macro names

New LaTeX users are often suprised that macro definitions containing non-letters, such as

\newcommand{\cul8r}{Goodbye!}

fail to compile. The reason is that the TeX macro language, unlike most programming languages, allows nothing but letters in macro names.

There are a number of techniques for defining a macro with a name like \cul8r. Unfortunately, none of the techniques is particularly good:

1. Use \csname\endcsname to define and invoke the

  macro:
  ```latex
  \expandafter\newcommand\csname cul8r\endcsname{Goodbye!}
  I said, ``\csname cul8r\endcsname''.
  ```
  
  1. Pro: No unexpected side effects
  2. Con: So verbose as to be unusable

2. Define a “special-command generator”, and use the resulting

  commands:
  
  ```latex
  \newcommand{\DefineRemark}[2]{%
    \expandafter\newcommand\csname rmk-#1\endcsname{#2}%
  }
  \newcommand{\Remark}[1]{\csname rmk-#1\endcsname}
  ...
  \DefineRemark{cul8r}{Goodbye!}
  ...
  \Remark{cul8r}
  ```
  
  1. Pro: Straightforward to use, not too untidy
  2. Con: It's hardly doing what we set out to do (experts will

see that you are defining a macro, but others likely won't)

3. Convince TeX that 8 is a letter:

  ```latex
  \catcode`8 = 11 
  \newcommand{\cul8r}{Goodbye!}
  I said, ``\cul8r''.
  ```
  
  1. Pro: \cul8r can be used directly
  2. Con: Likely to break other uses of 8 (such as

numbers or dimensions; so

    ''\setlength{''\paperwidth''}{8in}'' tells us:
  ```latex
  ! Missing number, treated as zero.
  <to be read again> 
                     8
  ```
  
  As a general rule, changing category codes is something to use
  //in extremis//, after detailed examination of options.  It is
  conceivable that such drastic action could be useful for you, but
  most ordinary users are well advised not even to try such a
  technique.

4. Define a macro \cul which must always be followed by

  ''8r'':
  ```latex
  \def\cul8r{Goodbye!}
  I said, ``\cul8r''.
  ```
  
  1. Pro: \cul8r can be used directly
  2. Con #1: Breaks if \cul is followed by anything other

than 8r, with a confusing diagnostic —

    ''\cul99'' produces:
  ```latex
  ! Use of \cul doesn't match its definition.
  <*> \cul9
           9
  ```
    (which would confuse someone who hadn't even realised there
    //was// a definition of ''\cul'' in the document).
  - Con #2: Silently redefines existing ''\cul'', if any;
    as a result, the technique cannot be used to define both a
    ''\cul8r'' and, say, a ''\cul123'' macro in the same
    document.
  

Technique 3 is in fact commonly used — in a limited form — within most LaTeX packages and within LaTeX itself. The convention is to use @ within the names of internal macros to hide them from the user and thereby prevent naming conflicts. To this end, LaTeX automatically treats @ as a letter while processing classes and packages and as a non-letter while processing the user's document. The key to this technique is the separation: internally a non-letter is used for macro names, and the user doesn't see anything of it, while the status remains “frozen” in all the definitions created within the class or package. See ''\@'' and ''@'' in macro names for more information.

Note that analogous use of technique 3 in this example would give us

\begingroup
  \catcode`8 = 11 
  \gdef\cul8r{Goodbye!}
  \gdef\later{\cul8r}
\endgroup
I said, ``\later''.

which works, but rather defeats the object of the exercise. (\later has the “frozen” catcode for “8”, even though the value has reverted to normal by the time it's used; note, also, the use of the primitive command \gdef, since \newcommand can't make a macro that's available outside the group.)

Recommendation: Either choose another mechanism (such as \DefineRemark above), or choose another name for your macro, one that contains only ordinary letters. A common approach is to use roman numerals in place of arabic ones:

\newcommand{\culVIIIr}{Goodbye!}

which rather spoils the intent of the joke implicit in the example \cul8r!


Source: Non-letters in macro names

composition/macros/caracteres_non_alphabetiques_dans_les_noms_de_macros.txt · Dernière modification: 2018/12/04 00:23 par jejust
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