(eplain.info)Category codes

---

Next: Allocation macros Up: Programming definitions

---

Enter node , (file) or (file)node

7.1 Category codes
==================

Plain TeX defines '\active' (as the number 13) for use in changing
category codes. Although the author of 'The TeXbook' has "intentionally
kept the category codes numeric", two other categories are commonly
used: letters (category code 11) and others (12). Therefore, Eplain
defines '\letter' and '\other'.

   Sometimes it is cleaner to make a character active without actually
writing a '\catcode' command. The '\makeactive' command takes a
character as an argument to make active (and ignores following spaces).
For example, here are two commands which both make '\' active:

     \makeactive\\   \makeactive92

   Sometimes you might want to temporarily change the category code of
the '@' character to '\letter', so that you can use or define macros
which are normally inaccessible to the user. For such situations, Eplain
provides the '\makeatletter' command. It sets the category code of '@'
to '\letter' (11) and defines '\resetatcatcode' to restore the category
code to whatever it was before the call to '\makeatletter'. For example:

     \makeatletter
     \def\@hidden@macro{This macro cannot normally be
                        called / redefined by the user}
     \resetatcatcode

There is also '\makeatother' which works similarly but sets the category
code of '@' to '\other' (12).

   Usually, when you give a definition to an active character, you have
to do so inside a group where you temporarily make the character active,
and then give it a global definition (cf. the definition of
'\obeyspaces' in 'The TeXbook'). This is inconvenient if you are writing
a long macro, or if the character already has a global definition you do
not wish to transcend. Eplain provides '\letreturn', which defines the
usual end-of-line character to be the argument. For example:

     \def\mymacro{... \letreturn\myreturn ... }
     \mymacro hello
     there

The end-of-line between 'hello' and 'there' causes '\myreturn' to be
expanded.

   'The TeXbook' describes '\uncatcodespecials', which makes all
characters which are normally "special" into "other" characters, but the
definition never made it into plain TeX. Eplain therefore defines it.
For notes on the usage, Note: Verbatim listing.

   Finally, '\percentchar' expands into a literal '%' character. This is
useful when you '\write' TeX output to a file, and want to avoid
spurious spaces. For example, Eplain writes a '\percentchar' after the
definition of cross-references. The macros '\lbracechar' and
'\rbracechar' expand similarly.