1

The following plain TeX manuscript

\def\mymacro{\newtoks\mytoks}\bye

fails to compile and generates the following error:

Runaway definition?
->
! Forbidden control sequence found while scanning definition of \mymacro.
<inserted text> 
                }
<to be read again> 
                   \newtoks 
l.1 \def\mymacro{\newtoks
                         \mytoks}\bye

What's the problem?

Evan Aad
  • 11,066

1 Answers1

3

Line 232 of plain.tex has

\outer\def\newtoks{\alloc@5\toks\toksdef\@cclvi}

Look in the TeXbook for \outer. Page 206, first doubly dangerous paragraph:

The \par-forbidding mechanism doesn’t catch all conceivable missing-brace errors, however; you might forget the } at the end of a \def, and the same problem would arise. In this case it’s harder to confine the error, because \par is a useful thing in replacement texts; we wouldn’t want to forbid \par there, so TeX has another mechanism: When a macro definition is preceded by ‘\outer’, the corresponding control sequence will not be allowed to appear in any place where tokens are being absorbed at high speed. An \outer macro cannot appear in an argument (not even when \par is allowed), nor can it appear in the parameter text or the replacement text of a definition, nor in the preamble to an alignment, nor in conditional text that is being skipped over.

See also Why are \new... defined as \outer in the plain TeX format? and Why is \newcount declared \outer?

If you really need to do \newtoks as part of a plain TeX definition:

\def\mymacro{\csname newtoks\endcsname\mytoks}

will do.

egreg
  • 1,121,712