When I run

\errmessage{My \pet\ is {sick}}
My \pet\ is {sick}.

the error message is:

My dog\ is {sick}.

but the PDF output reads:

My dog is sick.

What's going on here? Why is \pet expanded, but \␣ isn't? Why are those braces printed in the error message? Shouldn't I have to write \{ and \} to get braces?

  • 1
    Meta: <code>\&nbsp;</code> prints as \ . (I like \␣, too.) – Qrrbrbirlbel May 24 '13 at 18:16

The document clsguide.pdf, in section 4.9 mentions \space, which is not (that is backslash followed by a space).

      Your hovercraft is full of eels,\MessageBreak
      and \protect\foo\space is \foo
      Oh dear! Something's gone wrong.\MessageBreak
      \space \space Try typing \space <return>
      \space to proceed, ignoring \protect\foo.

Don't confuse \space with ; the former expands to a space, the second one is unexpandable; you find, in the LaTeX kernel,

\def\space{ }

Not quite an answer to the question, but related enough to warrant writing it. LaTeX3 provides ways to format nicely messages. In particular, within the command \iow_wrap:nnnN (and hence within all messages), is defined to expand to a space (just like \space). The list of available formatting commands is currently

  • expands to a space
  • \{ expands to a {
  • \} expands to a }
  • \# expands to a #
  • \% expands to a %
  • \~ expands to a ~
  • \\ goes to the next line
  • \iow_indent:n takes one argument and indents it by four spaces.

Example (well, I should probably have made \rescale do something rather than always throwing an error...).

\usepackage{expl3, xparse}
\msg_new:nnnn { mypkg } { percentage-too-high }
  { The~fuction~\token_to_str:N\rescale\ cannot~scale~more~than~1000\% ! }
    \iow_indent:n {#1} \\ \\
  { \msg_error:nnnn { mypkg } { percentage-too-high } {#1} {#2} }

What do you think \edef\temp{My \pet\space is sick} gets expanded to?

  \edef\temp{My \pet\ is {sick}}
\temp=macro:->My dog\ is {sick}.

According to [clsguide.pdf, §4.9], the correct way to enter a space in error messages is to write \space rather than \␣.

To understand what's going on here, one must bear in mind that \errmessage outputs to a text console, while most LaTeX commands output to the pdf being produced. Commands like \␣ are already fully expanded; all that's left to do is to execute them. But executing this sort of command doesn't make sense in a text-only situation. Hence, only a few commands, like \space, are programmed to make sense inside \errmessage.

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