Why the end-of-line % in macro definitions? [duplicate]

Question

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• What is the use of percent signs (%) at the end of lines? 4 answers

I found two things peculiar in the common style of macro definitions:

1. Ending lines in macro definitions with '%';
2. Not wrapping a macro name in braces.

Example:

\newcommand\examplemacro{%
    \somecommand%
    \othercommand%
}

Being a software engineer first and a LaTeX user a distant second, I would expect:

\newcommand{\examplemacro}{
    \somecommand{}
    \othercommand{}
}

Is there a reason for doing it the former way, or is it merely a matter of habit?

Answer 1

The comment at the end of certain lines removes the space otherwise inserted by the end-of-line character (which is usually turned into a space). It is not needed after macros (control words), because they remove spaces themselves. You do not need braces {} after a macro. I think you relate here to the () added in programming languages like C after functions. Macros aren't functions, even if they can often used in similar ways. If your \somecommand doesn't take an argument the {} will not be touched by it and the source code line break after it will be turned into an normally unwanted space. Normally a orphaned {} doesn't hurt, but in certain places, like in an expandable context, they might cause trouble. You should definitely not wrap all macros in braces, like {\somecommand}, because that makes them locally scoped.

So you should write your macros like:

\newcommand\examplemacro{%
    \somecommand
    \commandwitharg{somearg}%
}

Here the two % are required to remove the end-of-line-turned-space, but the \somecommand line doesn't require one because all spaces after control words are removed. This is done, because you might need to add at least one space after it to separate it from following text and multiple spaces are always reduced to one.

It should be noted that these (La)TeX rules can be dynamically changed, e.g. all line endings or even all spaces can be ignored by changing the required \endlinechar and catcodes. This is done for the LaTeX3 syntax, which avoids the need for this kind of comments.

Answer 2

As Martin has already commented, adding % to the end of a line prevents TeX from turning the end-of-line character into a space. This is needed at the end of every line except those where TeX is already skipping spaces, for example after a macro name which takes no arguments:

\newcommand{\examplemacro}{%
    \somecommand{}%
    \othercommand
}

Here, the line with \othercommand does not need a % as TeX will skip the space here anyway.

The use of braces for the first argument of \newcommand is 'optional' due to the way TeX grabs arguments. We use a brace group in LaTeX to indicate a single argument, but TeX will grab either a brace group or a single token as an undelimited argument. In the example, \examplemacro is a single token (a control sequence), and so TeX will grab it in one go.

There are places where the braces are required. For example

\newcommand\test{a}

will work but

\newcommand{\test}b

will not, as the 'replacement text' argument for \newcommand has to be given in braces.

As LaTeX is built on TeX, some TeX ideas leak through. The formal LaTeX syntax always includes braces, but as TeX does not always need them people 'in the know' take shortcuts.

Answer 3

I am not 100% sure but in one of my macros I could so suppress unwanted spaces in the output.

About question 2 why the braces are missing, I think because they are optional.