Line break preserving macros

In general LaTeX ignore single line breaks. It requires two line breaks in a row in order to end a line (paragraph). However, in certain cases you may wish to write blocks where the line breaks are preserved as they are written. An example of this is in the letter preamble when writing a multi-line address.

In inline LaTeX the \obeylines macro can be used to cause LaTeX to break lines on a single line break. Extending this idea it seems natural that the following macro, which places \obeylines before some text should also preserve line breaks.

\newcommand{\keepLineBreaks}[1]{\obeylines #1}

However, the \keepLineBreaks macro does not in fact work. This is because LaTeX reads the arguments to the macro in without \obeylines active in order to do the argument substitution into the macro. As a result, when it reads it the second time (this time with \obeylines active), all of the single line breaks have already been lost.

In order to address this, the \obeylines macro must be used, before the text is read. As a result two macros are required to implement this functionality. The first macro does not read in the arguments. It only invokes \obeylines and a second macro which will actually read in the arguments. Note that it starts with a \begingroup. This is to ensure that the effect of \obeylines does not extend beyond the expected range.

\newcommand{\keepLineBreaks}{\begingroup\obeylines\keepLineBreak@doStuff}

The second command reads in the arguments and actually does whatever processing needs to happen in the context of \obeylines. Note that it needs to end with \endgroup to close the group that was started by the first command.

\newcommand{\keepLineBreak@doStuff}[1]{#1\endgroup}