spacing
Is it possible to make it so that LaTeX will ignore newlines in the source? e.g. it shouldn't do anything to the output if I use an empty line.
I like to keep my source fairly spaced out for easier reading, but that becomes fairly difficult since an empty line forces a new paragraph.
The answer to the question in the title is technically "they are not processed" but I don't think that's the answer you want.
If you modify your definitions to
\newcommand{\oneArg}[1]{\long\def\a{[#1]}\typeout{\meaning\a}}
\newcommand{\twoArgs}[2]{\long\def\a{[#1][#2]}\typeout{\meaning\a}}
\newcommand{\threeArgs}[3]{\long\def\a{[#1][#2][#3]}\typeout{\meaning\a}}
you will see
\long macro:->[\par ]
\long macro:->[\par ][B]
\long macro:->[\par ][B][\par ]
Assuming normal catcodes are in force a blank line is turned by TeX into the token \par (literally the command name token \par not the primitive paragraph end function) It does this at a very early stage as characters are being tokenised, so before any token lists are passed to a macro. So a macro never sees a blank line in its argument. The behaviour is always as if you replace the blank line by \par in the input file.
Space tokens are similarly processed at this early stage. Spaces at the end of the line and the beginning of the next are discarded and never tokenised at all so macros have no record of them. (You can not prevent the discarding of space at the end of the line even if you change the catcode of space) and runs of spaces characters only produce one space token. It is the tokens not the file characters that are passed to a macro.
If you have non-delimited arguments as in your example any spaces tokens are skipped while looking for the argument, if you want a space to be the argument you need { }. \par can be an undelimited argument if the macro is \long or an error otherwise.
Whether or not there is "more space" depends on where you measure from. If (as in your example) the rules are placed on equally spaced baselines, then there is less space between the tops of the letters and the top rule than from the bottoms of the letters and the bottom rule. However it does mean that the space between the rules depends only on the number of lines, not on the content of the text.
You can remove the equal baseline requirement but then the top rule position moves if there are no uppercase letters on the first line, and the bottom moves if there are descenders.
There are of course variants where you adjust the space one way or another, but first you have to define "tight".
\documentclass{article}
\setlength\parindent{0pt}
\setlength\textwidth{5cm}
\begin{document}
\rule{\linewidth}{0.4pt}
1 This is a paragraph, possibly with more than 1 line of text.
\rule{\linewidth}{0.4pt}
\bigskip
\rule{\linewidth}{0.4pt}
\nointerlineskip
2 This is a paragraph, possibly with more than 1 line of text.
\nointerlineskip
\rule{\linewidth}{0.4pt}
\bigskip
\rule{\linewidth}{0.4pt}
3 This is a paragraph, possibly with more than 1 ggg.
\rule{\linewidth}{0.4pt}
\bigskip
\rule{\linewidth}{0.4pt}
\nointerlineskip
4 This is a paragraph, possibly with more than 1 ggg.
\nointerlineskip
\rule{\linewidth}{0.4pt}
\bigskip
\rule{\linewidth}{0.4pt}
5 this is a paragraph, possibly with more than 1 line of text.
\rule{\linewidth}{0.4pt}
\bigskip
\rule{\linewidth}{0.4pt}
\nointerlineskip
6 this is a paragraph, possibly with more than 1 line of text.
\nointerlineskip
\rule{\linewidth}{0.4pt}
\end{document}
Best Answer
if you use comment sign between lines latex will interpret it as a single paragraph.
example: