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From: marcel@cs.caltech.edu (Marcel van der Goot)
Newsgroups: comp.text.tex
Subject: obeylines (Re: easy macro for novice)
Summary: catcodes are assigned when a character is first read
Keywords: category codes, \obeylines
Message-ID: <1990Dec6.001129.10115@nntp-server.caltech.edu>
Date: 6 Dec 90 00:11:29 GMT
References: <DAVIS.90Dec5022859@pacific.mps.ohio-state.edu>
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In <DAVIS.90Dec5022859@pacific.mps.ohio-state.edu> John E. Davis
(davis@pacific.mps.ohio-state.edu) defines

> \def\dolines#1{\medskip\obeylines#1\medskip}

which doesn't work.

> [Apparently] TeX is converting the 'newlines' to spaces before
> substitution into the macro.  If this is the correct diagnosis, [...]

No, it isn't. What TeX reads (chapter 7) are pairs
(character code, category code). Of course, your file contains only
charcodes, so when TeX reads a character it uses the current catcode
of that character. When it reads the newlines before substitution in
your macro, <return> has catcode 5, so the newlines are replaced
by (<return>, 5). In your macro, \obeylines changes the current catcode
of <return> to 13 (active), but then it is already too late, since the
argument already consists of (charcode, catcode) pairs. The conversion
of <return> to a space is only done when the text is actually typeset.

You can check this by doing (with your definition of \dolines):
	{\catcode`\^^M=\active
	 \dolines{...} % read argument with current catcode (13) of <return>
	}

In other words, the solution is to do the \obeylines before you
read the argument. Here are macros that do that:
	\def\dolines{\bgroup\obeylines\dothelines}
	\def\dothelines#1{\medskip#1\medskip\egroup}
Usage:
	\dolines{
	This is line 1.
	This is line 2.
	This is line 3.
	}

					Marcel van der Goot
					marcel@vlsi.cs.caltech.edu