New lines and TeX: difference between ^^J and ^^M

Question

What is a new line for TeX in the following contexts:

1. When reading from a file.
2. When writing to a file.
3. After having read a % character.
4. In a \scantokens.

I am asking in particular because the following code only typesets A:

\documentclass{minimal}
\begin{document}
\catcode`\%=12
\def\foo{\scantokens{A%    

B}}
\show\foo
\catcode`\%=14
\foo
\end{document}

So my main question is: how does % know where to stop gobbling characters?

EDIT: Adding the two lines

\catcode`\^^M=12 
\newlinechar`\^^M

before the definition of \foo is instructive: then the definition actually contains new-lines, and the comment stops gobbling where we expect.

EDIT2: pdflatex sets \newlinechar``\^^J and \endlinechar``\^^M (see Harald's concise answer below for what these are).

Answer 1

1. I think that what constitutes the end of line upon reading from a file is hardcoded according to whatever operating system you are running on. And that end of line is represented by the character whose number is \endlinechar.
2. When writing, the character whose number is \newlinechar will trigger the end of a line. Again, the exact result in the output file is hard coded, depending on your operating system.
3. See #1.
4. Usually, the argument to \scantokens is treated as a single line. Thus a percent sign in the argument to \scantokens will end input from this argument. However, any occurrences of the character whose number is \newlinechar will be used to split the argument into several lines.

To bring all these ideas together, consider the plain TeX file

\newlinechar=2
{\catcode`\%=12
 \gdef\foo{\scantokens{abc%xyz^^Bdef}}}%
\endlinechar=`X
\foo%
\bye%

which will typeset the text “abcdefX” .

(Edited to take into account what I learned about #4 from the comments.)

Answer 2

So you asked several questions, but let me first answer your main question.

What happens here is that when the definition for \foo is being parsed, it's identical to

\def\foo{\scantokens{A%\par B}}

Now when \scantokens executes, it's as if it read the line

A%\par B

from a file with the current catcodes in effect. Since you reset % to be a comment, the \par B is ignored.

For your other questions, individual installations of TeX can determine what is treated as a new line when reading and writing files. If I recall correctly, \n, \r, \r\n, and \n\r are treated as a newline for the purposes of input, at least that's what I recall from reading through the source for pdfTeX recently. For output, (i.e., writing files), I suspect it uses \n on *NIX and \r\n on Windows, but I haven't verified this.

After TeX reads a line of text from an input file—and before it begins to tokenize it—it removes all trailing space characters including \r and \n and appends the \endlinechar character which is normally ^^M (i.e., \r). This happens regardless of there being a % character in the line. When TeX encounters a % character in its input (note that there is no comment token), it ignores the rest of the line, including the trailing \r.

As far as I know, ^^J (i.e., \n) is not special in most contexts except it is often used as the \newlinechar for use in \write.

I forgot \scantokens. It is really treated like lines of input from a file, including the \endlinechar at the end of each. For a simple example of this, try

\endlinechar`X %
\scantokens{A}%
\bye%

(The space after the X is necessary because TeX looks for an optional space there.) Every line ends with a percent and yet the output shows AX because \scantokens has inserted that character.