Lout 3.18 tag.3.18

git-svn-id: http://svn.savannah.nongnu.org/svn/lout/tags/3.18@6 9365b830-b601-4143-9ba8-b4a8e2c3339c

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+@Section
+   @Title { Error messages }
+   @Tag { cpp_erro }
+@Begin
+@PP
+In order to understand the error messages produced by program
+printing, it is necessary to understand that Lout's first step when
+given a program text is to pass it to the separate {@Code prg2lout}
+program for analysis.  This separate program is the source of most
+of the error messages associated with program printing.
+@PP
+The {@Code prg2lout} program is quite happy to format a fragment of a
+computer program:  there is no need to supply a complete routine, or
+a complete statement, or any such thing.  However, it will complain if
+you supply only a fragment of one lexical unit, such as a comment or
+string without its terminating delimiter.  It will also complain if
+there is a character that cannot be classified as part of an identifier,
+number, etc. according to the rules of the language as they have been
+given to @Code prg2lout by the implementer.  Irrespective of the
+language rules, @Code prg2lout always interprets spaces, tabs, newlines,
+and formfeed characters in the usual way.
+@PP
+If an error message is generated by {@Code prg2lout}, it will contain
+a line and column number counting from the start of the program text
+involved.  Lout will precede this error message with a file name,
+line number, and column number pointing to the Lout symbol
+({@Code "@CP"}, {@Code "@Eiffel"} etc.) whose program text caused the
+error message, like this:
+@ID @Code @Verbatim {
+lout file "cpp_tabs" (from "cpp" line 80, from "all" line 45):
+   56,23: prg2lout 2,1: program text ended within comment
+}
+This is an actual message produced when formatting this chapter.  The
+program text in question has only one line, containing an incomplete comment,
+so when @Code "prg2lout" tried to start the second line and found nothing,
+it complained as shown.  In general, then, you have to add
+{@Code "prg2lout"}'s line number to Lout's line number, and use some
+initiative, to find the precise point of the problem.
+@End @Section