Path: utzoo!utgpu!watmath!iuvax!uxc.cso.uiuc.edu!tank!mimsy!chris
From: chris@mimsy.UUCP (Chris Torek)
Newsgroups: comp.text
Subject: Re: String functions in TeX: instr and extract
Message-ID: <19345@mimsy.UUCP>
Date: 31 Aug 89 06:16:17 GMT
References: <1989Aug30.133802.15579@cs.dal.ca> <19344@mimsy.UUCP>
Reply-To: chris@mimsy.umd.edu
Organization: U of Maryland, Dept. of Computer Science, Coll. Pk., MD 20742
Lines: 46

>>I would like to redefine the \input command so that an alternate file
>>extension is used when the file is not found.

In article <19344@mimsy.UUCP> I wrote:
>You cannot achieve this.  The only way to test for a file's existence
>is to open it; and open always succeeds.  If the file cannot be opened,
>TeX interacts with its operator to get a different name---and it will
>not take `no name' for an answer!  (I consider this a bug.)

This is not quite true (as ken@cs.rochester.edu pointed out).  You
can first open it with `\openin', and then use `\ifeof' to check for
immediate EOF, which `appears' true for nonexistent files (and, after
testing, it seems it appears false for empty files!).  Then, if and
only if it exists, you can \input it.  Something like this (which I
may start using myself...):

\catcode`@=11 % plain TeX only, not in LaTeX style files.

% Give #1 as an error, with help #2.  Stolen from LaTeX.
\def\@errhelp#1#2{\edef\@tempc{#2}\expandafter\errhelp\expandafter{\@tempc}
  \errmessage{#1}}

\let\@oldinput=\input
\newread\test@existence
% Is the `\immediate\closein' below unnecessary?
\def\@inerr#1{\immediate\closein\test@existence
  \@errhelp{Cannot read #1.tex}{The file you named does not exist,
  or cannot be read.^^JIf you want to use a different file,
  type \space I \string\input{file} <return> \space here.}}
\def\@inok#1{\immediate\closein\test@existence\@oldinput #1}
\def\input#1{\immediate\openin\test@existence=#1
  \ifeof\test@existence\let\next\@inerr\else\let\next\@inok\fi\next{#1}}

\catcode`@=12 % plain TeX only again.

Incidentally, the reason for using \next and \@inerr and \@inok is that
otherwise TeX's error-interaction lets one delete tokens from the
expansion of \input.  (My first version, before I experimented, used
\ifeof...\errmessage{Cannot read #1}\else...\fi; this let you delete
the \else...\fi part.)

(Now it is up to the original poster to decide how to look for
extensions in the argument to \input.)
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7163)
Domain:	chris@mimsy.umd.edu	Path:	uunet!mimsy!chris