[Caml-list] assert + -pp = wrong line number and filename in assert message

[Caml-list] assert + -pp = wrong line number and filename in assert message

[Paul Stodghill]

The "assert" construct generates an exception which contains the file
name and line number where the "assert" appears.

However, when a preprocessor is used (with the -pp option), the file name
and line number that are recorded are relative to the preprocessor
output file and not to original source file, even when the 
'# linenum "filename"' directives are inserted in the preprocessor
output.

A quick scanner of ocaml-3.02/parsing/lexer.mll seems to indicate that
the line directive is completely ignored.

milhouse$ uname -a
CYGWIN_NT-5.0 MILHOUSE 1.3.2(0.39/3/2) 2001-05-20 23:28 i686 unknown
milhouse$ ocamlc -v
The Objective Caml compiler, version 3.02
Standard library directory: /usr/local/lib/ocaml
milhouse$ 
milhouse$ 
milhouse$ cat preproc.sh 
#! /bin/sh

echo '# 1 "'$1'"'
cat $1
milhouse$ 
milhouse$ 
milhouse$ cat assert_bug.ml
assert false;;
milhouse$ 
milhouse$ 
milhouse$ ocamlc -o assert_bug.exe -pp './preproc.sh' assert_bug.ml 
milhouse$ ./assert_bug.exe 
Fatal error: uncaught exception Assert_failure("/home/stodghil/tmp/camlpp0", 20, 32)
milhouse$ 

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Re: [Caml-list] assert + -pp = wrong line number and filename in assert message

[Paul Stodghill]

> The "assert" construct generates an exception which contains the file
> name and line number where the "assert" appears.
> [ rest deleted ]

I should point out that the compiler does seem to respect '# line'
directives correctly when it comes to reporting syntax error. The
problem only appears to be with the assert construct.


milhouse$ cat bogus.ml 
this is totally bogus
milhouse$ ocamlc -c -pp './preproc.sh' bogus.ml 
File "bogus.ml", line 1, characters 0-4:
Unbound value this
milhouse$ 

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Re: [Caml-list] assert + -pp = wrong line number and filename in assert message

[Xavier Leroy]

> The "assert" construct generates an exception which contains the file
> name and line number where the "assert" appears.

Character number, actually.

> However, when a preprocessor is used (with the -pp option), the file name
> and line number that are recorded are relative to the preprocessor
> output file and not to original source file, even when the 
> '# linenum "filename"' directives are inserted in the preprocessor
> output.

Right.  OCaml has a peculiar way of treating positions in source code:
on parsing, it remembers minimal information (absolute character
numbers in the input, regardless of # lineno "filename" directives),
and it's only when an error is detected that it re-scans the input to
convert the character numbers into file name + line number.

This works well for error reporting, but causes problems with
"assert", the compiler-generated "Match_failure" exceptions, and
certain uses of Camlp4.

All this will have to be addressed at some point, although it's rather
painful to do right.

Best regards,

- Xavier Leroy
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