Subject: Re: [PATCH] grep: improve errors for unmatched ( and )

From: Junio C Hamano <[email-masked]>
Date: Fri, 22 Mar 2024 09:41:40 -0700

Ahelenia Ziemiańska <[email-masked]> writes:

> Imagine you want to grep for (. Easy:

Please have a blank line before and 

>   $ git grep '('
>   fatal: unmatched parenthesis

after a displayed text like this one (this applies to a few more
paragraphs in the proposed log message).

> uhoh. This is plainly wrong. Unless you know specifically that
> (a) git grep has expression groups and that
> (b) the only way to work around them is by doing -- '(' or -e '('

I do not think "--" (end of options and beginning of pathspec)
marker would work for that purpose, UNLESS you are talking about a
file whose name is an open parenthesis.  Just keep "-e '('" in the
description and drop the double-dash there.

> Similarly,
>   $ git grep ')'
>   fatal: incomplete pattern expression: )
> is somehow worse. ")" is a complete regular expression pattern.
> Of course, the error wants to say "group" here.

Nice problem description so far.

> These now return

Phrase it more like "Make them return" ...

>   $ ./git grep '('
>   fatal: unmatched ( for expression group
>   $ ./git grep ')'
>   fatal: incomplete pattern expression group: )

> which hopefully are clearer in indicating that it's not the expression
> that's wrong (since no pattern had been parsed at all), but rather that
> it's been misconstrued as a grouping operator.

Nicely done.

> Link: https://bugs.debian.org/1051205
> Signed-off-by: Ahelenia Ziemiańska <[email-masked]>
> ---
>  grep.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/grep.c b/grep.c
> index 5f23d1a..ac34bfe 100644
> --- a/grep.c
> +++ b/grep.c
> @@ -621,7 +621,7 @@ static struct grep_expr *compile_pattern_atom(struct grep_pat **list)
>  		*list = p->next;
>  		x = compile_pattern_or(list);
>  		if (!*list || (*list)->token != GREP_CLOSE_PAREN)
> -			die("unmatched parenthesis");
> +			die("unmatched ( for expression group");
>  		*list = (*list)->next;
>  		return x;
>  	default:
> @@ -792,7 +792,7 @@ void compile_grep_patterns(struct grep_opt *opt)
>  	if (p)
>  		opt->pattern_expression = compile_pattern_expr(&p);
>  	if (p)
> -		die("incomplete pattern expression: %s", p->pattern);
> +		die("incomplete pattern expression group: %s", p->pattern);
>  
>  	if (opt->no_body_match && opt->pattern_expression)
>  		opt->pattern_expression = grep_not_expr(opt->pattern_expression);

Thanks.  The changes to these two messages look good.