not a very informative error message

[Gregory Szorc]

On Sun, Aug 9, 2015 at 1:25 AM, Yuya Nishihara <yuya at tcha.org> wrote:

> On Sat, 08 Aug 2015 22:58:23 -0400, Matt Harbison wrote:
> > On Sat, 08 Aug 2015 22:13:04 -0400, Cameron Simpson <cs at zip.com.au>
> wrote:
> > > On 08Aug2015 21:11, Matt Harbison <mharbison72 at gmail.com> wrote:
> > >> On Fri, 07 Aug 2015 07:19:38 -0400, Neal Becker <ndbecker2 at gmail.com>
> > >> wrote:
> > >>
> > >>> $ hg id
> > >>> 1f8f96084e75+ (collision) tip
> > >>>
> > >>> $ hg cat -r 1f8f96084e75+ test_awgn.py > /dev/null
> > >>              01234567890123
> > >>
> > >>> hg: parse error at 13: not a prefix: end
>
> > I pretty much agree that it isn't helpful, but it looks like the same
> > parser code is used for revsets, filesets and templates.  Therefore it
> > really can't know about common mistakes specific to one of these in order
> > to make suggestions.  It's just parsing based on language tokens it has
> > been told about for which ever module is using it, and bails when it gets
> > confused.  I'm not sure if this part can be made better.
> >
> > That said, it would be nice if it would say that the revision arg is bad.
> > Debugging a command that used a revision, files and a template would be a
> > headache.  I wonder if we can catch the parse error and prefix the
> message
> > with 'revision:' or whatever.  Bad commands are caught and offer a
> > suggestion if you misspell cat, for example.  I'm not sure what would
> > happen if multiple --rev args are given, but we would be no worse off I
> > guess.
>
> Maybe a ParseError could have more verbose output:
>
>   % hg log -r 1f8f96084e75+
>   hg: parse error at 13: not a prefix: end
>   1f8f96084e75+
>               ~^


+1. I tried to implement this a while back, thinking it would be an easy
feature and gave up because it required some invasive changes for the
raised exceptions to capture context necessary for reporting. Maybe I
wasn't approaching the problem correctly. I'm sure Yuya would know better
than me.
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