not a very informative error message

[Cameron Simpson]

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
>
>Since this isn't a valid revision or revset, all it can tell you is 
>that the problem occurred at the 13th character.
>
>I'm not sure what you are trying to do here.  The '+' on the end of 
>the revision simply means that the working directory is dirty- it 
>isn't part of the revision value.

Well, as an outsider, I read Neal's email to indicate that here is a case where 
hg's error message in unhelpful to the user.

Specificly, I'd read it to indicate that:

(a) the error message, in this context, sucks. "position 13" in what? "not a 
prefix"? Oh, not a prefix of a revision hash. "end"? Huh?

(b) it is not an unnatural mistake to grab the whole "1f8f96084e75+" word from 
the output of "hg id". It might be good to recognise this explicitly in hg's 
revcode parser and suggest to the user that that is what they did.

Regarding (a), when I write error messages and when I read them, this:

  not a prefix: end

normally suggests that the literal text "end" is either the whole value being 
rejected, or the tail of the rejected value (i.e. the parse failed when it 
reached the string "end" in some larger string).

One, IMO genuine, problem with the error is that it does not even make it clear 
what hg is unhappy about. The "-r" option? The filename? The spelling of "cat"?  
Some context is very needed in this message.

Cheers,
Cameron Simpson <cs at zip.com.au>

Uhlmann's Razor: When stupidity is a sufficient explanation, there is no need
                 to have recourse to any other.
                        - Michael M. Uhlmann, assistant attorney general
                          for legislation in the Ford Administration