Re: [GSOC][PATCH] userdiff: add support for Scheme

[Phillip Wood <phillip.wood123@gmail.com>]

Hi Atharva

On 28/03/2021 12:51, Atharva Raykar wrote:
> On 28-Mar-2021, at 04:20, Junio C Hamano <gitster@pobox.com> wrote:
>>
>> Atharva Raykar <raykar.ath@gmail.com> writes:
>>
>>> +         /*
>>> +          * Scheme allows symbol names to have any character,
>>> +          * as long as it is not a form of a parenthesis.
>>> +          * The spaces must be escaped.
>>> +          */
>>> +         "(\\.|[^][)(\\}\\{ ])+"),
>>
>> One or more "dot or anything other than SP or parentheses"?  But
>> a dot "." is neither a space or any {bra-ce} letter, so would the
>> above be equivalent to
>>
>> 	"[^][()\\{\\} \t]+"
>>
>> I wonder...
> 
> A backslash is allowed in scheme identifiers, and I erroneously thought that
> the first part handles the case for identifiers such as `component\new` or
> `\"id-with-quotes\"`. (I tested it with a regex engine that behaves differently
> than the one git is using, my bad.)
> 
>> I am also trying to figure out what you wanted to achieve by
>> mentioning "The spaces must be escaped.".  Did you mean something
>> like (string->symbol "a symbol with SP in it") is a symbol?  Even
>> so, I cannot quite guess the significance of that fact wrt the
>> regexp you added here?
> 
> I initially tried using identifiers like `space\ separated` and they
> seemed to work in my REPL, but turns out space separated identifiers in
> scheme do not work with backslashes, and it was working because of the way
> my terminal handled escaping. Space separated identifiers are declared like
> `|space separated|` and this too only seems to work with Racket, not
> the other Scheme implementations.

I think the bar notation works with some other such as gambit and 
possibly guile (it's a while since I used the latter)

Best wishes

Phillip

  So I stand corrected here, and it's better
> to drop this feature altogether.
> 
> But somehow, the regexp you suggested, ie:
> 
> 	"[^][()\\{\\} \t]+"
> 
> does not handle the case of make\foo -> make\bar (it will only diff on foo).
> I am not too sure why it treats backslashes as delimiters.
> 
> This seems to actually do what I was going for:
> 
> 	"(\\\\|[^][)(\\}\\{ ])+"
> 
>> As we are trying to catch program identifiers (symbols in scheme)
>> and numeric literals, treating any group of non-whitespace letters
>> that is delimited by one or more whitespaces as a "word" would be a
>> good first-order approximation, but in addition, as can be seen in
>> an example like (a(b(c))), parentheses can also serve as such "word
>> delimiters" in addition to whitespaces.  So the regexp given above
>> makes sense to me from that angle, especially if you do not limit
>> the whitespace to only SP, but include HT (\t) as well.  But was
>> that how you came up with the regexp?
> 
> Yes, this is exactly what I was trying to express. All words should be
> delimited by either whitespace or a parenthesis, and all other special
> characters should be accepted as part of the word.
>