Re: [EXTERNAL] Re: Allow declaration after statement and reformat code to use it - Mailing list pgsql-hackers

Oops. I'm sorry. That previous email looked horrible in plaintext. Please
regard that as unsent. Same email, but second try:

> However, even if such an idea were to get the green light, I think I would
> take the obligatory regex jokes seriously, and instead use something like
> srcML [0] and do the analysis and modification on proper parse trees.

@Chapman I think that's a reasonable ask. This is the first time I heard of
srcML. It indeed looks like it would be a much better tool than a regex to do
this refactor. The refactor logic would still be roughly the same though
because the regex essentially extracts and modifies a parse tree too. Just one
that only has the bare minimum requirements for this specific refactor. I would
like to state, that we've used this regex to refactor both Citus and
pg_auto_failover, and no bugs turned up because of it so far. This is probably,
because a regex is such a horrible way of writing a parser in the first place.
So, any bugs in the regex will very likely create code that does not compile at
all, instead of code that can be compiled but has some obscure bug .

> I think effectively it would need to be run on all supported versions,
> which means the risk of introducing errors would have to be very low.

@Bruce @Tom @Chapman I agree that doing this refactor only on the current
master branch would cause way more pain due to backpatching, than the amount of
happiness it provides by having easier to reason about code. I like the idea of
avoiding this backpatching problem by running it . Luckily, running the
automatic refactor on all branches should be pretty easy. However, it seems
like PG11 and below only require a C89 compatible compiler. Am I correct in
assuming increasing that requirement to C99 is not really an option? If so, I
think it might be best if I revive this thread in ~27 months when PG11 is not
supported anymore.

> The rule against declaration-after-statement was kept
> after significant discussion, which you can find in the archives if you
> look. 

@Tom I guess my search skills in the archives are not very good, because I
cannot find any big discussion on about declaration-after-statement. The only
discussion I can find is this: 
https://www.postgresql.org/message-id/877efaol77.fsf@news-spur.riddles.org.uk 

If you're able to find the thread that you meant, could you please share it?

> C needs readability, not fewer lines.
> Aside from horrible code, it doesn't improve 0.1% on anything.

@Ranier I tried to explain in my initial email that it not only reduces lines,
but more importantly improves readability quite a bit IMHO:

> 2. Declarations are closer to the actual usage. This is advised by the
>    "Code Complete" book [2] and has the following advantages:
>    a. This limits variable scope to what is necessary. Which in turn makes
>       the mental model you have to keep of a function when reading the code
>       simpler.
>    b. In most cases it allows you to see the the type of a variable without
>       needing to go to the top of the function.
> 3. You can do input checking and assertions at the top of the function,
>    instead of having to put declarations in front of it. This makes it clear
>    which invariants hold for the function.  (as seen in the example above and
>    the changes for pg_file_rename[3])

If you disagree with those statements, could you explain why?