linux-kernel - Re: [PATCH] Drop 80-character limit in checkpatch.pl

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Message-ID: <Pine.LNX.4.64.0912171725370.21142@hs20-bc2-1.build.redhat.com>
Date:	Thu, 17 Dec 2009 17:37:57 -0500 (EST)
From:	Mikulas Patocka <mpatocka@...hat.com>
To:	Paul Mundt <lethal@...ux-sh.org>
cc:	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Alasdair G Kergon <agk@...hat.com>, dm-devel@...hat.com
Subject: Re: [PATCH] Drop 80-character limit in checkpatch.pl



On Thu, 17 Dec 2009, Paul Mundt wrote:

> On Tue, Dec 15, 2009 at 04:57:49PM -0500, Mikulas Patocka wrote:
> > (5) Wrapping makes long expressions harder to understand
> > --------------------------------------------------------
> > 
> > If I have a complex expression, I do not try to wrap it at predefined
> > 80-column boundaries, but at logical boundaries within the expression to make
> > it more readable (the brain can't find matching parentheses fast, so we can
> > help it by aligning the code according to topmost terms in the expression).
> > 
> > Example:
> > 				if (unlikely(call_some_function(s, value) != RET
> > _SUCCESS) ||
> > 				    (var_1 == prev_var_1 && var_2 == prev_var_2)
> >  ||
> > 				    flags & (FLAG_1 | FLAG_2) ||
> > 				    some_other_condition) {
> > 				}
> > 
> > Now, if we impose 80-column limit, we get this. One may argue that is looks
> > aesthetically better, but it is also less intelligible than the previous
> > version:
> > 				if (unlikely(call_some_function(s, value) !=
> > 				    RET_SUCCESS) || (var_1 == prev_var_1 &&
> > 				    var_2 == prev_var_2) || flags & (FLAG_1 |
> > 				    FLAG_2) || some_other_condition) {
> > 				}
> > 
> For starters, this is just crap. If you're writing code like this, then
> line wrapping is really the least of your concerns. Take your function
> return value and assign it to a variable before testing it in unlikely()
> as per existing conventions and most of this goes away in this example.

I wouldn't say that this is better:
				int csf_failed, vars_equal, flags_12;

				...

				csf_failed = call_some_function(s, value) != RET_SUCCESS;
				vars_equal = var_1 == prev_var_1 && var_2 == prev_var_2;
				flags_12 = flags & (FLAG_1 | FLAG_2);
				if (unlikely(csf_failed) || vars_equal ||
				    flags_12 || some_other_conditions) {
				}

If you think that it is better, it's OK, just write your code like that. 
And don't force it to everyone.

> If you're testing an absurd amount of conditions in a single block with
> needlessly verbose variable names, then yes, you will go over 80
> characters. Consequently, your "clean" example doesn't look much better
> than your purposely obfuscated one.

For me, breaking the expression into variables is worse because:
- adding/removing conditions must be done at 3 places (vs. my original 
example, where it would be done only on 1 place)
- when reading the conditions, your eyes must skip to two places (vs. my 
original example, where you only read it at one place)

But everyone has different brain, so it may be that for you, the extra 
variables are really more understandable. So write your code like that and 
don't preach.

Mikulas
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