What problem is solved by chaining? All I see here is cost for no benefit.
On Nov 16, 2011 5:02 PM, "Brion Vibber" <brion(a)pobox.com> wrote:
On Wed, Nov 16, 2011 at 3:15 AM, Owen Davis
<owen(a)wikia-inc.com> wrote:
John Du Hart <compwhizii <at>
gmail.com> writes:
It's just another style I've encountered
on other projects and I
personally like.
The syntax itself is fine, but at Wikia we have found (after a recent
post
mortem) that out of 23 "Fatal Error"
code defects found in production,
7 of them were due to method calls on null object references. If any
method in that chain returns null then the request fails. It most cases
core MediaWiki objects do return a valid stub object of some kind, but
not all of them do (and in some cases they return null. intermittently
so the code works "most of the time", which is in many ways the worse
scenario). Introducing a pattern like this in a code base this large is
therefore problematic. The "clean looking code" benefit is perhaps
outweighed by the fact that you need to add extra "if" conditions or
try/catch blocks everywhere to handle local null object exceptions.
In our case, we are trying to ensure during code reviews that we just
check for null objects in all "if" conditions, which does also lead to
more
highly nested and less readable code.
Any individual interface should generally have one of three patterns:
A) cannot fail, ever -- always returns a live object
B) always returns a live object -OR- throws an exception
C) returns a live object -OR- null/false as error
The most annoying problems seem to come from treating C code as if it's A:
everything works fine until you come across some invalid input, and then
somewhere else in your code you die, utterly surprised.
The thing I like about B) is that failing to handle the error case gives
more immediate feedback: the error output & backtrace tell you where the
*thing that didn't find its data* happened, instead of where some other
code tried to use that previously-fetched data.
My ideal unicorn-magic world tends to fall mostly in the A-B spectrum
(though exceptions do not always make sense in every case.).
Throwing chaining onto things like OutputPage could be done but seem
unlikely to be worthwhile; the operations are generally independent of each
other.
Chaining is good for things that work conceptually like a filter chain. For
instance in jQuery code we'll often do something like:
$(element).find('a')
.click(magicClickHandler)
.attr('title', 'Click for magic!');
The real magic actually came from the progress from $(element) to
.find('a') and then to performing operations on the resulting collection.
There's a much smaller difference when there's a single variable as a
common prefix:
$foo
.click(magicClickHandler)
.attr('title', 'Click for magic!');
vs
$foo.click(magicClickHandler)
$foo.attr('title', 'Click for magic!');
is only slightly "cleaner".
Everything is a tradeoff, but IMHO you do need to
check for null in
chains
like that, which means you can't really get
away with it (at least not
for
long).
jQuery's chaining API is explicitly built to make sure that you don't --
it's style A in the layout above. Each step in the chain returns a
collection, even if it's an empty collection... acting on an empty
collection may not do anything, but it doesn't cause a fatal error!
Any chaining APIs we do add should similarly be built explicitly this way,
such as the Message class which we have both PHP and JavaScript versions
of.
-- brion
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