Comments on: Syntactic sugar will occassionally kick your puppies.

By: draegtun

draegtun — Wed, 04 Mar 2009 13:01:46 +0000

Trailing if/unless statements come from Perl and these same problems are well known there when it comes to declaring variables ;-)

However this does allow declaration of state variables….

my $persist if 0;

This peculiarity can now be replaced in Perl 5.10 with the much nicer…

state $persist;

“Perl Best Practises” only recommends u use them with next, last & redo (and in fact not use “unless” at all!). I don’t go that far but I certainly make sure I never use variable declaration with them!

/I3az/

By: Chris Heald

Chris Heald — Tue, 03 Mar 2009 22:20:01 +0000

The more I think about it, the less convinced I am that a more idiomatic solution exists, because anything that does a left-hand assignment is going to initialize the variable to be assigned to before the right-hand side of the expression is evaluated.

The only way I could see this working would be if there were some kind of right-hand assignment operator in ruby, which I’m fairly certain there isn’t.

By: Rahim

Rahim — Tue, 03 Mar 2009 16:20:41 +0000

In fact

irb(main):002:0> bar = defined?(bar)
=> “local-variable”

Shows the behaviour more clearly. (MRI 1.8.7)

By: Rahim

Rahim — Tue, 03 Mar 2009 16:17:40 +0000

Interesting stuff, I’ve come across the same problem a few times and never been satisfied, we want a false (and/or nil) preserving equivalent of ||=

Surely there’s a cleaner or more idiomatic way of approaching this than using line separators?

Trashing around in IRB testing the behaviour I tried:

irb(main):002:0> foo = defined?(foo) || true
=> “local-variable”

which was another result I wasn’t expecting…

By: Chris Heald

Chris Heald — Mon, 02 Mar 2009 21:30:59 +0000

@Scott – yeah, I usually do. The problem here was that “false” is a valid initialization value for the variable I was interested in, and I didn’t want to clobber that if it was already set.

By: Scott

Scott — Mon, 02 Mar 2009 21:26:49 +0000

You can just use:
foobar ||= true
which will set foobar to true if it’s undefined, nil, or false, but otherwise leave it’s value the same.
It’s just the same as:
foobar = foobar || true

Still, it’s good to remember these tricky order of operations things.

By: Chris Heald

Chris Heald — Mon, 02 Mar 2009 21:26:09 +0000

Well, it’s a combination of the two, really. The real gotcha is that Ruby initializes variables as they’re parsed, not as the code is run – I wasn’t expecting that at all.

By: honzasterba

honzasterba — Mon, 02 Mar 2009 21:23:36 +0000

This not as much about the trailing condition but about the defined? method which itself is pretty tricky and should be avoided. I myself spent a sleepless night trying to check if something is defined and write the code in a way that would not accidentally define it before the actual defined? is evaluated.