If statement and if modifier not equivalent?


#1

I was under the impression that the if modifier and the
if statememt were equivalent, but for the following code
calling A.test raises an exception, while calling A.test2
succeeds. This is with Ruby 1.8.4.

class A
def A.test
a if a=1
end

def A.test2
if a=1
a
end
end
end


#2

On Sunday 19 February 2006 12:08, Mark J. wrote:

I was under the impression that the if modifier and the
if statememt were equivalent, but for the following code
calling A.test raises an exception, while calling A.test2
succeeds. This is with Ruby 1.8.4.
That’s because of the way Ruby decides wether a local variable exists or
not. Basically, a local variable is defined the first time it is
assigned
to, so in the first case
def A.test
a if a=1
end
a=1, which defines the local, is seen after the ‘a’ statement. So, ‘a’
does not exist and boom

In the second case,

def A.test2
if a=1
a
end
end
a=1 is seen before the ‘a’ statement, so it works

Regards,
Sylvain


#3

Sylvain J. wrote:

That’s because of the way Ruby decides wether a local variable exists or
not. Basically, a local variable is defined the first time it is assigned
to, so in the first case

def A.test
a if a=1
end

a=1, which defines the local, is seen after the ‘a’ statement. So, ‘a’
does not exist and boom

OK, thanks Sylvain. But I would have thought that any examination of
the guarded statement would be delayed until after the condition had
been evaluated. Is a parse like this possible?


#4

DÅ?a Nedeľa 19 Február 2006 12:58 Mark J. napísal:

OK, thanks Sylvain. But I would have thought that any examination of
the guarded statement would be delayed until after the condition had
been evaluated. Is a parse like this possible?

Personally, I’d be surprised if the parser paid any attention to much of
the
semantics of the parsed text at all - that’s what the interpreter is
supposed
to do. The variable declaration checking at parse-time seems to be the
exception here.

Syntax ge… gurus, feel free to elaborate.

David V.


#5

Hi,

In message “Re: If statement and if modifier not equivalent?”
on Sun, 19 Feb 2006 21:39:04 +0900, David V.
removed_email_address@domain.invalid writes:

|DÅ?a Nedeľa 19 Február 2006 12:58 Mark J. napísal:
|> OK, thanks Sylvain. But I would have thought that any examination of
|> the guarded statement would be delayed until after the condition had
|> been evaluated. Is a parse like this possible?
|
|Personally, I’d be surprised if the parser paid any attention to much of the
|semantics of the parsed text at all - that’s what the interpreter is supposed
|to do. The variable declaration checking at parse-time seems to be the
|exception here.
|
|Syntax ge… gurus, feel free to elaborate.

Agreed with you. But there’s a vague possibility that this issue will
be solved in Ruby 2.0 in different manner.

						matz.

#6

On 2/19/06, Mark J. removed_email_address@domain.invalid wrote:

def A.test2
if a=1
a
end
end
end

Do you really want ‘if a = 1’ there, and not ‘if a == 1’?


#7

Which makes the assignment a side-effect of the conditional, and
depending on the context can be an excellent example of bad
side-effects, particularly from a software-engineering point of view,
where the code has to be maintained over any sort of time, by a team.

I think of it as the imperative corollary to duck-typing: if it looks
like a duck and sounds like a duck, it should be a duck.

There’s a great public example of where this technique can break down
from an attempted hack on the linux kernal a couple of years ago. It’s
worth noting that it was only caught because of 1) very tight, very
paranoid peer review, and 2) the secure nature of the project. It
would have gone through unnoticed virtually anywhere else. In my past
life doing telecoms programming, I can remember this exact pattern
being responsible for bringing down a good portion of the US East
Coast’s 1-800 (and 900, etc) phone numbers.

To each their own, and if it’s code for your own use it’s unlikely to
matter, but assignment-as-conditional really, really smells bad to me.

http://kerneltrap.org/node/1584

  •   if ((options == (__WCLONE|__WALL)) && (current->uid = 0))
    
  •                   retval = -EINVAL;
    

Associated commentary:

Setting current->uid to zero when options __WCLONE and __WALL are set? The
retval is dead code because of the next line, but it looks like an attempt
to backdoor the kernel, does it not?


#8

On 2/19/06, Garance A Drosehn removed_email_address@domain.invalid wrote:

Do you really want ‘if a = 1’ there, and not ‘if a == 1’?

yes. I imagine so.

in a more complicated example

if ( a = get_something_that_might_be_nil_or_false )
puts a
end


#9

DÅ?a Pondelok 20 Február 2006 14:53 removed_email_address@domain.invalid napísal:

It sure does. Note “current->uid = 0”, not “current->uid == 0”.
Good eyes, I missed that. This function is sys_wait4() so by passing in
__WCLONE|__WALL you are root. How nice.

Ow.

David V.