From: Robert Klemme <bob.news@...>
Date: 2005-02-14T01:39:57+09:00
Subject: Re: evaluation order


"David A. Black" <dblack@wobblini.net> schrieb im Newsbeitrag 
news:Pine.LNX.4.61.0502130707250.22113@wobblini...
> Hi --
>
> On Sun, 13 Feb 2005, Martin DeMello wrote:
>
>> David A. Black <dblack@wobblini.net> wrote:
>>
>>> (For clarity I've deleted the warning about using '=' in conditional.
>>> [Matz, can't we get rid of that warning?])
>>
>> I'd love per-warning suppressibility, but if not, I think this particular
>> one does more good than harm. (But then, I find the if (a = f(x)) idiom
>> rather ugly to begin with).
>
> I'm trying to figure out the pattern behind when it gets triggered or
> not:
>
>   irb(main):002:0> while (x = 1); end
>   (irb):2: warning: found = in conditional, should be ==
>   irb(main):003:0> while (line = gets); end
>   abc
>   ^D
>   => nil
>   irb(main):004:0> if (m = /a/.match("b")); end
>   => nil
>   irb(main):005:0> if (m = /a/); end
>   (irb):5: warning: found = in conditional, should be ==
>   irb(main):006:0> if (m = "abc".replace("abc")); end
>   => nil
>   irb(main):007:0> if (m = "abc"); end
>   (irb):7: warning: found = in conditional, should be ==
>   => nil
>
> When the rhs is a literal, I guess?  That at least vastly increases
> the chances of its being a case of a typo.  Could it be that Matz is
> several steps ahead of me? :-)

:-))

Note also, that assignment in a conditional really only makes sense for 
loops.  With "if" and "unless" it's totally superfluous since you can always 
do

a=x
if a then...

instead of

if a=x then...

And it's also more readable.  So I would not mind if there was *always* a 
warning for assignment if it's "if" or "unless".  Just my 0.02 EUR though...

Whereas there is no easy equivalent replacement for

while ( line = gets ) ...

Typically, you would need two assignments which is somewhat ugly:

line = gets
while line
....
  line = gets
end

Kind regards

    robert