From: dblack@...
Date: 2007-07-22T18:44:55+09:00
Subject: Re: Is there a replacement for sub?

Hi --

On Sun, 22 Jul 2007, Michael W. Ryder wrote:

> dblack@wobblini.net wrote:
>> Hi --
>> 
>> On Sat, 21 Jul 2007, Michael W. Ryder wrote:
>> 
>>> Sebastian Hungerecker wrote:
>>>> Michael W. Ryder wrote:
>>>>> Martin DeMello wrote:
>>>> 
>>>>>> a = "a b c d e f"
>>>>>> b = a
>>>>>> a = a.sub(" ", "") # => a is now "ab c d e f", b is still "a b c d e f"
>>>>> Wrong. 
>>>> 
>>>> Nope.
>>>> 
>>>>> Unless you use b = a.dup b will be the same as a as it is pointing to 
>>>>> the same object. 
>>>> 
>>>> Yes.
>>>> 
>>>>> The result of the sub operation will change b as well as a.
>>>> 
>>>> No, sub won't change the actual string - that's what sub! would do. It 
>>>> will
>>>> create a new string which will be assigned to a, at which point a and b 
>>>> won't
>>>> be pointing at the same object anymore.
>>>> 
>>>> 
>>> My mistake.  Too many tests with and without the ! operator.  I guess that 
>>> my test used sub! instead of sub.  This just proves my point that the 
>>> whole thing is too confusing.
>> 
>> Keep in mind that the ! means that the method is the "dangerous"
>> version.  In this case, the danger consists of both the changing of
>> the object in place, and the returning of nil when there's no change.
>> Any time you see a ! in a method name you've been warned that there's
>> probably something you need to be particular aware of and careful
>> about.
>> 
>> 
> Unfortunately, my background has always been that a function never returns 
> nil if nothing is done.

Think about the implications of that, though: what you're saying, in
effect, is that Ruby should adapt itself in great detail to everyone's
background -- which really means that Ruby should do everything the
same way *all* other languages (since someone has a background in
every language) do it.  Of course, there's no such thing :-)

It's not that Ruby is above criticism, of course; but it's best to
take it on its own terms as much as possible, and you'll find that
things are very well planned and thought through.  The ! isn't
supposed to be a trap or a surprise; on the contrary, it's an explicit
indicator of exactly what you're concerned about -- namely, the fact
that the behavior of this method may do "dangerous" things that the
the non-! version doesn't do.

Have a look at: http://www.infoq.com/articles/coming-from-ruby for
more encouragement along these lines :-)

> C has the ++ operator which can be very dangerous but it never
> returns nil.

It also doesn't tell you, right in its name, that you should be
cautious about danger.

> It may be safer to use a = a + 1 or even better b = a + 1, but I
> don't have to worry that a++ will return 0 unless a was -1 before. I
> realize that the ! operator is a shortcut, but  if it is going to be
> a shortcut that is all it should be.

It's not a shortcut; as Robert K. said, it's just the last character
in the method name.  It has no language-level significance, but the
contract in place is that it means "dangerous".


David

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