From: "David A. Black" <dblack@...>
Date: 2009-11-01T02:03:58+09:00
Subject: Re: In-place parameter modification

Hi --

On Sun, 1 Nov 2009, Jordi Bunster wrote:

> On Oct 31, 2009, at 12:44 PM, Robert Gleeson wrote:
>
>> Yup, you can write your own mutating methods that mutate the
>> receiver(like chomp!, gsub!, sub!, and so on), but you can't mutate any
>> of the core classes because you cannot reassign self.
>
> Some core classes know how to replace themselves:
>
> class String
> def clobber!
>   replace ''
> end
> end
>
> I wonder why Object (or BasicObject) doesn't know this trick.

I don't think "replace" has any generalizable semantics. You can't
replace 2 with 3, for example (fortunately :-)

> I also wonder why "replace" isn't called "replace!" :)

The !/non-! methods come in pairs. The ! one is the "dangerous" one --
that is, the one that probably has side-effects, special-case
semantics, etc. A lot of the "danger" involves changing the receiver,
but not all of it (e.g., exit!).

Therefore, if it's just one method, not a pair, it simply has a name
that says what it does, without a !. String#replace has to be
destructive. Otherwise the name is absurd. (str.replace("abc") can be
better written as just "abc", if all it's doing is creating a
completely new string with the characters "abc".)

There's no one-to-one correspondence, in either direction, between
receiver-changing methods and the !. Array#pop and friends, the
various replace and clear methods, Hash#update -- there are lots of
them.

Here's some further reading on the !-convention (and its lamentable
near-disappearance):

http://dablog.rubypal.com/2007/8/15/bang-methods-or-danger-will-rubyist


David

-- 
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