Forum: Ruby attribute setters for instance variables

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Fc761ccaf6c0d7d977e2959f9bfebd06?d=identicon&s=25 unknown (Guest)
on 2006-03-11 18:19
(Received via mailing list)
Hello,

The Pickaxe book presents the following example:

class Incorrect
  attr_accessor :one, :two
  def initialize
    one = 1
    self.two = 2
  end
end

obj = Incorrect.new

p obj.one
p obj.two

The first thing printed is 'nil' and not '1' since "one = 1" assigns to
a local variable of method initialize instead of the instance variable.
So far it's clear.
Now, they propose a solution: to use self.one instead of just one.
Won't be using @one simpler and also correct ? What am I missing ?

Thanks
Ec594ddcaeae2771ee9d0dd7f0d598ed?d=identicon&s=25 Hans Fugal (Guest)
on 2006-03-11 18:26
(Received via mailing list)
eliben@gmail.com wrote:
> end
> Won't be using @one simpler and also correct ? What am I missing ?
>
> Thanks
>

@one = 1 is setting the instance variable itself, and that's fine for
accessors. If you want to do something special on =() then you will want
to use the self.one style. e.g.

def Foo
   def one=(n)
     @one = n+42
   end
   def initialize(n)
     self.one = 0 # @one == 42
     @one = 0     # @one == 0
   end
end
Fc761ccaf6c0d7d977e2959f9bfebd06?d=identicon&s=25 unknown (Guest)
on 2006-03-11 18:26
(Received via mailing list)
Hans Fugal wrote:
> >   end
> > Now, they propose a solution: to use self.one instead of just one.
>    def one=(n)
>      @one = n+42
>    end
>    def initialize(n)
>      self.one = 0 # @one == 42
>      @one = 0     # @one == 0
>    end
> end

Thanks for your swift and insightful reply.
Cb48ca5059faf7409a5ab3745a964696?d=identicon&s=25 unknown (Guest)
on 2006-03-11 21:12
(Received via mailing list)
On Sun, 12 Mar 2006 eliben@gmail.com wrote:

> end
> Won't be using @one simpler and also correct ? What am I missing ?
def one= arg
     log{ "somthing about one" }
     validate{ arg }
     notify_observers :one, arg
     @one = arg
   end

just a few examples.

this become even more true when the number attribues increases since you
can

   class C
     ATTRIBUTES = %w[ one two three four five six seven ]
     def initialize kws = {}
       kws.each{|kw, val| send "#{ kw }=", val}
     end
   end

in one line even if the number of attributes is huge.


i'd also argue that the simplest thing is to simply make the getter take
and
optional argument so one can do

   def initialize
     one 1
     two 2
   end

but that's a matter of taste.

hth - cheers.

-a
06db81f02537377c4bb207cdf8fd18ec?d=identicon&s=25 Mike Austin (Guest)
on 2006-03-12 02:26
(Received via mailing list)
Hans Fugal wrote:
>>   end
>> Now, they propose a solution: to use self.one instead of just one.
>   def one=(n)
>     @one = n+42
>   end
>   def initialize(n)
>     self.one = 0 # @one == 42
>     @one = 0     # @one == 0
>   end
> end

I really wish Ruby required explicit locals declaration.  There are many
advantages to this, including fix of the problem above, hard to debug
spelling
mistakes, clearer code, etc.  A simple 'var' declaration would be
simple:

def Foo
   def one=(n)
     @one = n+42
   end
   def initialize(n)
     var x, y = n * n, 1 / n
     one = 0      # @one == 42
     @one = 0     # @one == 0
     z = 10       # error
   end
end


Regards,
Mike
Ec594ddcaeae2771ee9d0dd7f0d598ed?d=identicon&s=25 Hans Fugal (Guest)
on 2006-03-12 05:10
(Received via mailing list)
Mike Austin wrote:
>>>     self.two = 2
>>> So far it's clear.
>> def Foo
> advantages to this, including fix of the problem above, hard to debug
> spelling mistakes, clearer code, etc.  A simple 'var' declaration would
> be simple:

What problem? I don't see any problem, I quite like that behavior. I
also like not declaring variables, the inconvenience from spelling
errors is minimal, especially when the telltale nil problems are almost
as useful as a compiler error most of the time.
06db81f02537377c4bb207cdf8fd18ec?d=identicon&s=25 Mike Austin (Guest)
on 2006-03-12 07:05
(Received via mailing list)
Hans Fugal wrote: [full message below]
 >> I really wish Ruby required explicit locals declaration.  There are
 >> many advantages to this, including fix of the problem above, hard to
 >> debug spelling mistakes, clearer code, etc.  A simple 'var'
 >> declaration would be simple:
 >
 > What problem? I don't see any problem, I quite like that behavior. I
 > also like not declaring variables, the inconvenience from spelling
 > errors is minimal, especially when the telltale nil problems are
almost
 > as useful as a compiler error most of the time.

I can't imagine coding thousands of lines of code without variable
declarations.  If you catch the nil errors caused by spelling mistakes
right
away, sure, you can easily track it down.  But I'd rather have the
compiler
tell me that a variable doesn't exist than to give me an nil error who
knows
where.  'var' is only 4 more letters, and I'm not saying limit them to
the
beginning of the method or block.  The only change would be prepending
'var'.

Mike
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