Re: Why nil

On Mon, 3 Dec 2018 at 14:47, Ken D'Ambrosio <[email protected]> wrote:
>
> On 2018-07-04 08:06, dade wrote:
>
>> Hello, let's look at an example
>>
>> class A
>>   attr_accessor :a
>>
>>   def initialize(a)
>>     @a = a
>>   end
>>
>>   def method1(val)
>>     a = val
>>   end
>>
>>   def method2(val)
>>     a = a + val
>>   end
>> end
>>
>> a = A.new(1)
>> a.method1(2) #=> 3
>> a.method2(2) #=> undefined method `+' for nil:NilClass
>>
>> Why the second a is nil?
>
>
> NOTE: I am not a programmer; I just dabble.  But, unless I miss my mark, the "a" being used in the initialization isn't available elsewhere in the class.  So, when you say
> a = a + val
> you are, in essence, saying
> undefinedvariable = undefinedvariable + val

Two things:

1. there's no such thing as an undefined variable to the left of an
assignment.  `a =` explicitly defines the variable `a`

2. by the time the interpreter gets to the second `a` it *is* defined,
because of the `a =` that immediately precedes it.  However it's not
*initialised* yet.

> Which is a non-starter.
>
[snip]
>
> The fact that (if I had used attr_accessor) ":a" can be accessible as foo.a is simply a nicety of attr_accessor; it doesn't reflect what's going on in the class, itself.
>

Third thing:

3. the `attr_accessor :a` has defined `#.a` and `#.a=` methods,
however in this code neither of them are being called because Ruby
biases towards local variables; so `a =` will always define/assign to
a local variable, and any bare `a` (i.e. with no receiver nor
parentheses) that comes after it will always use that local variable.

These would work as (presumably?) intended:

~~~ruby
  def method1(val)
    self.a = val
  end

  def method2(val)
    self.a = self.a + val
    # OR:
    #self.a = a() + val
  end
~~~

>
> I'm sure, if I'm mistaken, someone will show me the error of my ways,
>
> -Ken
>
>> dade.

Not mistaken, just glossing some of the finesse.

Cheers
-- 
  Matthew Kerwin
  https://matthew.kerwin.net.au/

Unsubscribe: <mailto:[email protected]?subject=unsubscribe>
<http://lists.ruby-lang.org/cgi-bin/mailman/options/ruby-talk>