Marnen Laibow-Koser wrote:
>>
>> When you are confused about something, it is sometimes hard to distill
>> all your thoughts down to the essence of the issue and ask the right
>> question. But I am trying to define a CONSTANT using the result of an
>> instance method
>
> No you're not. The constant you're trying to define is clearly a class
> property.
>
> If you think about it, this is as it should be. The constant is going
> to have the same value for every instance of the class. In other words:
>
> a = PaymentType.new
> b = PaymentType.new
>
> a and b are now separate instances of PaymentType -- but surely you
> never want a.PAYMENT_TYPES to be different from b.PAYMENT_TYPES ?
>
The syntax of a.PAYMENT_TYPES won't work. I am not sure if you meant it
to work or you were just using it for the sake of explanation.
>> and Ruby just doesn't allow that and I don't understand
>> why.
>
> It does allow it, but a "constant" that's different for different
> instances is not really a constant and should probably not be
> represented as such. OTOH, if the constant is *not* different for
> different instances, then it should be a class property, not an instance
> property. Either way, you are asking for something that appears to have
> no utility whatsoever. Why bother?
>
Without trying to be arguementative, Ruby won't allow this.
class PaymentType
def payment_types
["Check", "Credit Card", "Purchase Order"]
end
PAYMENT_TYPES = payment_types
end
This will error: NoMethodError
But I get your point, which has enhanced my understanding. Things that
relate to the class in general, things that shouldn't be redefined, like
Math:PI are controlled only by class methods. So, in the above example,
if the payment_types were to change dynamically as the program executed,
I would not use a constant, I would setup getter and setter methods.
>>
>> Why does it have to be a class method? Is it because a constant is
>> "class-only". It can't be accessed by an instance. You could define an
>> instance method that would return the same information as the constant
>> but there is no syntax to access the constant directly from an instance
>> object.
>
> Sure there is: self.class::CONSTANT.
Again, this syntax won't work, if using self.payment_types
pt = PaymentType.new
pt.PaymentType::PAYMENT_TYPES
Error: NoMethodError: undefined method ‘PaymentType’
>
>>
>> So to define a constant, I can use a literal or a class method but not
>> an instance method.
>
> As I explained above, you can use an instance method, but there is no
> practical value to doing so.
See above.
>
>>
> And it can only be accessed via Class::Constant.
>>
>> Buzz
>
> Best,
> --
> Marnen Laibow-Koser
> http://www.marnen.org
> [email protected]
--
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