Redundancy of notation in instance_variable_set, instance_variable_get

Question

When I set or get instance variables using some name, for example @foo, I can do something like:

instance_variable_set("@foo", some_value)
...
instance_variable_get("@foo")

But often, I use a variable for the method name, which does not include the @ prefix, so that I end up doing:

method = :foo
...
instance_variable_set("@#{method}", some_value)
...
instance_variable_get("@#{method}")

But since all instance variables are prefixed with @, I think it redundant to have to type "@#{method}" instead of simply typing method. Why are the methods instance_variable_set and instance_variable_get not designed to accept string/symbol without @ as its first argument like this:

method = :foo
...
instance_variable_set(method, some_value)
...
instance_variable_get(method)

where the variable to be actually set will be @foo rather than foo?

Is there any advantage with the way it is?

:"hello?" is a valid symbol - is it a valid variable name? — Andrew Grimm
– Andrew Grimm, Commented Oct 9, 2011 at 22:37
@Grimm. It is not. My question was, why is it not designed to automatically prefix the string/symbol with "@". I added examples to make my question clearer. — sawa
– sawa, Commented Oct 9, 2011 at 22:45
If there are some valid symbols that would be converted into invalid instance variable names, could that be a reason? — Andrew Grimm
– Andrew Grimm, Commented Oct 9, 2011 at 23:30
@Grimm I thought about that, but if it would always be used without @, then there will be no problem. I was wondering why it could not be like attr_(accessor|reader|writer), where you don't put @. — sawa
– sawa, Commented Oct 10, 2011 at 0:09

molf · Accepted Answer · 2011-10-10 23:31:17Z

3

The reason is, quite simply, that the instance variable is named @foo, not foo. The @ is part of the variable name, just as the $ is part of the global variable name $foo.

The reason that @ is not necessary when calling attr_accessor and friends is because they define attribute methods, so it makes sense to provide the method names, not the variable names.

Of course there is no technical reason instance_variable_set cannot prepend the @ itself. However, the method accepts a symbol that corresponds to the variable name. A symbol by definition represents the identifier with the given name. So the only symbol that corresponds to the instance variable @foo is :@foo. That is why you have to include the @, because we know that :foo does not correspond to any instance variable identifier at all. (And if you supply a string, it will be converted to a symbol internally first.)

Update: In the C Ruby implementation (MRI), there is actually no mention of @ anywhere in the code that handles instance variables. Only the parser knows instance variables start with a @. So it seems that separating code parsing from implementation is another possible reason.

edited Oct 10, 2011 at 23:31

answered Oct 10, 2011 at 19:54

molf

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3 Comments

sawa Over a year ago

Maybe this boils down to difference in philosophy.

molf Over a year ago

I think it's more about semantics. Hopefully my updated answer makes it more clear (and also introduces another possible reason).

sawa Over a year ago

I am not convinced with your original answer, but the updated part convinces me. Thanks.

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