From: Ruby-Lang@...
Date: 2016-02-21T12:01:02+00:00
Subject: [ruby-core:73909] [Ruby trunk Feature#11361] proposal for easy method to nil-guard for generated variable name.

Issue #11361 has been updated by J��rg W Mittag.


masaki yamada wrote:
> It's easy to 'nil-guard' for normal variable.

> ~~~ruby
> def user
>   @user ||= User.find(1)
> end
> ~~~
> but it's not simple for generated variable name.

> ~~~ruby
> def user(id)
>   variable_name = "@user_#{id}"
>   instance_variable_set(variable_name, User.find(id)) unless instance_variable_defined?(variable_name)
>   instance_variable_get(variable_name)
> end
> ~~~

Note that the two examples you gave are *not* equivalent: the first assigns to the variable if it evaluates to `false` or `nil` regardless of whether it is defined or not, the second only assigns if it is undefined. Which one of the two do you want?

----------------------------------------
Feature #11361: proposal for easy method to nil-guard for generated variable name.
https://bugs.ruby-lang.org/issues/11361#change-57068

* Author: masaki yamada
* Status: Open
* Priority: Normal
* Assignee: 
----------------------------------------
It's easy to 'nil-guard' for normal variable.

~~~
def user
  @user ||= User.find(1)
end

~~~


but it's not simple for generated variable name.

~~~
def user(id)
  variable_name = "@user_#{id}"
  instance_variable_set(variable_name, User.find(id)) unless instance_variable_defined?(variable_name)
  instance_variable_get(variable_name)
end
~~~


I want to write it like this.

~~~
def user(id)
  instance_variable_get_or_set("@user_#{id}") { User.find(1) }
end
~~~


it can be implemented by this code.

~~~
def instance_variable_get_or_set(variable_name, &block)
  instance_variable_set(variable_name, block.call) unless instance_variable_defined?(variable_name)
  instance_variable_get(variable_name)
end
~~~



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