What's the cleanest way to define a constant string that involves a variable in Ruby?

Question

For some context, a lot of my code has the same lines of text throughout it (we are using Calabash to do iOS automation, if that gives you an idea).

For example: "all label marked:'#{name}'" is used 8 times in a particular class.

I would prefer to be able to have a constant that uses that text, but if I throw it at the top of the class, of course the variable "name" has not been set yet. Without defining a method that takes a parameter and returns a string, is there a way to do something essentially like this that can exist at the top of the class, but not be evaluated until it's used?:

class ClassName
  extend Calabash::Cucumber::Operations

  @NAME_CONSTANT = "all label marked:'#{name}'"

  def self.method_name(name)
    query("#{@NAME_CONSTANT} sibling label marked:'anotherLabel' isHidden:0")
  end
end

If you use the syntax I mentioned, you get this error: undefined local variable or method `name' for ClassName

Answer 1

You could use String#% to insert the string later.

class ClassName
    @NAME_CONSTANT = "all label marked:'%{name}'"

    def self.method_name(insert_name)
        query("#{@NAME_CONSTANT} sibling label marked:'anotherLabel' isHidden:0" % {name: insert_name})
    end

    def self.query(string)
        puts string
    end 
end 

ClassName.method_name('test')
#=> "all label marked:'test' sibling label marked:'anotherLabel' isHidden:0"

Answer 2

I agree with @Sergio. Don't define a constant string that includes a variable. Just use a method. Including a variable in a constant seems like a bad idea. Constants shouldn't be dynamic, by definition.

If you really want to include a variable in a constant string, you can assign a lambda to a contstant, like so:

class ClassName
  extend Calabash::Cucumber::Operations

  NAME_CONSTANT = ->(name) { "all label marked:'#{name}'" }

  def self.method_name(name)
    query("#{NAME_CONSTANT.call(name)} sibling label marked:'anotherLabel' isHidden:0")
  end
end

I removed the @ before the constant, since including it creates a class-level instance variable, not a constant.

I really wouldn't use the code sample I posted, though. Just use a method. Avdi Grimm has a good post called "Do we need constants?" where he describes some of the benefits of using methods instead of constants.