Initialize an object using a block instead of constructor arguments

Question

I have a class with a constructor that takes two parameters and sets the fields to those values. I want to also be able to initialize objects from this class by setting fields explicitly inside of a block passed to the constructor.

The constructor:

class Grammar
    attr_reader :rules, :start, ...
    def initialize(rules, start)
        @rules = rules
        @start = start
        ...
    end
end

The parameters passed to the constructor involve creating several objects that are only used as intermediate building blocks for the parameters rules and start and it would make sense to limit the existence of these objects to the block passed to the constructor. In this block I would expect to first build the intermediate objects and then directly set the fields of the new object.

I want to be able to instantiate Grammar like so:

grammar = Grammar.new { |g|
    ...
    # build rules and start
    ...
    g.start = start
    g.rules = rules
}

How should I modify the constructor to allow both methods of initialisation?

tadman · Accepted Answer · 2019-05-03 22:17:32Z

5

It's actually really easy to add this functionality to most classes provided they already have attr_writer and/or attr_accessor in place:

class Grammar
  attr_accessor :rules, :start, ...

  def initialize(rules, start)
    @rules = rules
    @start = start
    ...

    yield self if block_given?
  end
end

Where you can now do exactly what you wanted. The yield self part will supply the object being initialized to the block, and block_given? is only true if you've supplied a block to the new call. You'll want to run this after setting all your defaults.

edited May 3, 2019 at 22:17

answered May 3, 2019 at 22:03

tadman

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3 Comments

max pleaner Over a year ago

I think they would need attr_writer or attr_accessor here.

Jan Parzydło Over a year ago

So do I have to change the access to these variables in order to be able to set them from within a block? I want to leave this data structure immutable.

tadman Over a year ago

You could do it with a short-term proxy object but that's a whole other solution.

Josh Brody · Accepted Answer · 2019-05-04 04:44:52Z

3

You might be looking for Object#Tap which is built-in to Ruby — yields self to the block, and then returns self.

class Person
  attr_accessor :name 
end

person = Person.new.tap do |p|
  p.name = "Jan"
end

puts person.name # hi

edited May 4, 2019 at 4:44

answered May 3, 2019 at 22:37

Josh Brody

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1 Comment

tadman Over a year ago

Clever, but fairly unconventional, so best reserved for cases where a first-class yield in the initializer is not an option, like using a third-party library.

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