Question

In an Array subclass (just an array that does some coercing of input values) I’ve defined #concat to ensure values are coerced. Since nobody ever uses #concat and is more likely to use #+= I tried to alias #+= to #concat, but it never seems to get invoked. Any ideas?

Note that the coercing is actually always to objects of a particular superclass (which accepts input via the constructor), in case this code seems not to do what I describe. It’s part of an internal, private API.

  class CoercedArray < Array
    def initialize(type)
      super()
      @type = type
    end

    def push(object)
      object = @type.new(object) unless object.kind_of?(@type)
      super
    end

    def <<(object)
      push(object)
    end

    def concat(other)
      raise ArgumentError, "Cannot append #{other.class} to #{self.class}<#{@type}>" unless other.kind_of?(Array)
      super(other.inject(CoercedArray.new(@type)) { |ary, v| ary.push(v) })
    end

    alias :"+=" :concat
  end

#concat is working correctly, but #+= seems to be completely by-passed.

Answer 1

Since a += b is syntactic sugar for a = a + b I’d try to overwrite the + method.