max pleaner max pleaner - 1 month ago 8
Ruby Question

catch-all getter method on openstruct?

If I have an OpenStruct:

require 'ostruct'
open_struct = OpenStruct.new


I can overwrite
[]
which works in some cases

open_struct.define_singleton_method(:[]) do |*args|
puts args.map(&:class)
puts args
end

open_struct.a = 1
open_struct[:a]
# => Symbol
# a


But this
[]
method is not called when using the dot-method syntax:

open_struct.a
# => 1


I am trying to make a class which inherits from OpenStruct and works more like a Javascript object (basically I'm trying to remove the necessity to run
call
on a proc that's stored as a value)

Answer

First of all - OpenStruct already functions very much like JavaScript (given that #[] is a synonym of #call):

JS:

foo = {}
foo.bar = function() { console.log("Hello, world!"); };
foo.bar();
// => Hello, world!

Ruby:

foo = OpenStruct.new
foo.bar = proc { puts "Hello, world!" }
foo.bar[]
# => Hello, world!

If you mean function more like Ruby... you can override new_ostruct_member:

require 'ostruct'

class AutoCallableOpenStruct < OpenStruct
  protected def new_ostruct_member(name)
    name = name.to_sym
    unless respond_to?(name)
      define_singleton_method(name) {
        val = @table[name]
        if Proc === val && val.arity == 0
          val.call
        else
          val
        end
      }
      define_singleton_method("#{name}=") { |x| modifiable[name] = x }
    end
    name
  end
end

a = AutoCallableOpenStruct.new
a.name = "max"
a.helloworld = proc { puts "Hello, world!" }
a.hello = proc { |name| puts "Hello, #{name}!" }

a.name              # non-Proc, retrieve
# => max
a.helloworld        # nullary proc, autocall
# => Hello, world!
a.hello[a.name]     # non-nullary Proc, retrieve (#[] invokes)
# => Hello, max!

Just be aware that OpenStruct in Ruby slows down your program, and shouldn't be used if you can avoid it.

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