Sweet instance var assignment in the object initializer
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I'm very excited about this feature in CoffeeScript, and think it might be a nice-to-have thing in Ruby 2.0.
That's how I think it would look like:
class Me def initialize(@name, @age, @location); end end
So we can declare
@variables in the initializer method parameters definition to avoid assigning instance variables from method arguments by hand, like:
class Me def initialize(name, age, location) @name = name @age = age @location = location end end
Want to hear what do you guys think, does that feature worth being included in 2.0?
#9 Updated by Robert A. Heiler over 2 years ago
Is this even possible with the Parser?
It would eliminate a few lines of code.
Also, I am not sure if this violates the principle of least matz surprise.
If I initially see
def initialize(@name, @age, @location)
I wonder a bit, because it does not feel consistent.
Perhaps it would be different if some kind of
attr* could be
attr_initialize :name, :age, :location
And then the above could work. Where the name would work just
similar to an
attr_writer, but different in that it assumes
default values passed to initialize to automatically go towards
those instance variables (the order must be the same of course)
#12 Updated by Thomas Sawyer about 2 years ago
I think it's really not such a good idea. Often you'll just end up having to redo it anyway when you finally decide to coerce and/or validate arguments to make your code more robust, e,g,
def initialize(@name, @age, @location); end
def initialize(name, age, location) @name = name.to_s @age = age.to_i @location = Location.new(location) end
Might as well write it out from the get-go in preparation.
#14 Updated by Boris Stitnicky over 1 year ago
Well... I like the sweetness... But to have such a feature working syntactically from
inside of #
initialize method, but not from other methods... I don't know.
It's not like this is my suggestion, but since it is up for discussion, let me try to
straighten this proposal according to my own thinking:
These "attributes on steroids" are a thing to be done not at the level of the #
method, but at the level of the module, along with #
attr_accessor and friends.
Imho, it would be necessary to update #
attr_accessor & friends to accept :autoinit
attr_reader :name, :age, autoinit: true
Now there are two flavors of this candy, one with ordered arguments, one with named.
So we could have to specify it:
attr_reader :name, autoinit: :ordered attr_reader :age, autoinit: :named
I'm sure you know what I mean here. Default option could be eg. :named.
autoinit method, or rather, #
would have to be added to the Module, for those times, when we want to
autoinit, but don't want the reader/writer/accessor:
autoinit_named :age autoinit_ordered :name
Afaik, current #
attr_accessor & friends work by defining instance methods on
the module. How #
autoinit should work, is a question. Two possibilities come
to my mind:
By patching #
By creating and including a mixin patching #
newclass method, that would
set the appropriate instance variables right after creating a new instance,
in effect something like this:
module AgeNamedArgAutoinit def new *args, &block named_args = args.extract_options! age = named_args.delete :age modified_args = args + named_args.empty? ?  : [named_args] new_instance = super *modified_args, &block new_instance.instance_variable_set :@age, age end end module NameOrderedArgAutoinit def new *args, &block name = args.shift new_instance = super *args, &block new_instance.instance_variable_set :@name, name end end class MyClass include AgeNamedArgAutoinit include NameOrderedArgAutoinit end
MyClass.new( "John Smith", :whatever, age: 35, other_stuff: :whatever ) should
behave in the expected way.
Again, I have not come up with this proposal, I do not give +1 or -1 to it,
I am only trying to iron it to be more consistent, leaving the decision to others.
#15 Updated by Matthew Kerwin over 1 year ago
alexeymuranov (Alexey Muranov) wrote:
+1, why only initialize?
I agree. Is there a reason not to specify something other than a local variable as the receiver for a method parameter?
# precondition: ??? # postcondition: updates @instance_var and $global_var def some_thing(local_var, @instance_var, $global_var) # ... end
I know it's not safe, in that it makes it quite easy to shoot one's self in the foot, but I don't know that it's necessarily a bad thing unless someone tells me it is.
#16 Updated by Prem Sichanugrist about 1 year ago
I think this is a good feature, so I'd like to support this (and possibly, provide a patch for this)
Reading from all the comments, I saw that someone has some concern about having this feature on another method definition (not just initialize) as well. I think we should implement it for any type of method definition if that's going to make the code cleaner, but the main focus here is for the initialize method.
As per comment 12, I think it's OK to start up with
def initialize(@foo, @bar, @baz) and then refactor it if you need to perform any method on those arguments before store it to an instance variable.
I think adding support for only local and instance variables make sense, and I think we need to make sure that we're not supporting global variable like in comment 15.
Anyway, the benefit I'm seeing here is that we're not cluttering the initializer with all those obvious instance variable assignments. I also think it's a good syntactic sugar and it make sense after I saw all the usage from CoffeeScript. I hope I can help to make this happen in the next minor.
#19 Updated by Tyler Rick about 1 month ago
I would love to see this feature in Ruby. Assigning an argument to an instance variable in a constructor is something that we do in almost every constructor we write, so I think this should be made as easy and simple as possible, by adding a little syntactic sugar to the language.
I shouldn't have to repeat myself and type out each argument name 3 times in every constructor I write, for something as mundane as this:
def initialize(name₁, …) @name₂ = name₃ … end
This constant repetition feels inelegant to me and goes against one of the Ruby community's most fundamental values (Don't Repeat Yourself).
This method could be simplified to simply this:
def initialize(@name, …) end
I think CoffeeScript solved this problem quite nicely. Many constructors in CoffeeScript end up being beautiful, simple one-liners!
constructor: (@name) ->
And with the rising popularity of CoffeeScript, there are going to be more and more Rubyists not only wishing for this but also expecting this same feature to exist in Ruby as well. :)
Here are a few more "votes" for this feature:
Plus various attempts at removing the duplication from assign variables in constructors, using only pure Ruby:
* http://redsquirrel.com/cgi-bin/dave/dynamic/def_init.html (
def_init :arg1, :arg2)
* http://blog.jayfields.com/2007/04/ruby-assigning-instance-variables-in.html (
initializer :arg1, :arg2)
* https://github.com/rubyworks/facets/blob/master/lib/core/facets/kernel/assign.rb (
* https://github.com/sheldonh/magic_options (
(But pure Ruby solutions can only go so far, and none of those solutions really solve the problem nicely enough...)