Feature #5825
openSweet instance var assignment in the object initializer
Description
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 @variable
s 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?
Updated by lisovskyvlad (Vlad Lisosvky) almost 13 years ago
I like it. No stupid assigns.
Updated by rue (Eero Saynatkari) almost 13 years ago
Would be nice, and should be able to coexist with normal parameters:
def foo(bar, @baz, quux = @moomin)
…
end
And so on. What about splat- and block arguments? It gets a little ugly:
def foo(bar, *@baz, &@quux)
…
end
Updated by rosenfeld (Rodrigo Rosenfeld Rosas) almost 13 years ago
+1 - too common use case
Updated by shyouhei (Shyouhei Urabe) almost 13 years ago
I liked this 1.8-specific trick:
define_method(:intialize){|@foo, @bar|}
but it was abondoned for any reason.
Updated by alexeymuranov (Alexey Muranov) almost 13 years ago
+1, why only initialize?
Updated by Eregon (Benoit Daloze) almost 13 years ago
I think most of the time you need to parse or check your arguments, in which case this syntax would not be practical.
Otherwise, you could use Struct to avoid the duplication:
class Me < Struct.new(:name, :age, :location)
end
Updated by goshakkk (Gosha Arinich) almost 13 years ago
Alexey Muranov wrote:
+1, why only initialize?
It's just too common case. It could work for other methods as well.
Updated by shevegen (Robert A. Heiler) almost 13 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
used.
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)
Updated by naruse (Yui NARUSE) almost 13 years ago
- Status changed from Open to Assigned
- Assignee set to matz (Yukihiro Matsumoto)
Updated by ko1 (Koichi Sasada) over 12 years ago
- Target version set to 2.0.0
if my memory serves me right, matz dislikes such a style of arguments.
Matz, could you explain the reason again?
I know some people like this style.
Updated by trans (Thomas Sawyer) over 12 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
Becomes
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.
Updated by ko1 (Koichi Sasada) about 12 years ago
- Target version changed from 2.0.0 to 2.6
I changed target to next minor.
I think someone who want to introduce it need to persuade matz.
Updated by Anonymous almost 12 years 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 #initialize
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
named argument:
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.
Also, #autoinit
method, or rather, #autoinit_named
, #autoinit_ordered
,
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 #
initialize
method. -
By creating and including a mixin patching #
new
class 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
Now 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.
Updated by phluid61 (Matthew Kerwin) almost 12 years 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?
For example:
# 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.
Updated by sikachu (Prem Sichanugrist) over 11 years 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.
Updated by nobu (Nobuyoshi Nakada) over 11 years ago
- Description updated (diff)
Updated by nobu (Nobuyoshi Nakada) over 11 years ago
- Category set to syntax
- Target version changed from 2.6 to 3.0
Updated by TylerRick (Tyler Rick) over 10 years 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:
- http://stackoverflow.com/questions/10856191/ruby-automatically-set-instance-variable-as-method-argument
- http://stackoverflow.com/questions/9597249/in-ruby-can-i-automatically-populate-instance-variables-somehow-in-the-initializ/10855962
- http://stackoverflow.com/questions/16072965/why-do-method-arguments-not-work-for-assignment
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 (
assign(hash)
) -
https://github.com/sheldonh/magic_options (
magic_initialize
andmagic_options(hash)
)
(But pure Ruby solutions can only go so far, and none of those solutions really solve the problem nicely enough...)
Updated by nobu (Nobuyoshi Nakada) over 10 years ago
- Description updated (diff)
Tyler Rick wrote:
I think CoffeeScript solved this problem quite nicely. Many constructors in CoffeeScript end up being beautiful, simple one-liners!
constructor: (@name) ->
Sorry, it doesn look beautiful to me.
Updated by phluid61 (Matthew Kerwin) over 10 years ago
Does anyone have a link to discussions/logs that lead to the decision to remove instance/global variables from block parameters?
Updated by nobu (Nobuyoshi Nakada) almost 9 years ago
- Has duplicate Feature #12023: Allow ivars to be used as method arguments added
Updated by nobu (Nobuyoshi Nakada) over 8 years ago
- Has duplicate Feature #12578: Instance Variables Assigned In parameters ( ala Crystal? ) added
Updated by nobu (Nobuyoshi Nakada) about 8 years ago
- Has duplicate Feature #12820: Shorter syntax for assigning a method argument to an instance variable added
Updated by mame (Yusuke Endoh) about 6 years ago
- Related to Feature #15192: Introduce a new "shortcut assigning" syntax to convenient setup instance variables added
Updated by nobu (Nobuyoshi Nakada) over 5 years ago
- Related to deleted (Feature #15192: Introduce a new "shortcut assigning" syntax to convenient setup instance variables)
Updated by nobu (Nobuyoshi Nakada) over 5 years ago
- Has duplicate Feature #15192: Introduce a new "shortcut assigning" syntax to convenient setup instance variables added
Updated by nobu (Nobuyoshi Nakada) over 5 years ago
- Has duplicate Feature #16095: 2 Features: remove (simplify) 'new' keyword and Property Shorthand added
Updated by nobu (Nobuyoshi Nakada) over 5 years ago
- Has duplicate deleted (Feature #16095: 2 Features: remove (simplify) 'new' keyword and Property Shorthand)
Updated by mame (Yusuke Endoh) over 3 years ago
- Related to Feature #17942: Add a `initialize(public @a, private @b)` shortcut syntax for defining public/private accessors for instance vars as part of constructor added
Updated by eike.send@gmail.com (Eike Send) over 2 years ago
I would really love to see native support for an initializer instance variable assignment shorthand without adding the problems when inheriting from Struct.
Updated by nobu (Nobuyoshi Nakada) about 1 year ago
- Has duplicate Feature #19898: Special syntax for instance variable assignment added
Updated by matheusrich (Matheus Richard) about 1 year ago
Maybe a different proposal, but if the def initialize(@a, @b)
is ugly, how about a new method similar to attr_{reader,writter}
like
class User
init_with :name, :age
# or maybe
attributes :name, :age
end
User.new("Matz", 21)
We could even have an option like keyword_init: true
for keyword args.
On the other hand, a simple alternative is using the new Data
class, which already provides this nicety:
User = Data.define(:name, :age)
User.new("matz", 21)
# or
User.new(name: "matz", age: 21)