From: "RubyBugs (A Nonymous)" <noreply@...>
Date: 2022-11-30T19:58:59+00:00
Subject: [ruby-core:111098] [Ruby master Feature#19000] Data: Add "Copy with changes method" [Follow-on to #16122 Data: simple immutable value object]

Issue #19000 has been updated by RubyBugs (A Nonymous).





For comparison, you can plug in the following example code for a faster implementation of `#with` and it should work the same:



```ruby

Point = Data.define(:x, :y) do

  # Example only, too slow. Real implementation should eliminate allocations.

  # def with(**args)

  #   raise ArgumentError unless args.keys.all? { |k| members.include? k }

  #   self.class.new(**(to_h.merge(args)))

  # end  



  # A trick to avoid `nil` for defaults so that `nil` can be valid

  DEFAULT = class.new.new.freeze



  # An example of a faster implementation that could inspire auto-generation

  def with(x: DEFAULT, y: DEFAULT)

    self.class.new(

      x: x.equal?(DEFAULT) ? self.x : x),

      y: y.equal?(DEFAULT) ? self.y : y),

    )

  end  

end

```



----------------------------------------

Feature #19000: Data: Add "Copy with changes method" [Follow-on to #16122 Data: simple immutable value object]

https://bugs.ruby-lang.org/issues/19000#change-100364



* Author: RubyBugs (A Nonymous)

* Status: Open

* Priority: Normal

----------------------------------------

*As requested: extracted a follow-up to #16122 Data: simple immutable value object from [this comment](http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-core/109815)*





# Proposal: Add a "Copy with changes" method to Data



Assume the proposed `Data.define` exists.

Seeing examples from the [[Values gem]](https://github.com/ms-ati/Values):



```ruby

# A new class

Point = Data.def(:x, :y)



# An immutable instance

Origin = Point.with(x: 0, y: 0)



# Q: How do we make copies that change 1 or more values?

right = Origin.with(x: 1.0)

up = Origin.with(y: 1.0)

up_and_right = right.with(y: up.y)



# In loops

movements = [

  { x: +0.5 },

  { x: +0.5 },

  { y: -1.0 },

  { x: +0.5 },

]



# position = Point(x: 1.5, y: -1.0)

position = movements.inject(Origin) { |p, move| p.with(**move) }

```



## Proposed detail: Call this method: `#with`



```ruby

Money = Data.define(:amount, :currency)



account = Money.new(amount: 100, currency: 'USD')



transactions = [+10, -5, +15]



account = transactions.inject(account) { |a, t| a.with(amount: a.amount + t) }

#=> Money(amount: 120, currency: "USD")

```



## Why add this "Copy with changes" method to the Data simple immutable value class?



Called on an instance, it returns a new instance with only the provided parameters changed.



This API affordance is now **widely adopted across many languages** for its usefulness. Why is it so useful? Because copying immutable value object instances, with 1 or more discrete changes to specific fields, is the proper and ubiquitous pattern that takes the place of mutation when working with immutable value objects.



**Other languages**



C# Records: ���immutable record structs ��� Non-destructive mutation��� ��� is called `with { ... }`

https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/builtin-types/record#nondestructive-mutation



Scala Case Classes ��� is called `#copy`

https://docs.scala-lang.org/tour/case-classes.html



Java 14+ Records ��� Brian Goetz at Oracle is working on adding a with copy constructor inspired by C# above as we speak, likely to be called `#with`

https://mail.openjdk.org/pipermail/amber-spec-experts/2022-June/003461.html



Rust ���Struct Update Syntax��� via `..` syntax in constructor

https://doc.rust-lang.org/book/ch05-01-defining-structs.html#creating-instances-from-other-instances-with-struct-update-syntax



## Alternatives



Without a copy-with-changes method, one must construct entirely new instances using the constructor. This can either be (a) fully spelled out as boilerplate code, or (b) use a symmetrical `#to_h` to feed the keyword-args constructor.



**(a) Boilerplate using constructor**



```ruby

Point = Data.define(:x, :y, :z)

Origin = Point.new(x: 0.0, y: 0.0, z: 0.0)



change = { z: -1.5 }



# Have to use full constructor -- does this even work?

point = Point.new(x: Origin.x, y: Origin.y, **change)

```



**(b) Using a separately proposed `#to_h` method and constructor symmetry**



```ruby

Point = Data.define(:x, :y, :z)

Origin = Point.new(x: 0.0, y: 0.0, z: 0.0)



change = { z: -1.5 }



# Have to use full constructor -- does this even work?

point = Point.new(**(Origin.to_h.merge(change)))

```



Notice that the above are not ergonomic -- leading so many of our peer language communities to adopt the `#with` method to copy an instance with discrete changes.







-- 

https://bugs.ruby-lang.org/

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