From: "tomstuart (Tom Stuart)" Date: 2022-11-16T10:02:52+00:00 Subject: [ruby-core:110774] [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 tomstuart (Tom Stuart). While it���s undeniable that this method duplicates its receiver in some sense, I���d like to add my support for the `#with` naming choice. I don���t have any technical objection to using `#dup`, it���s just a stylistic preference: in the interests of programmer happiness, to me it���s more natural for the name to communicate the intent of the sender (���I want an object just like `ORIGIN`, except `#with` an `x` of `1.0`���) rather than the lower-level implementation detail of what that operation involves (���I want a shallow clone of `ORIGIN` via `#allocate` & `#initialize_copy`, except assigning `1.0` as the value of the `@x` instance variable���). Perhaps another way of putting this is that I���d estimate most Ruby programmers rarely call `Object#dup` in the course of their work, whereas I would expect users of `Data` to need this new ���copy with changes��� operation somewhat frequently, especially if they���re migrating existing code from `Struct`. But of course I have no data to support that guess! ---------------------------------------- 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-100119 * 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/ Unsubscribe: