Feature #19001
closedData: Add #to_h symmetric to constructor with keyword args [Follow-on to #16122 Data: simple immutable value object]
Description
Extracted a follow-up to #16122 Data: simple immutable value object
Proposal: Add a #to_h
method symmetric to a constructor accepting keyword arguments¶
This allows round-trip between a Hash
and a Value object instance, for example:
Point = Data.define(:x, :y, :z)
points = [
Point.new(x: 1, y: 0, z: 0),
Point.new(x: 0, y: 1, z: 0),
Point.new(x: 0, y: 0, z: 1),
]
hashes = points.map(&:to_h)
points_2 = hashes.map { |h| Point.new(**h) }
points_2 == points
#=> true
Why?¶
Having symmetric operation between #to_h
and a keyword-args constructor is a major ergonomic factor in usage of immutable value objects.
To play with code that works like this, you may take a look at the Values gem
Alternatives¶
If there is no symmetric construction and de-construction along these lines, a number of use cases become more complicated and less ergonomic.
Updated by RubyBugs (A Nonymous) about 2 years ago
Per @matz (Yukihiro Matsumoto) here, the preference would be for the constructor to take either:
- Only keyword args
- Either keyword args OR positional args
The Values gem provides separate positional and keyword args constructors:
-
.new
-- positional constructor -
.with
-- keyword args constructor
Given the high need for ergonomics related to the keyword args constructor, my recommendation is that this one is more important than a positional constructor.
For example, when using simple immutable value objects, a keyword args constructor combined with a #with
method to copy an instance with discrete changes (see #19000), can completely replace the need for default values, as follows:
Point = Data.define(:x, :y, :z)
Default = Point.new(x: 1, y: 2, z: 3)
p = Default.with(x: 42)
#=> Point(x: 42, y: 2, z: 3)
Updated by zverok (Victor Shepelev) about 2 years ago
There isn't any need for this ticket as a separate request, as far as I am concerned.
It works in the initial implementation of data already as submitted in https://bugs.ruby-lang.org/issues/16122#note-68, and even works with ol' good Struct, too:
Point = Struct.new(:x, :y, :z)
Point[1, 0, 0].to_h.then { Point[**_1] } == Point[1, 0, 0] # => true
Updated by RubyBugs (A Nonymous) 5 months ago
Agreed, this need ended up being met by the initial implementation! Thank you, closing this ticket.
zverok (Victor Shepelev) wrote in #note-2:
There isn't any need for this ticket as a separate request, as far as I am concerned.
It works in the initial implementation of data already as submitted in https://bugs.ruby-lang.org/issues/16122#note-68, and even works with ol' good Struct, too:Point = Struct.new(:x, :y, :z) Point[1, 0, 0].to_h.then { Point[**_1] } == Point[1, 0, 0] # => true
Updated by hsbt (Hiroshi SHIBATA) 5 months ago
- Status changed from Open to Closed