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Support pattern matching on instances #70

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Support pattern matching on instances #70

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4 changes: 4 additions & 0 deletions lib/immutable-struct.rb
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Original file line number Diff line number Diff line change
Expand Up @@ -121,6 +121,10 @@ def merge(new_attrs)
end
(attribute_values + [self.class]).hash
end

def deconstruct_keys(keys)
to_h.slice(*keys)
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From the ruby docs:

keys are passed to deconstruct_keys to provide a room for optimization in the matched class: if calculating a full hash representation is expensive, one may calculate only the necessary subhash

I don't think we need or want to slice a subhash here - it's actually less optimized that way. There's also a bug this introduces, since pattern matching with "any key" (e.g. **rest) will pass keys = nil and expects to get the entire hash, but slice(*nil) # => { }. A test could be written for this case which would fail given this implementation.

https://zverok.space/blog/2022-12-20-pattern-matching.html#dont-forget-about-the-any-key-option recommends not optimizing this, and just returning the full hash in most cases, and I think this is one of those cases. Note: this would reflect a change in the returns a hash with the specified keys test.

end
end
klass.class_exec(&block) unless block.nil?

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20 changes: 20 additions & 0 deletions spec/immutable_struct_spec.rb
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Expand Up @@ -190,6 +190,26 @@ def location_near?(other_location)
end
end

describe "#deconstruct_keys" do
it "returns a hash with the specified keys" do
klass = ImmutableStruct.new(:a, :b, :c)
instance = klass.new(a: 1, b: 2, c: 3)
expect(instance.deconstruct_keys([:a, :c])).to eq({ a: 1, c: 3 })
end

it "allows an instance to be used with pattern matching" do
klass = ImmutableStruct.new(:a, :b, :c)
instance = klass.new(a: 1, b: 2, c: 3)
expect {
case instance
in { a: 1 }
# good!
end
# a NoMatchingPatternError would be raised if the pattern didn't match
}.not_to raise_error
end
end

describe "merge" do
it "returns a new object as a result of merging attributes" do
klass = ImmutableStruct.new(:food, :snacks, :butter)
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