This is part 2 in this series on Ruby's Data Class. Part 1 covered Syntax and part 3 explores immutability workarounds with Data.
In practice, a main difference between instances of Data and Struct is mutability:
- Instances of Data are immutable*
- Instances of Struct are mutable
*Mutable values passed to instances of Data remain immutable by default.
Data also has fewer (and different) 'built-in' methods than Struct.
This post explores differences in syntax and behavior between Data and Struct.
Syntax comparison: Create New Instance
Data
House = Data.define(:rooms, :area, :floors) ranch = House.new(rooms: 5, area: 1200, floors: 1) # => #<data House rooms=5, area=1200, floors=1> Struct
House = Struct.new(:rooms, :area, :floors, keyword_init: true) ranch = House.new(rooms: 5, area: 1200, floors: 1) # => #<struct House rooms=5, area=1200, floors=1> Mutability: Attempt to Renovate the house...
Data
House = Data.define(:rooms, :area, :floors) ranch = House.new(rooms: 5, area: 1200, floors: 1) # => #<data House rooms=5, area=1200, floors=1> ranch.floors = 2 # (irb):3:in `<main>': undefined method `floors=' for an instance of House (NoMethodError) This tells us there is no 'writer' (sometimes called "setter" in other
languages) method to use.
If we try to define a writer method on House and update that value, FrozenError is raised.
House = Data.define(:rooms, :area, :floors) do def floors=(quantity) @floors = quantity end end # => House cottage = House.new(rooms: 5, area: 700, floors: 1) # => #<data House rooms=5, area=700, floors=1> cottage.floors = 2 # (irb):21:in `floors=': can't modify frozen House: #<data House rooms=5, area=700, floors=1> (FrozenError) cottage.frozen? # => true What we can do with a Data object is clone it and update any of the attribute values.
House = Data.define(:rooms, :area, :floors) cottage = House.new(rooms: 5, area: 1200, floors: 1) # => #<data House rooms=5, area=1200, floors=1> two_story_cottage = cottage.with(floors: 2) # => #<data House rooms=5, area=1200, floors=2> Struct
With Struct, we can renovate.
House = Struct.new(:rooms, :area, :floors, keyword_init: true) ranch = House.new(rooms: 5, area: 1200, floors: 1) # => #<struct House rooms=5, area=1200, floors=1> ranch.floors = 2 # => 2 ranch # => #<struct House rooms=5, area=1200, floors=2> ranch.frozen? # => false Built-in Methods
Data Class
>> Data.methods(false) => [:define] >> Data.instance_methods(false) => [:deconstruct_keys, :pretty_print, :pretty_print_cycle, :deconstruct, :hash, :==, :inspect, :members, :to_s, :with, :eql?, :to_h] Struct
>> Struct.methods(false) => [:new] >> Struct.instance_methods(false) => [:deconstruct_keys, :==, :members, :to_a, :to_s, :[], :[]=, :values_at, :eql?, :to_h, :select, :filter, :pretty_print, :pretty_print_cycle, :deconstruct, :hash, :inspect, :each_pair, :values, :length, :dig, :size, :each] Comparison
>> struct_methods - data_methods => [:to_a, :[], :[]=, :values_at, :select, :filter, :each_pair, :values, :length, :dig, :size, :each] >> data_methods - struct_methods => [:with] 
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