[nauticacom]

Sure, that works for some specific problems where you're computing a value from a defined set of data types. "Subset of this data" was an example I've encountered in the past and used here because it has clearly distinct cases—give me the whole thing, give me some index-delimited range, possibly others—but there are plenty of other examples that don't fit a polymorphic function model (and let's forget that I've never even used a language with polymorphic functions).

As another example I've encountered in the past, let's say you have some object that can dynamically define fields. Once you define a field, you can retrieve its value or maybe some default value e.g.

    model = Model.new
    model.define("points", default: 1)
    model.store("points", 10)
    points = model.retrieve("points")
    puts points # => 10

Let's say doing anything with an undefined field is invalid. Here's my first pass at an implementation:

    class Model
        def initialize
            @fields = {}
        end

        def define(name, default: nil)
            @fields[name] = Field.new(name, default)
        end

        def retrieve(name)
            @fields[name].value
        end

        def store(name, value)
            @fields[name].value = value
        end
    end

    class Field
        attr_reader :name
        attr_accessor :value

        def initialize(name, value)
            @name  = name
            @value = value
        end
    end

Works great! One day a requirement comes along that default values need to be lambdas, too, which are called every time the value is retrieved. How do we implement that? One way is to add a conditional to the Field class:

    class Field
        attr_reader :name
        attr_writer :value

        def initialize(name, value)
            @name  = name
            @value = value
        end

        def value
            if value.is_a?(Proc)
                @value.call
            else
                @value
            end
        end
    end

But now Field knows that it can be passed a lambda, so testing it needs to account for that case (among many other considerations, probably, in a real-world system). And any time we add more cases for default values, let alone changes to regular values like type casting or something, the Field class becomes more complicated. I'd probably reach for a new object instead:

    class Model
        def initialize
            @fields = {}
        end

        def define(name, default: nil)
            @fields[name] = Field.new(name, nil, Default.for(default))
        end

        def retrieve(name)
            @fields[name].value
        end

        def store(name, value)
            @fields[name].value = value
        end
    end

    class Field
        def initialize(name, value, default)
            @name    = name
            @value   = value
            @default = default
        end

        def value
            if @value.nil?
                @default.value
            else
                @value
            end
        end
    end

    class Default
        def self.for(indicator)
            if indicator.is_a?(Proc)
                Default::Dynamic.new(indicator)
            elsif indicator.nil?
                Default::None.new
            else
                Default::Static.new(indicator)
            end
        end

        class Static < Default
            def initialize(value)
                @value = value
            end

            def value
                @value
            end
        end

        class Dynamic < Default
            def initialize(callable)
                @callable = callable
            end

            def value
                @callable.call
            end
        end

        class None < Default
            def value
                nil
            end
        end
    end

Now we've changed the conditional in the Field class to one that's actually relevant to it (do I have a value yet?) and won't change when the kinds of default values that it can accept change. Because we dependency-injected the Default object into the Field object, testing that conditional becomes a binary of retrieving the default value when no value is set, and retrieving the value once it's set. We can then test each kind of Default on its own, and changes to Default don't impact Field. If we really, really wanted to we could even eliminate the conditional in Field alltogether by unifying the interface for @default and @value such that they're both objects with a #value method (or maybe rename it to something else so we don't write @value.value). In either case we've made each piece simpler to reason about and pushed conditionals up the call stack so the resulting code is more straightforward.

I can probably recall more examples of simplifications like this, but this is where I find inheritance the most useful: a known set of things that each polymorphically conform to some interface. In these examples I don't actually use the superclass for any shared behavior, but you can imagine a case where I might.

One other benefit that I really like from the inheritance-object-modeling-as-pushing-up-conditionals perspective is that it makes you define what the different cases of something are as distinct objects, and give names to them. It's a similar benefit that falls out of using named sum types instead of signal values or tagged unions or something, but has the opposite effect (overall reduction of conditionals rather than proliferation).