72: Functional Core, Imperative Shell

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The content

To get started, we the example class we start with is a Tweet class.

require 'values'

class Tweet < Value.new(:tid, :username, :text)
    def initialize(tid, username, text)
        super(tid.to_i, username, text)
    end

    def line_count
        lines.count
    end

    def lines
        text.split("\n")
    end
end

# Another file
class Timeline
    attr_reader :tweets

    def initialize(tweets)
        @tweets = tweets
    end

    def add(tweets)
        existing_ids = @tweets.map(&:tid).to_set
        new_tweets = tweets.reject { |t| existing_ids.include?(t.tid) }
        Timeline.new(new_tweets + @tweets)
    end
end
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There is an even more Cursor class that is explained in the video, but in the video he is happy that Cursor is immutable.

As a last example of the functional core, there is a TweetRenderer class.

They are all examples of the "functional core". The only difference is the re-binding of local variables.

The imperative shell will be encapsulated in a single file (mostly at least) that manages the work like updating the database, etc. This is a file that deals with side-effects.

An example of an imperative shell:

class Toot
    def initialize(screen)
        @screen = screen
    end

    def run
        TwittlerLib.authenticate

        database = Database.default
        timeline = Timeline.new(database.timeline)
        timeline_queue = SelectableQueue.new
        # start_timeline_stream(timeline, timeline_queue)
        world = World.new(timeline)

        view = TimelineView.new(@screen)
        EventLoop.new(database, world, timeline_queue, view).run
    end

    def start_timeline_stream(timeline, timeline_queue)
        timeline_stream = TimelineStream.new(timeline, timeline_queue)
        Thread.new { timeline_stream.run }
    end
end
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