Functional take on Conway's Game of Life

functional_gol.rb

module Cell
  def cell
    { state: :alive, neighbours: [] }
  end

  def dead_cell
    {state: :dead, neighbours: []}
  end

  def neighbours_of(cell, neighbour_info)
    cell.merge!(neighbours: neighbour_info[:are])
  end

  def evolve_cell(cell)
    if underpopulated?(cell) or overpopulated?(cell)
      dead_cell
    else
      cell
    end
  end

  def underpopulated?(cell)
    neighbour_count(cell) < 2
  end

  def overpopulated?(cell)
    neighbour_count(cell) > 3
  end

  def neighbour_count(cell)
    cell[:neighbours].length
  end
end

module Organism
  include Cell

  def organism(cells = {})
    cells
  end

  def evolved(organism)
    organism.each_with_object({}){ |(name, cell), evolved| evolved[name] = evolve_cell(cell) }
  end

  def cell_named(name, organism_info)
    organism_info[:from][name]
  end
end

RSpec.configure do |config|
  config.include(Organism)
end

RSpec::Matchers.define :be_alive do
  match do |cell|
    cell[:state] == :alive
  end
end

RSpec::Matchers.define :be_dead do
  match do |cell|
    cell[:state] == :dead
  end
end

describe 'evolution' do
  context 'in an organism with a cell without neighbours' do
    it 'kills that cell' do
      cell_named(:c1, from: evolved(organism(c1: cell))).should be_dead
    end
  end

  context 'in an organism with a cell with 2 neighbours' do
    it 'keeps that cell alive' do
      c1, c2, c3 = cell, cell, cell
      neighbours_of c1, are: [c2, c3]
      neighbours_of c2, are: [c1]
      neighbours_of c3, are: [c1]

      evolved_organism = evolved(organism(c1: c1, c2: c2, c3: c3))

      cell_named(:c1, from: evolved_organism).should be_alive
      cell_named(:c2, from: evolved_organism).should be_dead
      cell_named(:c3, from: evolved_organism).should be_dead
    end
  end

  context 'in an organism with a cell with 3 neighbours' do
    it 'keeps that cell alive' do
      c1, c2, c3, c4 = cell, cell, cell, cell
      neighbours_of c1, are: [c2, c3, c4]
      neighbours_of c2, are: [c1, c3]
      neighbours_of c3, are: [c1, c2]
      neighbours_of c4, are: [c1]

      evolved_organism = evolved(organism(c1: c1, c2: c2, c3: c3, c4: c4))

      cell_named(:c1, from: evolved_organism).should be_alive
      cell_named(:c2, from: evolved_organism).should be_alive
      cell_named(:c3, from: evolved_organism).should be_alive
      cell_named(:c4, from: evolved_organism).should be_dead
    end
  end

  context 'in an organism with a cell with 4 neighbours' do
    it 'kills that cell' do
      c1, c2, c3, c4, c5 = cell, cell, cell, cell, cell
      neighbours_of c1, are: [c2, c3, c4, c5]
      neighbours_of c2, are: [c1, c3]
      neighbours_of c3, are: [c1, c2]
      neighbours_of c4, are: [c1]
      neighbours_of c5, are: [c1]

      evolved_organism = evolved(organism(c1: c1, c2: c2, c3: c3, c4: c4, c5: c5))

      cell_named(:c1, from: evolved_organism).should be_dead
      cell_named(:c2, from: evolved_organism).should be_alive
      cell_named(:c3, from: evolved_organism).should be_alive
      cell_named(:c4, from: evolved_organism).should be_dead
      cell_named(:c5, from: evolved_organism).should be_dead
    end
  end
end

Nice, quite readable.

You have a couple of bugs, though:

• A dead cell with 3 neighbours should come to life, which I don't think your implementation does.
• You don't keep the neighbours on evolving to a dead_cell, so subsequent generations will deviate from the expected results.

Also, there are two things I would do different:

• Neighbourship is reciprocal, so I don't like the fact that you have two define it both ways.
• evolve_cell returns a new dead_cell or the cell itself. It's a bit subtle, since the parameter name is the same as the "new cell" function. Maybe a more symmetrical behaviour would be clearer.