Board put_at (https://twitter.com/TotherAlistair/status/960018354168913925)

board.rb

class Board
  class BoardOccupiedError < StandardError
  end
  
  attr_reader :board
  attr_reader :last_step_successful

  def initialize
    @board = []
    @last_step_successful = true
  end

  def occupied?(location)
    !@board[location]
  end

  def put_at(stone, location)
    if occupied?(location)
      @last_step_successful = false
    else
      @board[location] = stone
      @last_step_successful = true
    end

    @board
  end

  def put_at!(stone, location)
    put_at(stone, location)
    raise BoardOccupiedError, "Board at #{location} is occupied!" unless @last_step_successful
  end

  def clear_at(location)
    @board[location] = nil
    
    @board
  end
end

## Use cases

# 1. Silent moves. Also chaining!

board.put_at(stone, location).put_at(stone2, location2)

# 2. Silent moves, control flow 
# ("as a user, if the move is impossible, I get a message")

board = board.put_at(stone, location)
if board.last_step_successful
  redirect_to :next_move
else
  flash[:error] = "Board is occupied!"
end

# 3. Moves with exception handling

board.put_at!(stone, location)

# 3.1 we don't expect any errors and only see them in logs/error catching services

# ...

# 3.2 we use exceptions for flow control, yay!

# ...
  board.put_at!(stone, location)
rescue BoardOccupiedError
  flash[:error] = "Board is occupied!"
# ...

## Alternative approach

# `put_at` returns `true` or `false`, `board` is accessble via `#board`
# 
# Cons: non-chainable
# 
# Pros: easier code
# 
# Pros 2: you probably don't really need chaining anyway since you need to
# check on errors after every move

class Board
  class BoardOccupiedError < StandardError
  end
  
  attr_reader :board

  def initialize
    @board = []
  end

  def occupied?(location)
    !@board[location]
  end

  def put_at(stone, location)
    success = false
    unless occupied?(location)
      @board[location] = stone
      success = true
    end

    success
  end

  def put_at!(stone, location)
    put_at(stone, location) || 
      raise BoardOccupiedError, "Board at #{location} is occupied!"
  end
end

## Use cases

if board.put_at(stone.location)
  flash[:message] = "Done!"
else
  flash[:error] = "Yikes!"
end