* Refactor.

Use a specific exception instead of ArgumentError, thus saving us from:
* Rescuing errors we didn't mean to rescue
OR
* Having an ugly being-rescue block at the start of #place.

robot.rb

@@ -54,7 +54,7 @@ def place x, y, direction
     return unless @@table.contain? new_coords.x, new_coords.y
 
     @coords = new_coords
-  rescue ArgumentError
+  rescue StateArgumentError
   end
 
   def report

spec/state_spec.rb

@@ -6,4 +6,9 @@
     state = Robot::State.new 0, 0, 'NORTH'
     expect(state.to_s).to eq '0,0,NORTH'
   end
+
+  it 'raises State if a constructor argument is invalid' do
+    expect { Robot::State.new 'a', 0, 'NORTH' }.to \
+      raise_error Robot::StateArgumentError
+  end
 end

state.rb

@@ -1,10 +1,14 @@
 require './direction'
 
 class Robot
+  class StateArgumentError < ArgumentError; end
+
   State = Struct.new(:x, :y, :direction) do
     def initialize x, y, direction
       self.x, self.y, self.direction =
 	Integer(x), Integer(y), Direction(direction)
+    rescue ArgumentError
+      raise StateArgumentError, $!.message
     end
 
     def to_s