1 files changed, 196 insertions, 0 deletions
diff --git a/simple/simple.rb b/simple/simple.rb
new file mode 100644
index 0000000..a31e001
--- /dev/null
+++ b/simple/simple.rb
@@ -0,0 +1,196 @@
+#!/usr/bin/ruby
+
+class Value < Struct.new(:value)
+  def to_s
+    value.to_s
+  end
+  def inspect
+    "<#{self}>"
+  end
+  def reducible?
+    false
+  end
+end
+
+class Number < Value
+end
+
+class Boolean < Value
+end
+
+class Variable < Struct.new(:name)
+  def to_s
+    name.to_s
+  end
+
+  def inspect
+    "<#{self}>"
+  end
+
+  def reduce(environment)
+    environment[name]
+  end
+
+  def reducible?
+    true
+  end
+end
+
+class BiOp < Struct.new(:left, :right)
+  def inspect
+    "<#{self}>"
+  end
+  def reducible?
+    true
+  end
+end
+
+class Add < BiOp
+  def to_s
+    "#{left} + #{right}"
+  end
+  def reduce(environment)
+    if left.reducible?
+      Add.new(left.reduce(environment),right)
+    elsif right.reducible?
+      Add.new(left,right.reduce(environment))
+    else
+      Number.new(left.value+right.value)
+    end
+  end
+end
+
+class Multiply < BiOp
+  def to_s
+    "#{left} * #{right}"
+  end
+  def reduce(environment)
+    if left.reducible?
+      Add.new(left.reduce(environment),right)
+    elsif right.reducible?
+      Add.new(left,right.reduce(environment))
+    else
+      Number.new(left.value*right.value)
+    end
+  end
+end
+
+class LessThan < BiOp
+  def to_s
+    "#{left} < #{right}"
+  end
+
+  def reduce(environment)
+    if left.reducible?
+      LessThan.new(left.reduce(environment), right)
+    elsif right.reducible?
+      LessThan.new(left, right.reduce(environment))
+    else
+      Boolean.new(left.value < right.value)
+    end
+  end
+end
+
+class DoNothing
+  def to_s
+    'do-nothing'
+  end
+
+  def inspect
+    "<#{self}>"
+  end
+
+  def ==(other_statement)
+    other_statement.instance_of?(DoNothing)
+  end
+
+  def reducible?
+    false
+  end
+end
+
+class Assign < Struct.new(:name, :expression)
+  def to_s
+    "#{name} = #{expression}"
+  end
+
+  def inspect
+    "<#{self}>"
+  end
+
+  def reducible?
+    true
+  end
+
+  def reduce(environment)
+    if expression.reducible?
+      [Assign.new(name, expression.reduce(environment)), environment]
+    else
+      [DoNothing.new, environment.merge({name => expression })]
+    end
+  end
+end
+
+class If < Struct.new(:condition, :consequence, :alternative)
+  def to_s
+    "if (#{condition}) { #{consequence} } else { #{alternative} }"
+  end
+
+  def inspect
+    "<#{self}>"
+  end
+
+  def reducible?
+    true
+  end
+
+  def reduce(environment)
+    if condition.reducible?
+      [If.new(condition.reduce(environment), consequence, alternative), environment]
+    else
+      case condition
+        when Boolean.new(true)
+          [consequence, environment]
+        when Boolean.new(false)
+          [alternative, environment]
+      end
+    end
+  end
+end
+
+class Sequence < Struct.new(:first, :second)
+  def to_s
+    "#{first}; #{second}"
+  end
+
+  def inspect
+    "<#{self}>"
+  end
+
+  def reducible?
+    true
+  end
+
+  def reduce(environment)
+    case first
+      when DoNothing.new
+        [second,environment]
+      else
+        reduced_first, reduced_environment = first.reduce(environment)
+        [Sequence.new(reduced_first, second), reduced_environment]
+  end
+
+class Machine < Struct.new(:statement, :environment)
+  def step
+    self.statement, self.environment = statement.reduce(environment)
+  end
+
+  def run
+    while statement.reducible?
+      puts "#{statement}, #{environment}"
+      step
+    end
+
+    puts "#{statement}, #{environment}"
+  end
+end