Example of the functional approach to problem decomposition in Ruby. https://class.coursera.org/proglang-002/forum/thread?thread_id=1962

section8.rb

BadResult = Class.new Exception

Int = Struct.new :i
Negate = Struct.new :e1
Add = Struct.new :e1, :e2

def eval e
  case e
  when Int    then e
  when Negate then case v1 = (eval e.e1)
                   when Int then Int.new -v1.i
                   else raise BadResult, "non-int in negation"
                   end
  when Add    then case v1 = (eval e.e1)
                   when Int then case v2 = (eval e.e2)
                                 when Int then Int.new v1.i + v2.i
                                 end
                   end or raise BadResult, "non-int in negation"
  else raise ArgumentError, e
  end
end

def toString e
  case e
  when Int    then e.i.to_s
  when Negate then "-(" + (toString e.e1) + ")"
  when Add    then "(" + (toString e.e1) + " + " + (toString e.e2) + ")"
  else raise ArgumentError, e
  end
end

def hasZero e
  case e
  when Int    then e.i == 0
  when Negate then hasZero e.e1
  when Add    then hasZero e.e1 or hasZero e.e2
  else raise ArgumentError, e
  end
end

def noNegConstants e
  case e
  when Int    then if e.i < 0 then Negate.new Int.new -e.i else e end
  when Negate then Negate.new noNegConstants e.e1
  when Add    then Add.new (noNegConstants e.e1), (noNegConstants e.e2)
  else raise ArgumentError, e
  end
end

session.sh

$ ruby -r ./section8.rb -e 'p (toString (Add.new (Int.new 5), (Add.new (Negate.new (Int.new 3)), (Int.new 8))))'
"(5 + (-(3) + 8))"

$ ruby -r ./section8.rb -e 'p (eval (Add.new (Int.new 5), (Add.new (Negate.new (Int.new 3)), (Int.new 8))))'
#<struct Int i=10>

$ ruby -r ./section8.rb -e 'p (hasZero (Add.new (Int.new 5), (Add.new (Negate.new (Int.new 3)), (Int.new 8))))'
false

$ ruby -r ./section8.rb -e 'p (toString (Add.new (Int.new -5), (Int.new -3)))'
"(-5 + -3)"

$ ruby -r ./section8.rb -e 'p (toString (noNegConstants (Add.new (Int.new -5), (Int.new -3))))'
"(-(5) + -(3))"

$ ruby -r ./section8.rb -e 'p (eval (Add.new (Int.new 5), "hello"))'
./section8.rb:19:in `eval': hello (ArgumentError)
        from ./section8.rb:15:in `eval'
        from -e:1:in `'