Ruby quizes!

quiz_01.rb

# implement the functions `amb` and `assert`.
# `amb` must choose a value from a given set,
# such that all assertions pass.
# as an example, the following program:

a = amb [1,2,3,4,5,6,7]
b = amb [1,2,3,4,5,6,7]
c = amb [1,2,3,4,5,6,7]

# a, b and c must define a legal right triangle:
assert c**2 == a**2 + b**2

# and side b is smaller than side a
assert b < a

# print the results:
puts [a,b,c].join " "

# RESULT: 4 3 5

solution_01.rb

require 'continuation'

def fail_stack
  (@fail_stack ||= [])
end

def failure
  throw "back tracking stack exhausted!" if fail_stack.empty?

  cc = fail_stack.shift
  cc.call cc
end

def amb choices
  cc = callcc {|cc| cc}

  failure if choices.empty?
  choice = choices.shift
  fail_stack.unshift cc
  choice
end

def assert condition
  failure unless condition
  true
end

# let's test it out
a = amb [1,2,3,4,5,6,7]
b = amb [1,2,3,4,5,6,7]
c = amb [1,2,3,4,5,6,7]

assert c**2 == a**2 + b**2
assert b < a

puts [a,b,c].join " "