Task: Amb
C#: using System;
using System.Collections.Generic;

public class Amb : IDisposable
{
    List<IValueSet> streams = new List<IValueSet>();
    List<IAssertOrAction> assertsOrActions = new List<IAssertOrAction>();
    volatile bool stopped = false;

    public IAmbValue<T> DefineValues<T>(params T[] values)
    {
        return DefineValueSet(values);
    }

    public IAmbValue<T> DefineValueSet<T>(IEnumerable<T> values)
    {
        ValueSet<T> stream = new ValueSet<T>();
        stream.Enumerable = values;
        streams.Add(stream);
        return stream;
    }

    public Amb Assert(Func<bool> function)
    {
        assertsOrActions.Add(new AmbAssert()
        {
            Level = streams.Count,
            IsValidFunction = function
        });
        return this;
    }

    public Amb Perform(Action action)
    {
        assertsOrActions.Add(new AmbAction()
        {
            Level = streams.Count,
            Action = action
        });
        return this;
    }

    public void Stop()
    {
        stopped = true;
    }

    public void Dispose()
    {
        RunLevel(0, 0);
        if (!stopped)
        {
            throw new AmbException();
        }
    }

    void RunLevel(int level, int actionIndex)
    {
        while (actionIndex < assertsOrActions.Count && assertsOrActions[actionIndex].Level <= level)
        {
            if (!assertsOrActions[actionIndex].Invoke() || stopped)
                return;
            actionIndex++;
        }
        if (level < streams.Count)
        {
            using (IValueSetIterator iterator = streams[level].CreateIterator())
            {
                while (iterator.MoveNext())
                {
                    RunLevel(level + 1, actionIndex);
                }
            }
        }
    }

    interface IValueSet
    {
        IValueSetIterator CreateIterator();
    }

    interface IValueSetIterator : IDisposable
    {
        bool MoveNext();
    }

    interface IAssertOrAction
    {
        int Level { get; }
        bool Invoke();
    }

    class AmbAssert : IAssertOrAction
    {
        internal int Level;
        internal Func<bool> IsValidFunction;

        int IAssertOrAction.Level { get { return Level; } }

        bool IAssertOrAction.Invoke()
        {
            return IsValidFunction();
        }
    }

    class AmbAction : IAssertOrAction
    {
        internal int Level;
        internal Action Action;

        int IAssertOrAction.Level { get { return Level; } }

        bool IAssertOrAction.Invoke()
        {
            Action(); return true;
        }
    }

    class ValueSet<T> : IValueSet, IAmbValue<T>, IValueSetIterator
    {
        internal IEnumerable<T> Enumerable;
        private IEnumerator<T> enumerator;

        public T Value { get { return enumerator.Current; } }

        public IValueSetIterator CreateIterator()
        {
            enumerator = Enumerable.GetEnumerator();
            return this;
        }

        public bool MoveNext()
        {
            return enumerator.MoveNext();
        }

        public void Dispose()
        {
            enumerator.Dispose();
        }
    }
}

public interface IAmbValue<T>
{
    T Value { get; }
}

public class AmbException : Exception
{
    public AmbException() : base("AMB is angry") { }
}

C++: #include <iostream>
#include <string_view>
#include <boost/hana.hpp>
#include <boost/hana/experimental/printable.hpp>

using namespace std;
namespace hana = boost::hana;



constexpr auto Amb(auto constraint, auto&& ...params)
{
  
  auto possibleSolutions = hana::cartesian_product(hana::tuple(params...));

  
  auto foldOperation = [constraint](auto a, auto b)
  {
    bool meetsConstraint = constraint(a);
    return meetsConstraint ? a : b;
  };

  return hana::fold_right(possibleSolutions, foldOperation);
}

void AlgebraExample()
{
  
  constexpr hana::tuple x{1, 2, 3};
  constexpr hana::tuple y{7, 6, 4, 5};

  
  constexpr auto constraint = [](auto t)
  {
    return t[hana::size_c<0>] * t[hana::size_c<1>] == 8;
  };

  
  auto result = Amb(constraint, x, y);

  cout << "\nx = " << hana::experimental::print(x);
  cout << "\ny = " << hana::experimental::print(y);
  cout << "\nx * y == 8: " << hana::experimental::print(result);
}

void StringExample()
{
  
  constexpr hana::tuple words1 {"the"sv, "that"sv, "a"sv};
  constexpr hana::tuple words2 {"frog"sv, "elephant"sv, "thing"sv};
  constexpr hana::tuple words3 {"walked"sv, "treaded"sv, "grows"sv};
  constexpr hana::tuple words4 {"slowly"sv, "quickly"sv};

  
  
  constexpr auto constraint = [](const auto t)
  {
    auto adjacent = hana::zip(hana::drop_back(t), hana::drop_front(t));
    return hana::all_of(adjacent, [](auto t)
    {
      return t[hana::size_c<0>].back() == t[hana::size_c<1>].front();
    });
  };


  
  auto wordResult = Amb(constraint, words1, words2, words3, words4);

  cout << "\n\nWords 1: " << hana::experimental::print(words1);
  cout << "\nWords 2: " << hana::experimental::print(words2);
  cout << "\nWords 3: " << hana::experimental::print(words3);
  cout << "\nWords 4: " << hana::experimental::print(words4);
  cout << "\nSolution: " << hana::experimental::print(wordResult) << "\n";
}

int main()
{
  AlgebraExample();
  StringExample();
}