/******************************************************************************
 * Top contributors (to current version):
 *   Tim King, Mathias Preiner
 *
 * This file is part of the cvc5 project.
 *
 * Copyright (c) 2009-2021 by the authors listed in the file AUTHORS
 * in the top-level source directory and their institutional affiliations.
 * All rights reserved.  See the file COPYING in the top-level source
 * directory for licensing information.
 * ****************************************************************************
 *
 * This provides a templated Maybe construct.
 *
 * This class provides a templated Maybe<T> construct.
 * This follows the rough pattern of the Maybe monad in haskell.
 * A Maybe is an algebraic type that is either Nothing | Just T
 *
 * T must support T() and operator=.
 *
 * This has a couple of uses:
 * - There is no reasonable value or particularly clean way to represent
 *   Nothing using a value of T
 * - High level of assurance that a value is not used before it is set.
 */
#include "cvc5_public.h"

#ifndef CVC5__UTIL__MAYBE_H
#define CVC5__UTIL__MAYBE_H

#include <ostream>

#include "base/exception.h"

namespace cvc5 {

template <class T>
class Maybe
{
 public:
  Maybe() : d_just(false), d_value(){}
  Maybe(const T& val): d_just(true), d_value(val){}

  Maybe& operator=(const T& v){
    d_just = true;
    d_value = v;
    return *this;
  }

  inline bool nothing() const { return !d_just; }
  inline bool just() const { return d_just; }
  explicit operator bool() const noexcept { return just(); }

  void clear() {
    if(just()){
      d_just = false;
      d_value = T();
    }
  }

  const T& value() const
  {
    if (nothing())
    {
      throw Exception("Maybe::value() requires the maybe to be set.");
    }
    return d_value;
  }

 private:
  bool d_just;
  T d_value;
};

template <class T>
inline std::ostream& operator<<(std::ostream& out, const Maybe<T>& m){
  out << "{";
  if(m.nothing()){
    out << "Nothing";
  }else{
    out << "Just ";
    out << m.value();
  }
  out << "}";
  return out;
}

}  // namespace cvc5

#endif /* CVC5__UTIL__MAYBE_H */