let.hpp

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#pragma once
 
#include <functional>
#include <optional>
 
namespace CppSpec {

class LetBase {
 protected:
  bool delivered{false};
 
 public:
  constexpr LetBase() noexcept = default;
  LetBase(const LetBase& copy) = default;
  void reset() noexcept { delivered = false; }
  [[nodiscard]] constexpr bool has_result() const noexcept { return this->delivered; }
};

template <typename T>
class Let : public LetBase {
  using block_t = std::function<T()>;
  std::optional<T> result;
 
  block_t body;
 
  void exec();
 
 public:
  explicit Let(block_t body) noexcept : LetBase(), body(body) {}
 
  T* operator->() {
    return std::addressof(value());
  }
 
  T& operator*() & { return value(); }
  T& value() &;
};

template <typename T>
void Let<T>::exec() {
  if (!delivered) {
    result = body();
    delivered = true;
  }
}

template <typename T>
T& Let<T>::value() & {
  exec();
  return result.value();
}
 
}  // namespace CppSpec