text: #ifndef STAN_MATH_PRIM_FUN_VALUE_OF_REC_HPP
  #define STAN_MATH_PRIM_FUN_VALUE_OF_REC_HPP
  
  #include <stan/math/prim/meta.hpp>
  #include <stan/math/prim/fun/Eigen.hpp>
  #include <complex>
  #include <cstddef>
  #include <vector>
  
  namespace stan {
  namespace math {
  
  /**
   * Return the value of the specified scalar argument
   * converted to a double value.
   *
   * <p>See the <code>primitive_value</code> function to
   * extract values without casting to <code>double</code>.
   *
   * <p>This function is meant to cover the primitive types. For
   * types requiring pass-by-reference, this template function
   * should be specialized.
   *
   * @tparam T Type of scalar.
   * @param x Scalar to convert to double.
   * @return Value of scalar cast to a double.
   */
  template <typename T, typename = require_stan_scalar_t<T>>
  inline double value_of_rec(const T x) {
    return static_cast<double>(x);
  }
  
  /**
   * Return the specified argument.
   *
   * <p>See <code>value_of(T)</code> for a polymorphic
   * implementation using static casts.
   *
   * <p>This inline pass-through no-op should be compiled away.
   *
   * @param x Specified value.
   * @return Specified value.
   */
  inline double value_of_rec(double x) { return x; }
  
  /**
   * Recursively apply value-of to the parts of the argument.
   *
   * @tparam T value type of argument
   * @param[in] x argument
   * @return real complex value of argument
   */
  template <typename T>
  inline std::complex<double> value_of_rec(const std::complex<T>& x) {
    return {value_of_rec(x.real()), value_of_rec(x.imag())};
  }
  
  /**
   * Convert a std::vector of type T to a std::vector of doubles.
   *
   * T must implement value_of_rec. See
   * test/math/fwd/fun/value_of_rec.cpp for fvar and var usage.
   *
   * @tparam T Scalar type in std::vector
   * @param[in] x std::vector to be converted
   * @return std::vector of values
   **/
  template <typename T>
  inline std::vector<double> value_of_rec(const std::vector<T>& x) {
    size_t x_size = x.size();
    std::vector<double> result(x_size);
    for (size_t i = 0; i < x_size; i++) {
      result[i] = value_of_rec(x[i]);
    }
    return result;
  }
  
  /**
   * Return the specified argument.
   *
   * <p>See <code>value_of_rec(T)</code> for a polymorphic
   * implementation using static casts.
   *
   * <p>This inline pass-through no-op should be compiled away.
   *
   * @param x Specified std::vector.
   * @return Specified std::vector.
   */
  inline const std::vector<double>& value_of_rec(const std::vector<double>& x) {
    return x;
  }
  
  /**
   * Convert a matrix of type T to a matrix of doubles.
   *
   * T must implement value_of_rec. See
   * test/unit/math/fwd/fun/value_of_test.cpp for fvar and var usage.
   *
   * @tparam T Type of matrix
   * @param[in] M Matrix to be converted
   * @return Matrix of values
   **/
  template <typename T, typename = require_not_st_same<T, double>,
            typename = require_eigen_t<T>>
  inline auto value_of_rec(const T& M) {
    return M.unaryExpr([](auto x) { return value_of_rec(x); });
  }
  
  /**
   * Return the specified argument.
   *
   * <p>See <code>value_of_rec(T)</code> for a polymorphic
   * implementation using static casts.
   *
   * <p>This inline pass-through no-op should be compiled away.
   *
   * @tparam T Type of matrix.
   * @param x Specified matrix.
   * @return Specified matrix.
   */
  template <typename T, typename = require_st_same<T, double>,
            typename = require_eigen_t<T>>
  inline const T& value_of_rec(const T& x) {
    return x;
  }
  }  // namespace math
  }  // namespace stan
  
  #endif
  