text: #ifndef STAN_MATH_PRIM_MAT_FUN_DISTANCE_HPP
  #define STAN_MATH_PRIM_MAT_FUN_DISTANCE_HPP
  
  #include <stan/math/prim/mat/fun/Eigen.hpp>
  #include <stan/math/prim/mat/fun/squared_distance.hpp>
  #include <stan/math/prim/mat/err/check_vector.hpp>
  #include <stan/math/prim/arr/err/check_matching_sizes.hpp>
  #include <boost/math/tools/promotion.hpp>
  
  namespace stan {
  namespace math {
  
  /**
   * Returns the distance between the specified vectors.
   *
   * @param v1 First vector.
   * @param v2 Second vector.
   * @return Dot product of the vectors.
   * @throw std::domain_error If the vectors are not the same
   * size or if they are both not vector dimensioned.
   */
  template <typename T1, int R1, int C1, typename T2, int R2, int C2>
  inline return_type_t<T1, T2> distance(const Eigen::Matrix<T1, R1, C1>& v1,
                                        const Eigen::Matrix<T2, R2, C2>& v2) {
    using std::sqrt;
    check_vector("distance", "v1", v1);
    check_vector("distance", "v2", v2);
    check_matching_sizes("distance", "v1", v1, "v2", v2);
    return sqrt(squared_distance(v1, v2));
  }
  
  }  // namespace math
  }  // namespace stan
  #endif
  