text: #ifndef STAN_MATH_PRIM_FUN_TANH_HPP
  #define STAN_MATH_PRIM_FUN_TANH_HPP
  
  #include <stan/math/prim/meta.hpp>
  #include <stan/math/prim/fun/Eigen.hpp>
  #include <stan/math/prim/fun/exp.hpp>
  #include <stan/math/prim/core/operator_division.hpp>
  #include <stan/math/prim/functor/apply_scalar_unary.hpp>
  #include <stan/math/prim/functor/apply_vector_unary.hpp>
  #include <cmath>
  #include <complex>
  
  namespace stan {
  namespace math {
  
  /**
   * Structure to wrap `tanh()` so that it can be vectorized.
   *
   * @tparam T type of argument
   * @param x angle in radians
   * @return Hyperbolic tangent of x.
   */
  struct tanh_fun {
    template <typename T>
    static inline T fun(const T& x) {
      using std::tanh;
      return tanh(x);
    }
  };
  
  /**
   * Vectorized version of `tanh()`.
   *
   * @tparam Container type of container
   * @param x angles in radians
   * @return Hyperbolic tangent of each value in x.
   */
  template <typename Container,
            require_not_container_st<std::is_arithmetic, Container>* = nullptr,
            require_all_not_nonscalar_prim_or_rev_kernel_expression_t<
                Container>* = nullptr>
  inline auto tanh(const Container& x) {
    return apply_scalar_unary<tanh_fun, Container>::apply(x);
  }
  
  /**
   * Version of `tanh()` that accepts std::vectors, Eigen Matrix/Array objects
   *  or expressions, and containers of these.
   *
   * @tparam Container Type of x
   * @param x Container
   * @return Hyperbolic tangent of each value in x.
   */
  template <typename Container,
            require_container_st<std::is_arithmetic, Container>* = nullptr>
  inline auto tanh(const Container& x) {
    return apply_vector_unary<Container>::apply(
        x, [](const auto& v) { return v.array().tanh(); });
  }
  
  namespace internal {
  /**
   * Return the hyperbolic tangent of the complex argument.
   *
   * @tparam V value type of argument
   * @param[in] z argument
   * @return hyperbolic tangent of the argument
   */
  template <typename V>
  inline std::complex<V> complex_tanh(const std::complex<V>& z) {
    using std::exp;
    using stan::math::operator/;
    auto exp_z = exp(z);
    auto exp_neg_z = exp(-z);
    return stan::math::internal::complex_divide(exp_z - exp_neg_z,
                                                exp_z + exp_neg_z);
  }
  }  // namespace internal
  
  }  // namespace math
  }  // namespace stan
  
  #endif
  