Add monadic bind operators, >= and >

Signed-off-by: Wolfgang E. Sanyer <[email protected]>

include/tl/expected.hpp

@@ -1251,6 +1251,32 @@ class expected : private detail::expected_move_assign_base<T, E>,
   typedef E error_type;
   typedef unexpected<E> unexpected_type;
 
+  /**
+   *  The "monadic bind" operation. In short:
+   *
+   *  1. if `this` contains an Err, return `this`
+   *  2. otherwise, return result of calling the supplied function `f`
+   *     with the Val contained in `this` as an arguement
+   *
+   *  Since it returns another `expected` these can be chained.
+   */
+  template<typename F>
+  auto operator>=(F f) -> std::invoke_result_t<F, T>
+  {
+      return has_value() ? f(value()) : *this;
+  }
+
+  /**
+   *  Like `>=` operator, except the function does not take any
+   *  arguments. In other words, discard the results from the previous
+   *  computation.
+   */
+  template<typename F>
+  auto operator>(F f) -> std::invoke_result_t<F>
+  {
+      return has_value() ? f() : *this;
+  }
+
 #if defined(TL_EXPECTED_CXX14) && !defined(TL_EXPECTED_GCC49) &&               \
     !defined(TL_EXPECTED_GCC54) && !defined(TL_EXPECTED_GCC55)
   template <class F> TL_EXPECTED_11_CONSTEXPR auto and_then(F &&f) & {