Transform std::expected into error or value

[mathisloge]

Is it somehow possible to transform a result of type std::expected into either an error state or a valid value?
E.g.

stdexec::schedule(scheduler) | stdexec::then([](){
   std::expected<int, MyError> my_operation = do_some_work();
   if(my_operation.has_value()) { return *my_operation; };
   
   // now set an error for MyError
}) 
| stdexec::upon_error([](MyError error) { // handleError })
| stdexec::then([](int value){});
...

Hope you know what I'm trying to do.

[maikel]

In principle that is doable but not intrinsically with then. You can define your own adaptor (which is more optimized) either by hand or by using let_value in conjunction with variant_sender, just, and just_error (which might be easier to do).

[mathisloge]

Do you happen to know if there are some simple examples to custom adaptors?
E.g. is it always the pattern as in https://github.com/NVIDIA/stdexec/blob/main/examples/algorithms/then.hpp
Have a _<op>_sender and then a _<op>_receiver which either calls stdexec::set_value or stdexec::set_error?

[maikel]

The implementation in stdexec optimizes for various things.

I made this example not very long ago for a naive implementation of the then sender, maybe that helps you. https://godbolt.org/z/5qczb9dhh

I will try to make a tutorial on sender adaptors soonish, but maybe someone else has already done so.

[mathisloge]

Thank you