Conditionally choose return value or error

[BartolomeyKant]

I'm not sure if this topic was discussed anywhere, but I can't find the solution.

let's have this snippet

...
| let_value([](int val) {
   if (val % 2 == 0) {
     return just(val);
   }
   else {
     return just_error(val);
   }
});

As you may notice I'm trying to return different senders depend on val. And it's not compile.

So the question is how in runtime choose set_value or set_error?

The obvious solution is just make the new Sender with logic on its op_state::start. But it's a lot of boilerplate code for each case there I need it.

libunifex has generic create sender. Why don't we have same in stdexec?
Or maybe there something like:

{
  if (val %2 == 0) {
     return just<set_value_t(int), set_error_t(int)>::set_value(v);
  }
  else {
     return just<set_value_t(int), set_error_t(int)>::set_error(v);
  }
}

?

How do you usually solve such task?