Thanks!

It is possible to make the type inference smart enough to not need to explicitly type the types:

The reason that you have to provide all type arguments in railroad is not because they can’t be interference of the argument. It is because it doesn’t know that formatName is going to introduce a new error type in the then. Not declaring all the error types means that formatName cannot be composed with getUser because they have incompatible types for e.

The solution is relatively simple: update then so that the new RailRoad has e | newE as its error type. With this we can expand the possible errors in e which each call to then which makes it more composable. e will ways need to be listed because generic types are the only way to have a wildcard type we can infer (‘private’ generics would be ultimately cool to make to kind of constructs).