Create with Extractor.precondition. Each call registers extractors for both PreCondition<T> and OverridablePreCondition<T>.

Each precondition type used in routes must have a corresponding extractor registered in EndpointConfig.

Example

 let config: EndpointConfig<AppError> = {
     Preconditions = [
         yield! Extractor.precondition<UserId, _> (fun ctx ->
             match getAuthCookie ctx with
             | Some id -> Ok id
             | None -> Error NotAuthenticated)
     ]
     // ... other config
 }