gistfile1.txt

interface IWebApplication<abstract TResult>
{
   TResult MapGet(string route, Func<TResult, TResult> f);
}

class NotFound : IResult { }
class OK<TValue> : IResult { }

class MyWebApp : IWebApplication<IResult>
{
   IResult MapGet(string route, Func<TResult, TResult f) => f(OK<int>(200));
}