Henning Thielemann wrote:
You know, in Haskell we do not need a built-in exception handling facility because we can handle it with the elements of the language. Returning an exceptional value or throwing an exception is the same. We can only hide the exception propagation by appropriate binding of actions.
I don't consider exceptions in the IO monad to be a "built-in" concept. This is because, if we wanted to, we could implement IO exceptions purely in Haskell on top of a primitive IO monad without exceptions (indeed, that's what Hugs and other systems do, and what GHC did in the past). If you are implementing a Haskell system, you don't need to provide any primitive functionality to support exceptions, it can all be done in the implementation of the IO monad. Exceptions are no more built-in than the Either type. In practice, you might want to implement exceptions at a lower level for performance reasons, which is what GHC does. Cheers, Simon