On Mon, Nov 08, 2004 at 02:20:45PM +0000, Graham Klyne wrote:
I just found myself writing a function that looked like this:
isSubsumedByWith :: TBox c -> c -> c -> Bool isSubsumedByWith [] c d = isALSubsumedBy c d isSubsumedByWith _ _ _ = error "TBox reasoning not supported for AL"
and immediately noticed that I might also write this:
isSubsumedByWith :: TBox c -> c -> c -> Bool isSubsumedByWith [] = isALSubsumedBy isSubsumedByWith _ = error "TBox reasoning not supported for AL"
which led me to thinking about the difference between these two functions (I reason there must be a difference, because the call of 'error' is required to fulfil (terminology?) values of different types).
I think it is this: Suppose I evaluate an expression:
let s = isSubsumedByWith [foo] in seq s e
then I think the first case will return a legitimate function, albeit one that returns error when it is applied, and the second will cause an error to be returned immediately. Am I right? Is this all?
It is my understanding (someone correct me if I am wrong) that 'seq' is the one and only way to determine the difference between _|_ and \_ -> _|_ and this causes some theoretical problems. I would actually like to hear more about what sort of problems this causes from a theory point of view if anyone has some references. (I know the practical advantages of having seq in the language are great so this is just academic curiosity) John -- John Meacham - ⑆repetae.net⑆john⑈