[Haskell-cafe] Currying and errors

[Graham Klyne]

At 11:31 08/11/04 -0800, John Meacham wrote:
>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)

Developing this line... I think it may be more than *just* an academic 
curiosity.  My intuition is that the latter of my example might actually 
produce a more useful diagnostic in the event of an error being generated.

I used seq in my example to avoid getting tied up with laziness, but I can 
see it might not be possible to expose the difference without some kind of 
strictness being enforced.

#g


------------
Graham Klyne
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