defaults

[Malcolm Wallace]

Bernie Pope <bjpop at csse.unimelb.edu.au> wrote:

> > I don't see a proposal to remove defaulting defaulting altogether on
> > that page - has that been discussed already?
> >
> > Defaulting is one wart I would be glad to be rid of.
> 
> I would also be happy if it was removed.

Me too, in some ways.  But...

> 1) It makes teaching Haskell more difficult, because it is a special  
>     case mechanism. I would prefer consistency here.

Unfortunately, I suspect that teaching is _the_ major use-case for
defaulting.  Imagine, day one, lesson one, a student types

    Prelude> 1+2

into Hugs, and gets the response

    Unresolved overloading: Num a

Huh?  This is lesson one, and you already need to tell students about
type classes and overloading, before you have even covered simple
expressions fully?  I am certain this is the reason why defaulting was
introduced.

> 2) It makes source-to-source program transformations more difficult,
> as found in Hat etc.

Sure, I'd be glad to improve that.  Removing defaults altogether would
solve the problem for the average user.  But power users like Duncan
would like to _extend_ defaulting to work over GUI type classes, and
that too seems a reasonable request to me.  Does my proposal for
revision of defaults allow that, and still make Hat-style transformation
easier?  Yes, I think so.

Regards,
    Malcolm