[Haskell-cafe] resolving missing class instances @ compile time

[Samuel Bronson]

On 12/05/05, Greg Buchholz <haskell at sleepingsquirrel.org> wrote:
> Samuel Bronson wrote:
> > Aren't the warnings just about as usefull as failures? Anyway, you
> > could always use the -Werrror flag for ghc...
> >
> > In any case, I would not like to have to implement an entire typeclass
> > at once... it would interfere with incremental development.
> 
>     Hmm.  I guess I'm doing a terrible job of asking my question.  I
> don't want to implement the entire typeclass either.  Just the part that
> my program actually uses.  Why can't the fact that my program uses an
> unimplemented instance of a class be statically determined?  Is there a
> theoretical reason it can't be done?  Is it more convienient for
> compiler/specification writers this way?  Is it just because that's the
> way its always been done?

After thinking about it for a while, I'm positive it would be a LOT of
work to get that to work in general, if it is even possible. Even
getting it to work in only specific, limited cases (such as within a
module) would probably not be easy, since it is such an indirect kind
of thing. It probably wouldn't be all that usefull anyway, either.

In general, though, I think they don't implement stuff like this
unless someone specifically wants to *use* it.

-- Sam