S.D.Mechveliani writes:
> Dear ghc developers,
>
> Could you, please, tell me
>
> 1. What are the ghc plans concerning the overlapping instances ?
> For it looks like ghc-3.02 does not support them.
>
It does, but the 3.02 driver doesn't `re-export' the command-line option
expected by the compiler proper. Try invoking ghc with
-optC-fallow-overlapping-instances
and see if that helps.
> 2. What is the recent ghc developers vision of overlapping instances?
> - when compared with the Choice 3a,3b from the Jones & Jones
> paper "Type classes: an exploration of the design space".
>
I'll leave the visionary statements to the authors of that paper :-)
BTW, there's a way of simulating overlapping instances within the
boundaries of the current Haskell type classes design, see
Erik Meijer's note on this
http://x7.dejanews.com/getdoc.xp?AN=317315535&CONTEXT=896959042.1530724451&hitnum=12
(it is based on the trick/hack used for the Show class in the Standard
Prelude to deal with String.)
--Sigbjorn
