Daniel Brown wrote:
class Baz a b | a - b
instance Baz (a - b) (a - [b])
instance Baz a a
...but Baz fails with this error...
When confronted with overlapping instances, the compiler chooses the
most specific one (if it is unique), e.g. `Baz (a - b) (a - [b])` is
more specific
This is still an ad-hoc solution, cause you lose
the `most-specific' instance property. You really have to
impose a `fixed' ordering in which instance-improvement rules
fire.
Recap:
The combination of overlapping instances
and type improvement leads to a `non-confluent' system, i.e.
there're
Martin's dead right. GHC uses a less sophisticated mechanism to do
matching when it's thinking about functional dependencies than when it's
doing straight instance matching. Maybe something cleverer for fundeps
would make sense, as you point out. I hadn't thought of that before;
it's a good
Simon's dead right, too :) The issue raised here is of general nature and
doesn't depend on the particular (syntactic) formalism used to specify
type dependencies (let it be FDs, ATs,...). The consequence is that
instances and type dependencies are closer linked to each other
then one might think
