Hi Kirstin,
| Surely this is obvious, but I cannot figure out how to properly deal with
| class constraints and monads. For instance, when trying
|
| instance Monad Set.Set where
| xs >>= f = Set.unionSet (Set.map f xs)
| return x = Set.single x
| fail s = Set.empty
|
| hugs complains that it "Cannot justify constraints in instance member
| binding" for >>=. unionSet type is Eq a => Set (Set a) -> Set a
This is a long standing problem in Haskell. Off the top of my head, I
can think of a couple of papers that talk about ways of addressing it.
The first is John Hughes' paper on "Restricted Datatypes in Haskell"
from the 1999 Haskell Workshop:
http://www.cs.chalmers.se/~rjmh/Papers/restricted-datatypes.ps
The second is Simon Peyton Jones' paper on "Bulk types with class" from
the 1996 Glasgow FP Workshop:
http://research.microsoft.com/Users/simonpj/Papers/collections.ps.gz
Hope these references are of some help!
All the best,
Mark
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