Re: [Haskell-cafe] Adding Ord constraint to instance Monad Set?

31 Mar 2004


      On Tue, 30 Mar 2004, S. Alexander Jacobson wrote:
...
Following the declaration of "instance Monad []"
in the prelude, and puzzling over the absence of
its equivalent from Data.Set, I naively typed:
instance Monad Set where
  m >>= k = concatSets (mapSet k m)
  return x = unitSet x
  fail s = emptySet
concatSets sets = foldl union emptySet (setToList sets)
   instance (Eq b,Ord b) => Ord (Set b) where
  compare set1 set2 = compare (setToList set1) (setToList set2)
and got the following error:
Could not deduce (Ord b) from the context (Monad Set)
      arising from use of `concatSets' at dbMeta3.hs:242
    Probable fix:
        Add (Ord b) to the class or instance method `>>='
    In the definition of `>>=': >>= m k = concatSets (mapSet k m)
    In the definition for method `>>='
    In the instance declaration for `Monad Set'
Surprisingly this is exact the same problem I posed in my mail "Context
for type parameters of type constructors" just before. I wanted to create
the class VectorSpace which is the analogon to Monad in your example. I'm
excited if the answers to your question will help me. :-)

Re: [Haskell-cafe] Adding Ord constraint to instance Monad Set?

Henning Thielemann