[Haskell-cafe] Typeclass question

[Jonathan Tang]

I've got what's probably a beginner's question, but I'm out of ideas
for solving it.  It looks like it tripped me up in "Write Yourself a
Scheme..." too, since the code there seems like it's arranged so I
never ran into it...

I've got a couple functions:

binop :: (AmbrosiaData -> AmbrosiaData -> AmbrosiaM AmbrosiaData) ->
AmbrosiaM ()
binop operator = do
  second <- popStack
  first <- popStack
  result <- operator first second
  pushStack result

numNumOp ::  Num a => (a -> a -> a) -> AmbrosiaData -> AmbrosiaData ->
AmbrosiaM AmbrosiaData
numNumOp op (Number val1) (Number val2) = return $ Number $ op val1 val2
numNumOp op (Float val1) (Float val2) = return $ Float $ op val1 val2
numNumOp op _ _ = throwError . TypeMismatch $ "Number"

the intention being that I can define primitive operations for an
interpreter with   ("+", binop $ numNumOp (+)).  In the Number case,
val1 & val2 are Integers, while in the Float case, they're Floats.
However, when I try to compile, I get:

./Primitives.hs:82:
    Cannot unify the type-signature variable `a'
        with the type `Integer'
        Expected type: Integer
        Inferred type: a
    In the application `op val1 val2'
    In the second argument of `($)', namely `op val1 val2'

This makes no sense to me, since Integer is an instance of Num and op
is defined as type Num a => a -> a -> a.  Shouldn't the typechecker be
able to instantiate 'a' with Integer and give the resulting type
Integer -> Integer -> Integer, and then pass the final Integer to
Number?  Is my understanding of typeclasses off somehow?

Any help would be appreciated.

Regards,
Jonathan