[Haskell-beginners] Question regarding to type classes

[Johannes Engels]

Dear list members,

as an exercise, I tried to define a type class for structures with 
ordered key values. The key values could be numbers, but also strings. 
For instance, I intend to define an address list containing records with 
names as key values. So far, my type class definition contains a 
function "key", which should extract the key values from the structures, 
and a function which compares the key values. So I tried:

class StructsWithOrderedKeys a where
-- no default definition for key
    key :: (Ord b) => a -> b

    (<?) :: a -> a -> Bool
    x <? y = (key x) < (key y)

Here I get the following error message from GHCI:

"Ambiguous type variable 'b' in the constraint:
'Ord b' arising from a use of 'key' ...
Probable fix: add a type signature that fixes these type variable(s)"

Could anybody explain what ambiguity arises here? As the arguments of 
(<?) are of the same type, I expected also the results (key x) and (key 
y) to be of the same type, which should be by the type constraint for 
"key" an instance of Ord. Why I am not allowed to use "key" in the 
definition of (<?) ?

I would be grateful for any hint!
Johannes