On Saturday 24 September 2011, 19:21:17, Johannes Engels wrote:
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
This does not mean what I think you intend. This type signature means that key can produce any type belonging to Ord, whatever the caller desires. So, I need a String, key can produce it, Bool? too, from the same value, Integer? yes, also that...
( a -> Bool x
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 (
Because, as said above, key's type says it can produce values of different types from the same argument, so the compiler can't know which type to pick, should it produce x StructsWithOrderedKeys a where type Key a key ::a -> Key a ( a -> Bool x b" is the functional dependency saying that a -- uniquely determines b class (Ord b) => StructsWithOrderedKeys a b | a -> b where key :: a -> b ( a -> Bool x