Henning Thielemann
class PERSON a b where p1 :: a name :: a -> b
A multi-parameter typeclass is a relation over types...
instance PERSON Int String where p1 = 1 name p1 = "john" ^--note that this is just an unused paramter, it is clearer to specify 'name _ = "john"'
...so this instance gives the PERSON relation for Int and String. So other PERSONs would have different types, say: instance PERSON Double Int where p1 = 3.14 name = 200 Does that make sense? It looks a bit strange to me.
The problem is certainly, that a Haskell interpreter has no way to infer types, thus you have to annotate type of both argument and result of 'name', and type annotation for 'p1' is not possible at all, because 'b' does not occur in its type. You have to write
name (12::Int) :: String
And (correct me if I'm wrong) there's no way to do that for p1, since there is no way to specify which type 'b' you want. You could have instance PERSON Int String where p1 = 1 name = "john" and instance PERSON Int Double where p1 = 24 name = 3.14 then the system would have no way to figure out which value you want even if you specify (p1::Int). Are you sure you didn't want: data Person = P Int String ? -k -- If I haven't seen further, it is by standing in the footprints of giants