* TP
Hi,
In the program I am trying to write, I have a problem that can be reduced to the following dummy example:
-------------------------- {-# LANGUAGE GADTs #-} {-# LANGUAGE DataKinds #-} {-# LANGUAGE KindSignatures #-} {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE IncoherentInstances #-}
class PrettyPrint a where prettify :: a -> String
data Gender = Male | Female | Gender3 | Gender4
data Person :: Gender -> * where Person :: String -> Person b Child :: String -> Person a -> Person b -> Person c
instance PrettyPrint (Person a)
instance PrettyPrint (Person Male) where prettify (Person name) = "My name is " ++ (show name) ++ " and I am a male" prettify (Child name person1 person2) = "My name is " ++ (show name) ++ " and my parents are:" ++ (prettify person1) ++ ", " ++ (prettify person2)
main = do
let p1 = Person "Jim" :: Person Male let p2 = Person "Joe" :: Person Male let p3 = Child "Jack" p1 p2
print $ prettify p1 print $ prettify p2 print $ prettify p3 --------------------------
The idea is that I want to implement PrettyPrint only for a subset of the possible types in Gender (through promotion process). Why? It would be longer to explain (it is a bit more complicated in my real program).
Anyway, in the program above, I have found that with IncoherentInstances (and the empty instance definition for (Person a)), it is working, it is able to use the most specific instance corresponding to the current type (I don't know exactly why). For example, p1 and p2 are correctly printed above, because they are of type (Person Male) and because I have implemented PrettyPrint for (Person Male).
But it does not work for p3, I obtain an error at runtime: ----- $ runghc test.hs "My name is \"Jim\" and I am a male" "My name is \"Joe\" and I am a male" test_typelost.hs: test_typelost.hs:16:10-31: No instance nor default method for class operation Main.prettify -----
The reason is that the information that p1 and p2 are Male seems to be "lost" when we construct the child "Child "Jack" p1 p2", probably because GHC only sees that in the type signature of Child, we have a more general (Person a) -> (Person b). So he tries to find an implementation of prettify in PrettyPrint (Person a), but there is none.
Is there any workaround?
The rule of thumb is that you should never use IncoherentInstances. The proper way to do it is: data Person :: Gender -> * where Person :: String -> Person b Child :: (PrettyPrint a, PrettyPrint b) => String -> Person a -> Person b -> Person c Roman