[Haskell-cafe] question about type constructors

22 May 2013

      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?

Thanks,

TP