Re: [Haskell-cafe] question about type constructors

23 May 2013


      Roman Cheplyaka wrote:
...
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
Thanks a lot. Now I am using FlexibleContexts, and it works correctly (see 
code below). I think I have understood the problem.
However, I have still one question. In the code below, I have added data 
constructors "Child2", "Child3" (imagining a world where every people has 
three children). The problem is that I am compelled to repeat the context 
"(PrettyPrint (Person a), PrettyPrint (Person b))" for each one of the 
constructors. Is there any way to specify the context only once? I have 
tried using "forall", but without any success.

Thanks,

TP

-------------
{-# LANGUAGE GADTs #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE FlexibleContexts #-}

class PrettyPrint a where
    prettify :: a -> String

data Gender = Male | Female | Gender3 | Gender4

data Person :: Gender -> * where
    Person :: String -> Person b
    Child1 :: (PrettyPrint (Person a)
             , PrettyPrint (Person b)
             ) => String -> Person a -> Person b -> Person c
    Child2 :: (PrettyPrint (Person a)
             , PrettyPrint (Person b)
             ) => String -> Person a -> Person b -> Person c
    Child3 :: (PrettyPrint (Person a)
             , PrettyPrint (Person b)
             ) => String -> Person a -> Person b -> Person c

instance PrettyPrint (Person Male) where
    prettify (Person name) = "My name is " ++ (show name)
          ++ " and I am a male"
    prettify (Child1 name person1 person2) = "My name is " ++ (show name)
          ++ " and my parents are:" ++ (prettify person1) ++ ", "
          ++ (prettify person2)

main = do

let a = Person "Jim" :: Person Male
let b = Person "Joe" :: Person Male
let c = Child1 "Jack" a b :: Person Male

print $ prettify a
print $ prettify b
print $ prettify c