[Haskell] Problem with constructing instances for a class Pair

23 Dec 2005


      i want the operations on pairs to work for lists 
    applying to each pair in the list. 
    i prefer to achieve this with overloading, using classes and instances
    
    i cannot get from the code for the ordinary case:
            class P a where
                outl :: a b c -> b
                ...
            data P1 p q = P1 p q    deriving (Show, Eq, Ord)
                    -- a pair like constructor    
            instance P P1 where
                outl (P1 p q) = p
    to code for the mapped case:
    
            instance P [P1] where ....
    gives
    --     - Kind error: `P1' is not applied to enough type arguments 
    --     - In the instance declaration for `P [P1]'   
    
    no other combination of parameters i can think of works for an 
    instance of class P for lists.
    
    what do i not see? any help appreciated!
    
    andrew frank
    
    the remainder is the code i have tried:
-}
module PairTest where


class P a where
    outl :: a b c -> b
    outr :: a b c -> c
    make :: b -> c -> a b c
    
data P1 p q = P1 p q    deriving (Show, Eq, Ord)
type P2 = (,)

instance P P2 where
    outl = fst
    outr = snd
    make = (,)
    
instance P P1 where
    outl (P1 p q) = p
    outr (P1 p q) = q
    make = P1

p1list :: [P1 Int Bool]
p1list = [P1 3 True, P1 4 False] 
qlist = fmap outr p1list        -- or map 

--------------can this be done as overloading of outl, outr for lists?

outlPl = fmap outl

plist = outlPl p1list

--instance P [P1] where
--    outl = fmap outl
--     - Kind error: `P1' is not applied to enough type arguments 
--     - In the instance declaration for `P [P1]'   
    

--instance P [P1 p q] where
--    outl = fmap outl
    -- Kind error: Expecting kind `* -> * -> *', but `[P1 p q]' has kind `*'

    -- In the instance declaration for `P [P1 p q]'  
 
class P3 a b c where
    outl3 :: a b c -> b
   
--instance P3 [P1 p q]    
--     - Kind error: Expecting kind `* -> * -> *', but `[P1 p q]' has kind
`*' 
--     - In the instance declaration for `P3 [P1 p q]' 
 

Andrew U. Frank
Professor, Head of Department
Geoinformation and Cartography E127       phone: +43 1 588 01 12710
TU Vienna                                 secr. +43 1 588 01 12700
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http://www.geoinfo.tuwien.ac.at/persons/frank/frank.html
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