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{-
From: Kevin Hammond <kh>
To: partain
Subject: Nasty Overloading
Date: Wed, 23 Oct 91 16:19:46 BST
-}
module Main where
class Foo a where
o1 :: a -> a -> Bool
o2 :: a -> Int
-- o2 :: Int
-- Lennart: The type of method o2 does not contain the variable a
-- (and it must according to line 1 page 29 of the manual).
class Foo tyvar => Bar tyvar where
o3 :: a -> tyvar -> tyvar
-- class (Eq a, Foo a) => Baz a where
class (Ord a, Foo a) => Baz a where
o4 :: a -> a -> (String,String,String,a)
instance (Ord a, Foo a) => Foo [a] where
o2 x = 100
o1 a b = a < b || o1 (head a) (head b)
-- instance Bar [a] where
instance (Ord a, Foo a) => Bar [a] where
o3 x l = []
--
-- Lennart: I guess the instance declaration
-- instance Bar [w] where
-- o3 x l = []
-- is wrong because to be a Bar you have to be a Foo. For [w] to
-- be a Foo, w has to be Ord and Foo. But w is not Ord or Foo in
-- this instance declaration so it must be wrong. (Page 31, line
-- 7: The context c' must imply ...)
instance Baz a => Baz [a] where
o4 [] [] = ("Nil", "Nil", "Nil", [])
o4 l1 l2 =
(if o1 l1 l2 then "Y" else "N",
if l1 == l2 then "Y" else "N",
-- if o4 (head l1) (head l2) then "Y" else "N",
case o4 (head l1) (head l2) of
(_,_,_,l3) -> if (o1 (head l1) l3) then "Y" else "N",
l1 ++ l2 )
instance Foo Int where
o2 x = x
o1 i j = i == j
instance Bar Int where
o3 _ j = j + 1
instance Baz Int where
-- o4 i j = i > j
o4 i j = (if i>j then "Y" else "Z", "p", "q", i+j)
--simpl:o4 i j = ("Z", "p", "q", i+j)
{- also works w/ glhc! -}
main = if o4 [1,2,3] [1,3,2::Int] /= ("Y","N","Y",[1,2,3,1,3,2]) then
(print "43\n")
else (print "144\n")
{- works: glhc
main = case o4 [1,2,3] [1,3,2::Int] of
(s1,s2,s3,x) -> print s1
main = case o4 ([]::[Int]) ([]::[Int]) of
(s1,s2,s3,x) -> print s1
-}
{- simple main: breaks nhc, works w/ glhc
main = case o4 (3::Int) (4::Int) of (s1,s2,s3,x) -> print s1
-}
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