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-- !!! Show/Read deriving example given in the Haskell Report.
module Main(main) where
infix 4 :^:
data Tree a
= Leaf a | (Tree a) :^: (Tree a)
deriving (Show, Read)
val1 :: Tree Int
val1 = Leaf 2
val2 :: Tree Int
val2 = Leaf 2 :^: Leaf (-1)
main = do
print val1
print val2
print ((read (show val1))::Tree Int)
print ((read (show val2))::Tree Int)
print ((read (show val1))::Tree Integer)
print ((read (show val2))::Tree Integer)
{- What you'll want
instance (Show a) => Show (Tree a) where
showsPrec d (Leaf m) = showParen (d >= 10) showStr
where
showStr = showString "Leaf " . showsPrec 10 m
showsPrec d (u :^: v) = showParen (d > 4) showStr
where
showStr = showsPrec 5 u .
showString " :^: " .
showsPrec 5 v
instance (Read a) => Read (Tree a) where
readsPrec d r = readParen (d > 4)
(\r -> [(u:^:v,w) |
(u,s) <- readsPrec 5 r,
(":^:",t) <- lex s,
(v,w) <- readsPrec 5 t]) r
++ readParen (d > 9)
(\r -> [(Leaf m,t) |
("Leaf",s) <- lex r,
(m,t) <- readsPrec 10 s]) r
-}
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