Real World Haskell - 第3章 Exercises Part1

Real World Haskell で Haskell を継続的に勉強中。

Defining Types, Streamlining Functions
Chapter 3. Defining Types, Streamlining Functions

Exercises (Section "Recursive types" の最後にあるもの)

1. Write the converse of fromList for the List type: a function that takes a List a and generates a [a].

2. Define a tree type that has only one constructor, like our Java example. Instead of the Empty constructor, use the Maybe type to refer to a node's children.

1

data List a = Cons a (List a)
            | Nil
              deriving (Show)
--fromList (x:xs) = Cons x (fromList xs)
--fromList []     = Nil

test1 = Nil
test2 = Cons 0 Nil
test3 = Cons 1 (Cons 0 Nil)
test4 = Cons 2 (Cons 1 (Cons 0 Nil))
test5 = Cons 3 (Cons 2 (Cons 1 (Cons 0 Nil)))

toList (Cons x xs) = x : (toList xs)
toList Nil    = []

GHCi, version 6.10.1: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer ... linking ... done.
Loading package base ... linking ... done.
Prelude> :load ex3--1.hs
[1 of 1] Compiling Main             ( ex3--1.hs, interpreted )
Ok, modules loaded: Main.
*Main> toList test1
[]
*Main> toList test2
[0]
*Main> toList test3
[1,0]
*Main> toList test4
[2,1,0]
*Main> toList test5
[3,2,1,0]

2

{-
data Maybe a = Just a
            | Nothing

data Tree a = Node a (Tree a) (Tree a)
            | Empty
              deriving (Show)

simpleTree = Node "parent" (Node "left child" Empty Empty)
                           (Node "right child" Empty Empty)
-}

data Tree a = Node a (Maybe (Tree a)) (Maybe (Tree a))
              deriving (Show)

test1 = Node "parent" Nothing Nothing
test2 = Node "parent" Nothing (Just (Node "right child" Nothing Nothing))
test3 = Node "parent" (Just (Node "left child" Nothing Nothing)) Nothing
test4 = Node "parent" (Just(Node "left child" Nothing Nothing))
                      (Just(Node "right child" Nothing Nothing))
test5 = Node "parent" (Just(Node "left child" (Just(Node "grandchild" Nothing Nothing)) (Just(Node "grandchilde" Nothing Nothing))))
                      (Just(Node "righit child" (Just(Node "grandchild" Nothing Nothing)) (Just(Node "grandchilde" Nothing Nothing))))

GHCi, version 6.10.1: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer ... linking ... done.
Loading package base ... linking ... done.
Prelude> :load ex3--2.hs
[1 of 1] Compiling Main             ( ex3--2.hs, interpreted )
Ok, modules loaded: Main.
*Main> :info Tree
data Tree a = Node a (Maybe (Tree a)) (Maybe (Tree a))
        -- Defined at ex3--2.hs:13:5-8
instance (Show a) => Show (Tree a) -- Defined at ex3--2.hs:14:24-27
*Main> test1
Node "parent" Nothing Nothing
*Main> test2
Node "parent" Nothing (Just (Node "right child" Nothing Nothing))
*Main> test3
Node "parent" (Just (Node "left child" Nothing Nothing)) Nothing
*Main> test4
Node "parent" (Just (Node "left child" Nothing Nothing)) (Just (Node "right chil
d" Nothing Nothing))
*Main> test5
Node "parent" (Just (Node "left child" (Just (Node "grandchild" Nothing Nothing)
) (Just (Node "grandchilde" Nothing Nothing)))) (Just (Node "righit child" (Just
 (Node "grandchild" Nothing Nothing)) (Just (Node "grandchilde" Nothing Nothing)
)))
*Main>