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型や型引数を自分でつくろう

7.2 形づくる

• 宿題
  • 3点のPointから、それらを点を結んだ時の三角形の面積を求める関数を実装してください。

data Point = Point Float Float
data Triangle = Triangle Point Point Point

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5 高階関数　課題

下記length', all'をfoldl, foldr, foldl1, foldr1のいずれかを用いて実装せよ。 length', all'の挙動はlength, allを参照すること。

length' :: [a] -> Int

all' :: (a -> Bool) -> [a] -> Bool 

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length' :: String -> Int
length' = foldl (\ x _ -> x + 1) 0

last' :: Show a => [a] -> (Maybe a)
last' = foldl (\ _ x -> Just x) Nothing

head' :: Show a => [a] -> (Maybe a)
head' [] = Nothing
head' (x:_) = Just x

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import System.Random

dice :: StdGen -> Int
dice gen = num
    where (num, _) = diceWithRandomGen gen

diceWithRandomGen :: StdGen -> (Int, StdGen)
diceWithRandomGen = randomR (1, 6)

threeTimesDiceRoll :: StdGen -> (Int, Int, Int)