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module Shapes (Point, Shape, area, nudge, baseCircle, baseRect) where | |
-- data Shape = Circle Float Float Float | | |
-- Rectangle Float Float Float Float | |
-- data Shape = Circle Float Float Float | | |
-- Rectangle Float Float Float Float | |
-- deriving (Show) | |
data Point = Point Float Float deriving (Show) | |
data Shape = Circle Point Float | Rectangle Point Point deriving (Show) | |
-- area :: Shape -> Float | |
-- area (Circle _ _ r) = pi * r ^ 2 | |
-- area (Rectangle x1 y1 x2 y2) = (abs $ x2 - x1) * (abs $ y2 - y1) | |
area :: Shape -> Float | |
area (Circle _ r) = pi * r ^ 2 | |
area (Rectangle (Point x1 y1) (Point x2 y2)) = (abs $ x2 - x1) * (abs $ y2 - y1) | |
baseCircle :: Float -> Shape | |
baseCircle r = Circle (Point 0 0) r | |
baseRect :: Float -> Float -> Shape | |
baseRect w h = Rectangle (Point 0 0) (Point w h) | |
nudge :: Shape -> Float -> Float -> Shape | |
nudge (Circle (Point x y) r) a b = Circle (Point (x+a) (y+b)) r | |
nudge (Rectangle (Point x1 y1) (Point x2 y2)) a b = Rectangle (Point (x1+a) (y1+b)) (Point (x2+a) (y2+b)) |
import Shapes
-- main = do
-- print $ Circle (Point 10 20) 30
-- #=> Data constructor not in scope: Circle :: t0 -> Integer -> a0
-- #=> Data constructor not in scope: Point :: Integer -> Integer -> t0
main = do
print $ nudge (baseCircle 30) 10 20
コメントアウトした部分はエラーになる。 Circle Point Float
という値コンストラクタを隠蔽したため
module Shapes (Point(..), Shape(..), ...)
と書けば値コンストラクタもエクスポートされる
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すごいHaskellたのしく学ぼう! 『第7章 型や型クラスを自分で作ろう』より