- 関数型プログラミング言語Haskell Part29 [転載禁止]©2ch.net
412 ：デフォルトの名無しさん[sage]：2015/08/31(月) 00:21:09.78 ID:VGvD8wHt - ()もいいけど、型をラベルみたいにして情報載せるのは、ファントムタイプの出番ですよ。
data Umai a = Umai { defaultLevel :: Int,
season :: Season}
data Season = Spring | Summer | Autumn | Winter deriving Eq
data Nasu
data Tomato
class Level a where level :: a -> Int
instance Level (Umai Nasu) where level = defaultLevel * 100
instance Level (Umai Tomato) where level = defaultLevel * (liftA (== Summer) season ?) 200 100
{--
> level (Umai 1 Summer :: Umai Nasu)
100
> level (Umai 1 Summer :: Umai Tomato)
200
> level (Umai 2 Summer :: Umai Tomato)
400
--}
| - 関数型プログラミング言語Haskell Part29 [転載禁止]©2ch.net
413 ：412[sage]：2015/08/31(月) 00:22:07.09 ID:VGvD8wHt - ちな
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FlexibleInstances,TypeFamilies #-}
module Main where
import Control.Applicative
(412の中身)
instance Num n => Num (a -> n) where
(+)= liftA2 (+)
(-) = liftA2 (-)
(*) = liftA2 (*)
abs = liftA abs
signum = liftA signum
fromInteger = const . fromInteger
class Conditional q a where
type ConditionalExec q a
(?) :: q -> ConditionalExec q a -> ConditionalExec q a -> ConditionalExec q a
infixr 1 ?
instance Conditional Bool a where
type ConditionalExec Bool a = a
(?) b x y = if b then x else y
instance Conditional (a -> Bool) b where
type ConditionalExec (a -> Bool) b = a -> b
(?) = liftA3 (?)
main :: IO ()
main = print $ level (Umai 1 Spring :: Umai Tomato)
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