如何创建许多有界类型而不在Haskell中复制代码？

Question

对于一个项目,我创建了一个基于Int的类型,只要程序试图使用超出限制的值(在我的情况下为[0..127])就会抛出错误.下面的代码执行此操作,它适用于我.

是否有可能在Haskell中创建第二个有界类型(例如[0..255])而不重复此代码？

谢谢你的回答

{-# LANGUAGE GeneralizedNewtypeDeriving #-}
module Minitel.Type.MNatural (MNat, mnat, fromMNat) where

-- | The MNat type. The constructor is hidden.
newtype MNat = MakeMNat Int deriving (Real, Eq, Ord, Show)

-- | MNat is a bounded type
instance Bounded MNat where
    minBound = MakeMNat 0
    maxBound = MakeMNat 127

-- | Converts an Int into an MNat
mnat :: Int -> MNat
mnat x | loLimit <= x && x <= hiLimit = MakeMNat x
       | otherwise = error "Number out of bounds"
       where loLimit = fromIntegral (minBound :: MNat)
             hiLimit = fromIntegral (maxBound :: MNat)

-- | Converts an MNat into an Int
fromMNat :: MNat -> Int
fromMNat (MakeMNat i) = i

-- | Converts an Int binary function returning Int to a MNat binary function
--   returning an MNat
mfnat :: (Int -> Int) -> (MNat -> MNat)
mfnat f = mnat . f . fromMNat

mfnat2 :: (Int -> Int -> Int) -> (MNat -> MNat -> MNat)
mfnat2 f x y = mnat $ f (fromMNat x) (fromMNat y)

-- | You can do additions, substractions and multiplication with MNat
instance Num MNat where
    fromInteger = mnat . fromIntegral
    (+)         = mfnat2 (+)
    (-)         = mfnat2 (-)
    (*)         = mfnat2 (*)
    abs         = mfnat abs
    signum      = mfnat signum

-- | Allows to use toInteger with MNat
instance Integral MNat where
    quotRem x y = (fromInteger $ quot x' y', fromInteger $ rem x' y')
                  where (x', y') = (toInteger x, toInteger y)
    toInteger   = toInteger . fromMNat

-- | Allows to generate lists
instance Enum MNat where
    toEnum      = mnat
    fromEnum    = fromMNat

注意:

• 表演不是问题

Answer 1

您可以使用类型级文字在GHC 7.8中执行此操作:

{-# LANGUAGE DataKinds, PolyKinds, ScopedTypeVariables #-}
module SO26723035 where

import GHC.TypeLits
import Data.Proxy

newtype MNat (n :: Nat) = MakeMNat Int deriving (Eq, Ord, Show)

instance KnownNat n => Bounded (MNat n) where
  minBound = MakeMNat 0
  maxBound = MakeMNat . fromInteger $ natVal (Proxy :: Proxy n)

 

ghci> maxBound :: MNat 5
MakeMNat 5

您可以使用类型同义词来修复单个类型.其余的代码可以MNat通过机械更改来编译这种多态.您必须添加KnownNat到处上下文和使用ScopedTypeVariables在mnat.