模式匹配幻像类型

Question

我正在尝试实现一个 CurrencyQty 类型，它的作用类似于在编译时标记的数字：

data Currency = Usd | Eur | Gbp

data CurrencyQty (a :: Currency) = CurrencyQty Double deriving (Num)

现在我想实现一个动态查找汇率的通用转换函数。假设我有一些功能

currentExchangeRate :: Currency -> Currency -> IO Double

我要写

inUsd :: CurrencyQty a -> IO (CurrencyQty Usd)
inUsd (CurrencyQty Usd x) = return x
inUsd (CurrencyQty Eur x) = fmap (*x) $ currentExchangeRate Usd Eur
inUsd (CurrencyQty Gbp x) = fmap (*x) $ currentExchangeRate Usd Gbp

或者不知何故

inUsd :: CurrencyQty a -> IO (CurrencyQty Usd)
inUsd (CurrencyQty a x) = fmap (*x) $ currentExchangeRate Usd a

我使用的语法显然无效haskell...有没有办法做到这一点？

Answer 1

你不能为此使用幻影。 Phantom 类型在运行时消失，您的 inUsd 函数需要一些运行时信息。

通常的方法是使用 GADT 和单例类型。

-- the "index" type
data Currency = Usd | Eur | Gbp

-- the "singleton" type
-- If you want to autogenerate this, check out the singletons
-- package on Hackage
data SCurrency (a :: Currency) where
   SUsd :: Scurrency Usd
   SEur :: Scurrency Eur
   SGbp :: Scurrency Gbp

-- the "indexed" type
data CurrencyQty (a :: Currency) where
  CurrencyQty :: SCurrency a -> Double -> CurrencyQty a

instance Num (CurrencyQty a) where
   ... -- you have to manually write this, I guess?

inUsd :: CurrencyQty a -> IO (CurrencyQty Usd)
inUsd (CurrencyQty SUsd x) = return x
inUsd (CurrencyQty SEur x) = fmap (*x) $ currentExchangeRate Usd Eur
inUsd (CurrencyQty SGbp x) = fmap (*x) $ currentExchangeRate Usd Gbp

让我补充一下，您的代码没问题。如果 Haskell 具有完整的依赖类型，则可以在稍作调整后使用您的代码，并避免使用额外的单例类型。但目前Haskell无法避免，还需要付出更多努力。

模式匹配幻像类型

Pattern match phantom type

haskell

phantom-types

data-kinds