Question

我将如何处理以下情况？我有一个DU（例如货币）和一些记录类型。现在对于记录类型字段，我要求给定实例的实际值应该是相同的案例标识符（或者在Haskell中使用相同的值构造函数）

type Currency =
    | USD of decimal
    | EUR of decimal

type PositionalData = {
    grossAmount: Currency;
    pos1: Currency;
    pos2: Currency;
}

例如以下内容有效

let valid = {
    grossAmount = USD 10.0m;
    pos1 = USD 7.0m;
    pos2 = USD 3.0m;
}

此示例应该无效

let wrong = {
    grossAmount = USD 10.0m;
    pos1 = USD 7.0m;
    pos2 = EUR 3.0m;
    ^^^^^^^^^^^^^^^^
}

我知道这个特定的例子可以使用测量单位在F＃中解决。但很容易想象一个通过该机制无法解决的例子。所以我想请你考虑一个更通用的答案，而不一定只是解决了代码示例。

期待你的大脑转储; - ）

PS：对于所有Haskeleers来说 - 看看ADT（可能与更高级别的kinded类型组合）如何解决这个问题会很有趣。

Answer 1

“直接”翻译可以

{-# LANGUAGE GADTs, DataKinds, KindSignatures #-}

data Currency = USD | EUR deriving Show

-- We use `Currency` values to create `Amount` types
--   read about types in Haskell ([Kinds][1]: *, * -> *, ...)
--   here we fix one * to be Currency
data Amount :: Currency -> * where
     -- Data constructor, take one float and return some Amount
     Amount :: Float -> Amount a

-- Extract the specific currency symbol require extra effort
instance Show (Amount a) where
    show (Amount k) = show k

-- Many amounts (same currency)
-- `a` restrict `a1` and `a1` to have the same type => the same currency
data PData a = PData { a1 :: Amount a
                     , a2 :: Amount a
                     } deriving Show

-- Helpers
usd :: Float -> Amount USD
usd = Amount

eur :: Float -> Amount EUR
eur = Amount

main = do

    print $ PData (usd 3) (usd 4)  -- OK
    print $ PData (eur 3) (eur 4)  -- OK
    print $ PData (eur 3) (usd 4)  -- KO, Couldn't match type 'USD with 'EUR

（1）https://wiki.haskell.org/Kind

另一方面，@ TheInnerLight记住我可以使用phantom types

-- NOTE: this is not a "direct translation" since currencies are not
--       enumerated and is slightly different
data USD = USD
data EUR = EUR

data Amount c = Amount { amount :: Float }

instance Show (Amount c) where
    show (Amount a) = show a

data PData c = PData { c1 :: Amount c
                     , c2 :: Amount c }
                       deriving Show

usd :: Float -> Amount USD
usd = Amount

eur :: Float -> Amount EUR
eur = Amount

main = do

    print $ PData (usd 3) (usd 4)  -- OK
    print $ PData (eur 3) (eur 4)  -- OK
    print $ PData (eur 3) (usd 4)  -- KO, Couldn't match type 'USD with 'EUR

提取货币符号（或任何其他数据）的一种方法可能是

class    Symbol c   where symbol :: c -> String
instance Symbol USD where symbol _ = "USD"
instance Symbol EUR where symbol _ = "EUR"

instance Symbol c => Show (Amount c) where
    show s@(Amount a) = sym undefined s ++ " " ++ show a
                        where sym :: Symbol c => c -> Amount c -> String
                              sym k _ = symbol k

印刷

PData {c1 = USD 3.0, c2 = USD 4.0}
PData {c1 = EUR 3.0, c2 = EUR 4.0}

Answer 2

您无法直接比较F＃中的DU子类型（另请参阅此答案：https://stackoverflow.com/a/30841893/3929902），但您可以使用这种迂回方式实现它：

type USD =
    Amount of decimal
type EUR =
    Amount of decimal

type Currency = 
    | USD of USD
    | EUR of EUR

type PositionalData<'T> =
  {
      grossAmount: 'T
      pos1: 'T
      pos2: 'T
  }        

let valid = {
    grossAmount = USD.Amount 10.0m;
    pos1 = USD.Amount 7.0m;
    pos2 = USD.Amount 3.0m;
}

let wrong = {
    grossAmount = USD.Amount 10.0m;
    pos1 = USD.Amount 7.0m;
    pos2 = EUR.Amount 3.0m;
}

这个答案的一个明显问题是，PositionalData不限于Currency，但可以是任何类型

Answer 3

没有办法限制特定的联合案例，因为它们本身不是类型。这意味着特定值所属的联合案例在编译时不可用。

对于这个特殊情况，正如你在问题中指出的那样，我会使用度量单位而不是受歧视的联盟来解决F＃中的问题。

为了更普遍地解决这个问题，通常最好将关系转换为这样的事情：

type PositionalData = {
    grossAmount: decimal;
    pos1: decimal;
    pos2: decimal;
}

type Currency =
    | USD of PositionalData
    | EUR of PositionalData

您可以使用类似以下类型的PositionalData使其适用：

type Currency<'a> =
    | USD of 'a
    | EUR of 'a

对于Haskell案例，我建议您查看this answer。

如何将DU / ADT限制为某些案例标识符/值构造函数

3 个答案: