haskell - 如何将 DU/ADT 限制为某些案例标识符/值构造函数

Question

遇到以下情况我该如何处理？
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具有相同的大小写标识符(或在 Haskell 中相同的值构造函数)

type Currency =
    | USD of decimal
    | EUR of decimal

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

例如以下内容是有效的

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

这个例子应该是无效的

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

我知道可以使用测量单位在 F# 中解决这个特定示例。但很容易想象一个无法通过该机制解决的例子。因此，我想请您考虑一个更通用的答案，而不一定是仅解决给定代码示例的答案。

期待您的脑力转储;-)

PS:对于所有 Haskeleers 来说 - 看看 ADT(也许与更高种类的类型相结合)如何解决这个问题将会很有趣。

Answer 1

“直接”翻译可能是

{-# 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，你可以使用 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

打印

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