Haskell:如何解开数据中嵌套的可能

Question

这里是新手!

假设我有以下数据:

data Supplier = Supplier { 
    nameS :: String,
    country :: Maybe String
}

data Product = Product {
    nameP :: String,
    supplier :: Maybe Supplier
}

我需要一个函数来返回 Maybe 产品的国家/地区，或者如果链中没有 Nothing，则返回“未知”。我可以这样做:

productCountry :: Maybe Product -> String
productCountry product =
    case product of
    Just p -> case supplier p of
              Just s -> case country s of
                        Just c -> c
                        Nothing -> "unknown"
              Nothing -> "unknown"
    Nothing -> "unknown"

但这很尴尬。另一种方法是:

import Data.Maybe (fromMaybe)

productCountry2 :: Maybe Product -> String
productCountry2 product =
    let countryMaybe = do
        p <- product
        s <- supplier p
        c <- country s
        return c
    in fromMaybe "unknown" countryMaybe

我觉得一定有更好的方法，但我找不到。

“productCountry”的最佳惯用 Haskell 代码是什么？

Answer 1

如果您正在编写一个第一个参数为 Maybe Something 类型的函数，这通常是一个好兆头，表明您不必要地避免 Maybe 的 monad 实例。

data Supplier = Supplier { 
    nameS :: String,
    country :: Maybe String
}

data Product = Product {
    nameP :: String,
    supplier :: Maybe Supplier
}

productCountry :: Product -> Maybe String
productCountry p = supplier p >>= country
-- Or, using Kleisli composition (requires Control.Monad)
-- productCountry = supplier >=> country

如果您发现自己有一个 Maybe Product 实例，请再次使用 Maybe monad 将其提供给 productCountry。

maybeProduct >>= productCountry  -- Just "somelandia" or Nothing

此外，让任何调用 productCountry 担心如果他们返回什么都没有，是否需要占位符国家/地区名称。