如何将Opaleye中的新类型键引用为可空？

Question

我对我所有的桌子都使用新类型的键

newtype Key' a = Key a deriving (Show, Generic, Functor)
type Key = Key' Int64
type KeyR = Key' (Column PGInt8)
type KeyW = Key' (Maybe (Column PGInt8))
$(makeAdaptorAndInstance "pKey" ''Key')

我现在想要一个可空的引用，在另一个表中使用这样的键，但我正在挣扎。如何将密钥引用为可空？

Answer 1

将我的评论与@Forest凤的回答结合起来：

操作系统允许您在运行查询时将Column PGInt8转换为Int64。

这意味着它还允许您将Column (Nullable PGInt8)转换为Maybe Int64。

您正在将Column (Nullable PGInt8)包装在Key'新类型中(为了类型安全)，这将为您提供Key' (Column (Nullable PGInt8))。这意味着当您运行查询时，需要将其作为Key' (Maybe Int64)读取。

也许这张小桌子能让通信变得更清晰：

Opaleye side                  | Haskell side
----------------------------- | -------------
Column PGInt8                 | Int64
Column (Nullable PGInt)       | Maybe Int64
Key' (Column (Nullable PGInt) | Key' (Maybe Int64) 

一般而言：

Opaleye side                    | Haskell side
------------------------------- | -------------
Column o                        | h
Column (Nullable o)             | Maybe h
MyNewype' (Column (Nullable o)) | MyNewType' (Maybe h)

Answer 2

Opaleye有一个Nullable PGInt8到Maybe Int64的默认实例。

您要做的是将Key' (Column (Nullable PGInt8))转换为Maybe Key = Maybe (Key' Int64)。

要使默认实例正常工作，您需要转换

• 从Key' (Column (Nullable PGInt8)))到Key' (Maybe Int64)
• 从Column (Nullable (Key' PGInt8))到Maybe Key = Maybe (Key' Int64)