如何将Opaleye中的newtyped键引用为可空？

Question

我正在为所有表格使用新的键

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')

我现在想在另一张桌子上对这样一个键进行可空的引用，但我正在努力。如何将密钥引用为可空？

Answer 1

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

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

要使默认实例起作用，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)

Answer 2

将我的评论与@ ForestPhoenix的答案结合起来：

Opaleye允许您在运行查询时将Column PGInt8转换为Int64。

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

您将Column (Nullable PGInt8)包裹在Key'新类型（用于类型安全）中，它会为您提供Key' (Column (Nullable PGInt8))。这意味着当您运行查询时，您需要将其读作Key' (Maybe Int64)。

也许这张小桌子使通信更加清晰：

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

一般来说：

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