如何将 Opaleye 中的新类型键引用为可为空？

Question

我为所有 table

使用新类型的密钥

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')

我现在想在另一个 table 中对这样的键有一个可为 null 的引用，但我正在努力。如何将键引用为可为空？

Answer 1

Opaleye 有 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)

Answer 2

将我的评论与@ForestPhoenix 的回答结合起来：

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

这意味着它也允许您将Column (Nullable PGInt8)转换成Maybe Int64。

您将 Column (Nullable PGInt8) 包装在 Key' 新类型中（为了类型安全），它会为您提供 Key' (Column (Nullable PGInt8))。这意味着当您运行查询时，您需要将其读取为 Key' (Maybe Int64).

也许这个table使对应关系更清楚：

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

总的来说：

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

如何将 Opaleye 中的新类型键引用为可为空？

How to reference newtyped key in Opaleye as nullable?

postgresql

haskell

opaleye