Haskell "persistent"模型：如何正确定义交叉引用？

Question

假设您有两个模型（Foo 和 Bar）并且它们都相互引用（ Foo 有 barRef 类型 BarId 和 Bar 有 fooRef 类型 FooId)。一切顺利：

#!/usr/bin/env stack
{- stack script --resolver=lts-9.21 --package=persistent-template -}
{-# LANGUAGE GADTs, GeneralizedNewtypeDeriving, QuasiQuotes, TemplateHaskell #-}
{-# LANGUAGE TypeFamilies, MultiParamTypeClasses #-}

import Database.Persist.TH

mkPersist sqlSettings [persistLowerCase|
Foo
  barModel BarId
  fooField Int

Bar
  fooModel FooId
  barField Int
|]

main = pure ()

但是如果我将引用类型包装到 Maybe（只是一个例子，它可以是 Vector 或其他任何类型）：

#!/usr/bin/env stack
{- stack script --resolver=lts-9.21 --package=persistent-template -}
{-# LANGUAGE GADTs, GeneralizedNewtypeDeriving, QuasiQuotes, TemplateHaskell #-}
{-# LANGUAGE TypeFamilies, MultiParamTypeClasses #-}

import Database.Persist.TH

mkPersist sqlSettings [persistLowerCase|
Foo
  barModel (Maybe BarId)
  fooField Int

Bar
  fooModel FooId
  barField Int
|]

main = pure ()

失败并出现此错误：

Not in scope: type constructor or class ‘BarId’

我该如何解决这个问题？

Answer 1

您用于 Bar 模型的语法不正确。这样的事情应该有效：

#!/usr/bin/env stack
-- stack script --resolver lts-12.7

{-# LANGUAGE GADTs, GeneralizedNewtypeDeriving, QuasiQuotes, TemplateHaskell #-}
{-# LANGUAGE TypeFamilies, MultiParamTypeClasses #-}

import Database.Persist.TH

mkPersist sqlSettings [persistLowerCase|
Foo
  barModel BarId Maybe
  fooField Int

Bar
  fooModel FooId
  barField Int
|]

main = pure ()

参考official docs。

Haskell "persistent"模型：如何正确定义交叉引用？

Haskell "persistent" model: How to correctly define cross-reference?

database

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haskell-persistent