database - haskell "persistent"模型 : How to correctly define cross-reference?

Question

假设您有两个模型(Foo 和 Bar)并且它们都相互引用(Foo 有 barRef 类型为 BarId 和 Bar 有 fooRef 类型为 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为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’

我应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 ()

参见 official docs供引用。