如何从摘要功能的Singleton字段中获取表单值？

Question

我正在尝试使用具有消化功能的自定义数据类型。

此数据类型与数据库ID字段相关。我将其定义为：

dbkey :: (Monad m, Monoid v) => Maybe DBKey -> Form v m DBKey
dbkey p = Pure . Singleton $ (fromMaybe (DBKey 0) p)

现在剩下的问题是在“表单”中检索此值。我不能使用fieldInputText，因为它需要一个Text字段-commentId: expected (Text _), but got: (Singleton _)。

如何实现以上目标？必须编写自己的fieldInputText版本似乎很复杂。

---

更新到目前为止，我有以下内容：

fieldInputSingleton :: forall a v. Text -> View v -> Proxy a -> a
fieldInputSingleton ref (View _ _ form input _ method) p =
    queryField path form eval'
  where
    path       = toPath ref
    givenInput = lookupInput path input
    eval' field = case field of
        Singleton t -> evalField method givenInput (Singleton t)
        f      -> error $ T.unpack ref ++ ": expected (Singleton _), " ++ "but got: (" ++ show f ++ ")"

但是我遇到了：

    • Couldn't match type ‘a’ with ‘b’
      ‘a’ is a rigid type variable bound by
        the type signature for:
          fieldInputSingleton :: forall a v. Text -> View v -> Proxy a -> a
        at /home/chris/Projects/Haskell/Blog/src/View2.hs:26:1-65
      ‘b’ is a rigid type variable bound by
        a type expected by the context:
          forall b. Field v b -> a
        at /home/chris/Projects/Haskell/Blog/src/View2.hs:28:5-30
      Expected type: Field v b -> a
        Actual type: Field v a -> a
    • In the third argument of ‘queryField’, namely ‘eval'’
      In the expression: queryField path form eval'
      In an equation for ‘fieldInputSingleton’:
          fieldInputSingleton ref (View _ _ form input _ method) p
            = queryField path form eval'
            where
                path = toPath ref
                givenInput = lookupInput path input
                eval' field
                  = case field of
                      Singleton t -> evalField method givenInput (Singleton t)
                      f -> error
                             $ T.unpack ref
                                 ++ ": expected (Singleton _), " ++ "but got: (" ++ show f ++ ")"
    • Relevant bindings include
        eval' :: Field v a -> a
          (bound at /home/chris/Projects/Haskell/Blog/src/View2.hs:32:5)
        p :: Proxy a
          (bound at /home/chris/Projects/Haskell/Blog/src/View2.hs:27:56)
        fieldInputSingleton :: Text -> View v -> Proxy a -> a
          (bound at /home/chris/Projects/Haskell/Blog/src/View2.hs:27:1)
   |
28 |     queryField path form eval'
   |                          ^^^^^