找不到泛型的类型类实例

Question

我无法让 GHC 解析下面的通用类型类实例：（目标是实现与访问 point 的属性 x 相同的效果，例如 x point，而是利用类型级字符串)

data Label (l :: Symbol) =
  Get

class Has a l b | a l -> b where
  get :: a -> Label l -> b

instance (Generic r, GenericHas (Rep r) pl pt) => Has r pl pt where
  get = genericHas . from

class GenericHas a l b | a l -> b where
  genericHas :: a x -> Label l -> b

instance GenericHas (D1 a (C1 b (S1 (c ('Just pl) d e f) (Rec0 pt)))) pl pt where
  genericHas (M1 (M1 (M1 (K1 v)))) _ = v

data Point =
  Point
    { x :: Int
    }
  deriving (Show, Generic)

example :: Int
example = get (Point 1) (Get :: Label "x")

下面是错误：

    * No instance for (GenericHas
                         (D1
                            ('MetaData
                               "Point"
                               "Alba.Persistence"
                               "alba-0.1.0.0-w6KgEimatKAP1g0rWS7YT"
                               'False)
                            (C1
                               ('MetaCons "Point" 'PrefixI 'True)
                               (S1
                                  ('MetaSel
                                     ('Just "x")
                                     'NoSourceUnpackedness
                                     'NoSourceStrictness
                                     'DecidedLazy)
                                  (Rec0 Int))))
                         "x"
                         Int)
        arising from a use of `get'
    * In the expression: get (Point 1) (Get :: Label "x")
      In an equation for `example':
          example = get (Point 1) (Get :: Label "x")

Answer 1

问题是 GHC 无法将 c 与 'MetaSel 匹配，因为 'MetaSel 属于更高一类。默认情况下，假定新类型变量具有种类 *，因此此处匹配失败。

解决此问题的一种方法是将 c 替换为 'MetaSel:

instance GenericHas (D1 a (C1 b (S1 (`MetaSel ('Just pl) d e f) (Rec0 pt)))) pl pt where

另一种方法是启用PolyKinds。这将告诉 GHC not 假设新变量的种类 * 并且匹配将成功。

好处：TypeApplications 可以有更好的语法。您可以写 Get @"x" 而不是 Get :: Label "x"。或者你可以走得更远，定义一个包装 Get:

的函数

get' :: forall (l :: Symbol) a b. Has a l b => a -> b
get' a = get a (Get @l)

-- Usage:
example = get' @"x" (Point 1)

找不到泛型的类型类实例

Typeclass instance not found for generic

haskell

metaprogramming

typeclass