为类型家族的唱片写优雅的镜头？

Question

与Lenses and TypeFamilies 和How to derive instances for records with type-families 和How to make lenses for records with type-families 相关

每次我热情地尝试使用类型族来减少一些样板代码时，我都会把自己编程到一个角落里。这是最新的——如何在下面给出的代码 sn-p 中为 RecordPoly f 定义镜头？

{-# LANGUAGE TemplateHaskell, DeriveGeneric, MultiParamTypeClasses, FunctionalDependencies, UndecidableInstances, DataKinds #-}
module Try where

import Control.Lens

data PGInt
data PGText

data Selector = Haskell | DB


type family DBField (f :: Selector) hask db where
  DBField 'Haskell hask _ = hask
  DBField 'DB _ db = db

data RecordPoly f = RecordPoly
  { recFieldA :: !(DBField f Int PGInt)
  , recFieldB :: !(DBField f String PGText)
  }

class HasFieldA s a | s -> a where fieldA :: Lens' s a

instance HasFieldA (RecordPoly 'Haskell) Int where
  fieldA fctor (RecordPoly fa fb) = fmap (\x -> RecordPoly x fb) (fctor fa)

失败并出现以下错误：

• Illegal instance declaration for ‘HasFieldA (RecordPoly f) a’
    The liberal coverage condition fails in class ‘HasFieldA’
      for functional dependency: ‘s -> a’
    Reason: lhs type ‘RecordPoly f’ does not determine rhs type ‘a’
    Un-determined variable: a
• In the instance declaration for ‘HasFieldA (RecordPoly f) a’

似乎唯一可行的是：

instance HasFieldA (RecordPoly 'Haskell) Int where
  fieldA fctor (RecordPoly fa fb) = fmap (\x -> RecordPoly x fb) (fctor fa)

这是使它工作的唯一方法（从而导致一种新的样板爆炸）？这段代码的类似多态版本是如何工作的，但在使用类型族时却不行：

data RecordPoly fa fb = RecordPoly
  { recFieldA :: fa
  , recFieldB :: fb
  }

class HasFieldA s a | s -> a where fieldA :: Lens' s a

instance HasFieldA (RecordPoly fa fb) fa where
  fieldA fctor (RecordPoly fa fb) = fmap (\x -> RecordPoly x fb) (fctor fa)

Answer 1

终于明白了……

instance (fa ~ DBField f Int PGInt) => HasFieldA (RecordPoly f) fa where
  fieldA fctor (RecordPoly fa fb) = fmap (\x -> RecordPoly x fb) (fctor fa)