将函数调用移动到where子句中，使用OverlappingInstances中断类型检查器

Question

我使用OverlappingInstances来制作一个漂亮的打印类，当我没有为某些类型提供自定义实例时，它将默认为Show。

由于某些原因，当您使用where子句或let表达式时，这似乎会中断。

{-# LANGUAGE FlexibleInstances, UndecidableInstances #-}
class View a where
    view :: a -> String
instance {-# OVERLAPS #-} Show a => View a where
    view = show
-- Works just fine
instance (View a, View b) => View (a, b) where
    view (a, b) = "(" ++ view a ++ ", " ++ view b ++ ")"
-- Does not work
instance (View a, View b) => View (a, b) where
    view (a, b) = "(" ++ a' ++ ", " ++ b' ++ ")"
      where
        a' = view a
        b' = view b
-- Does not work
instance (View a, View b) => View (a, b) where
    view (a, b) = let
        a' = view a
        b' = view b
        in "(" ++ a' ++ ", " ++ b' ++ ")"

现在，如果我删除默认的重叠实例，所有其他实例都可以正常工作。

我希望有人能向我解释为什么会发生这种情况，或者它只是一个bug?

我得到的具体错误是:

Could not deduce (Show a) arising from a use of ‘view’ from the context (View a, View b) bound by the instance declaration at ...
Could not deduce (Show b) arising from a use of ‘view’ from the context (View a, View b) bound by the instance declaration at ...

所以出于某种原因，where/let欺骗类型检查器认为View需要Show，当它不需要时。

Answer 1

类型族方法如下:

{-# LANGUAGE TypeFamilies, MultiParamTypeClasses,
    FlexibleInstances, DataKinds, KindSignatures,
    ScopedTypeVariables #-}
import Data.Proxy

data Name = Default | Booly | Inty | Pairy Name Name
type family ViewF (a :: *) :: Name where
  ViewF Bool = 'Booly
  ViewF Int = 'Inty
  ViewF Integer = 'Inty --you can use one instance many times
  ViewF (a, b) = 'Pairy (ViewF a) (ViewF b)
  ViewF a = 'Default
class View (name :: Name) a where
  view' :: proxy name -> a -> String
instance (Show a, Num a) => View 'Inty a where
  view' _ x = "Looks Inty: " ++ show (x + 3)
instance a ~ Bool => View 'Booly a where
  view' _ x = "Looks Booly: " ++ show (not x)
instance Show a => View 'Default a where
  view' _ x = "Looks fishy: " ++ show x
instance (View n1 x1, View n2 x2) => View ('Pairy n1 n2) (x1, x2) where
  view' _ (x, y) = view' (Proxy :: Proxy n1) x ++ "," ++ view' (Proxy :: Proxy n2) y

view :: forall a name .
        (ViewF a ~ name, View name a)
     => a -> String
view x = view' (Proxy :: Proxy name) x
-- Example:
hello :: String
hello = "(" ++ view True ++ view (3 :: Int)
        ++ view "hi" ++ ")"

Answer 2

感谢@dfeuer提供的替代方法，但我认为我应该为问题本身编写相当快速的修复:

{-# LANGUAGE MonoLocalBinds #-}

一旦它被放在文件的顶部，一切都很好。从我所做的研究来看，似乎某些扩展(我猜包括OverlappingInstances)在本地绑定类型检查中戳了洞，而MonoLocalBinds牺牲了多态使用本地绑定来修复这些洞。