haskell - 将函数调用移动到 where 子句会破坏带有重叠实例的类型检查器

Question

我使用 OverlappingInstances 制作一个 pretty-print 类，每当我没有为某种类型提供自定义实例时，该类就会默认为 Show。

出于某种原因，每当您使用 where 子句或 let 表达式时，这似乎就会中断。

{-# 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，或者这只是一个错误？

我得到的每个错误的具体错误是:

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 ...

由于某种原因 where/let 欺骗类型检查器认为 View 需要 Show，当事实并非如此。

Answer 1

类型族方法如下所示:

{-# 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" ++ ")"