haskell - 使用 SYB 和 ad-hoc 多态性在 Haskell 中进行泛型编程

Question

我有一个与 Show 相同的类(class)我想为每个元组类型创建一个此类的实例。通常这是通过为每个元组类型单独编写实例来完成的

instance  (Show a, Show b) => Show (a,b)  where
  showsPrec _ (a,b) s = show_tuple [shows a, shows b] s

instance (Show a, Show b, Show c) => Show (a, b, c) where
  showsPrec _ (a,b,c) s = show_tuple [shows a, shows b, shows c] s

instance (Show a, Show b, Show c, Show d) => Show (a, b, c, d) where
  showsPrec _ (a,b,c,d) s = show_tuple [shows a, shows b, shows c, shows d] s
...

为每个元组类型编写一个实例会产生大量样板文件，并且很容易看到所有 showPrec 之间共享的通用模式。实现。为了避免这种样板，我想我可以使用 Data.Generics来自 Scrap your boilerplate并实现一个折叠元组，比如

showTuple = intercalate " " . gmapQ ("" `mkQ` show)

但是 showTuple由于某种原因不起作用

> showTuple (1,2)
" "

我认为问题在于 show是多态的，因为如果我专门 showTuple然后它工作

showTupleInt = intercalate " " . gmapQ ("" `mkQ` (show :: Int -> String))

> showTupleInt (1::Int,2::Int)
"1 2"

我检查了 gshow的代码这与我需要的类似，但我不知道它是如何工作的。如果我尝试将其代码导入 GHCI，则会收到错误消息:

> let gshows = (\t -> showChar '('
                      . (showString . showConstr . toConstr $ t)
                      . (foldr (.) id . gmapQ ((showChar ' ' .) . gshows) $ t)
                      . showChar ')'
                      ) `extQ` (shows :: String -> ShowS)
<interactive>:262:59:
Could not deduce (a ~ d)
from the context (Data a)
  bound by the inferred type of
           gshows :: Data a => a -> String -> String
  at <interactive>:(259,5)-(264,44)
or from (Data d)
  bound by a type expected by the context:
             Data d => d -> String -> String
  at <interactive>:262:33-65
  `a' is a rigid type variable bound by
      the inferred type of gshows :: Data a => a -> String -> String
      at <interactive>:259:5
  `d' is a rigid type variable bound by
      a type expected by the context: Data d => d -> String -> String
      at <interactive>:262:33
Expected type: d -> String -> String
  Actual type: a -> String -> String
In the second argument of `(.)', namely `gshows'
In the first argument of `gmapQ', namely
  `((showChar ' ' .) . gshows)'
In the second argument of `(.)', namely
  `gmapQ ((showChar ' ' .) . gshows)'

所以我有两个问题:

showTuple 有什么问题|以及如何修复它以使其适用于任何大小的元组

如何gshow有效，为什么如果我在 GHCI 上导入其代码会出现该错误？

编辑:我在学习 Data.Generics和一般的SYM，所以我想使用那个模块。只有当它仅使用该模块时，我才会接受答案。谢谢。

Answer 1

我更熟悉 GHC Generics，而不是 SYB，所以我提供了一个基于 Generics 的解决方案。虽然它不能直接回答您的问题，但我希望它也有用。

{-# LANGUAGE TypeOperators, FlexibleContexts, DefaultSignatures #-}
import Data.Sequence
import GHC.Generics

class Strs' f where
    strings' :: f a -> Seq String

instance Strs' U1 where
    strings' U1 = empty

instance Show c => Strs' (K1 i c) where
    strings' (K1 a) = singleton $ show a

instance (Strs' a) => Strs' (M1 i c a) where
    strings' (M1 a) = strings' a

instance (Strs' f, Strs' g) => Strs' (f :*: g) where
    strings' (a :*: b) = strings' a >< strings' b

class Strs a where
    strings :: a -> Seq String
    default strings :: (Generic a, Strs' (Rep a)) => a -> Seq String
    strings = strings' . from

-- Since tuples have Generic instances, they're automatically derived using
-- the above default.
instance Strs () where
instance (Show a, Show b) => Strs (a, b) where
instance (Show a, Show b, Show c) => Strs (a, b, c) where