为什么使用具有重叠实例的typeclass的函数在GHCI中表现不同？

Question

{-# LANGUAGE 
  NoImplicitPrelude,
  MultiParamTypeClasses,
  FlexibleInstances, FlexibleContexts,
  TypeFamilies, UndecidableInstances,
  AllowAmbiguousTypes
#-}

import Prelude(Char, Show, show, undefined, id)

data Nil
nil :: Nil
nil = undefined

instance Show Nil where
  show _ = "nil"

data Cons x xs = Cons x xs 
  deriving Show

class FPack f r where
  fpack :: f -> r

instance {-# OVERLAPPABLE #-} f ~ (Nil -> r) => FPack f r where
  fpack f = f nil

instance (FPack (x -> b) r, f ~ (Cons a x -> b)) => FPack f (a -> r) where
  fpack f a = fpack (\x -> f (Cons a x))

fpack id "a" "b" :: Cons [Char] (Cons [Char] Nil)

生成列表Cons“a”（Cons“b”nil)。

通常，我希望通过传递id作为其f参数（如上）来调用fpack，因此我希望将以下函数定义为速记：

pack = fpack id

如果我将上面的程序加载到GHCi中并执行上面的行，那么pack将根据需要定义，并且它的类型（由:t)是fpack(a->a)r=>r。所以我在我的程序中这样定义了函数：

pack :: FPack (a -> a) r => r
pack = fpack id

但当将所述程序加载到GHCI中时,会出现以下错误：

bugs\so-pack.hs:31:8: error:
    * Overlapping instances for FPack (a0 -> a0) r
        arising from a use of `fpack'
      Matching givens (or their superclasses):
        FPack (a -> a) r
          bound by the type signature for:
                     pack :: forall a r. FPack (a -> a) r => r
          at bugs\so-pack.hs:30:1-29
      Matching instances:
        instance [overlappable] (f ~ (Nil -> r)) => FPack f r
          -- Defined at bugs\so-pack.hs:24:31
        instance (FPack (x -> b) r, f ~ (Cons a x -> b)) =>
                 FPack f (a -> r)
          -- Defined at bugs\so-pack.hs:27:10
      (The choice depends on the instantiation of `a0, r')
    * In the expression: fpack id
      In an equation for `pack': pack = fpack id
   |
31 | pack = fpack id
   |     

这就引出了我的问题。为什么这个函数在GHCi中定义时起作用，而在程序中定义时却不起作用？有什么办法可以让这个程序正常运行吗？如果是，怎么做？

根据我对GHC和Haskell的理解，这个错误是因为pack可以解析为两个重叠实例中的任何一个，这就困扰了GHC。但是，我认为allowAmbiguoustypes选项应该通过将实例选择推迟到最终调用站点来解决这个问题。不幸的是，这显然是不够的。我很好奇为什么，但我更好奇的是为什么GHCi在它的REPL循环中接受这个定义，但当它在程序内部时却不接受它。

fpack id "a" "b" :: Cons [Char] (Cons [Char] Nil)

<interactive>:120:1: error:
    * Couldn't match type `Cons [Char] (Cons [Char] Nil)' with `()'
        arising from a use of `it'
    * In the first argument of `System.IO.print', namely `it'
      In a stmt of an interactive GHCi command: System.IO.print it

Answer 1

您需要手动实例化fpack。

pack :: forall a r . FPack (a -> a) r => r
pack = fpack @(a->a) @r id

这需要ScopedTypeVariables,TypeApplications,AllowAmbiguousTypes。

或者，提供id的类型。

pack :: forall a r . FPack (a -> a) r => r
pack = fpack (id :: a -> a)