存在量化构造函数列表

Question

我有以下数据类型:

{-# LANGUAGE ExistentialQuantification #-}
{-# LANGUAGE ExtendedDefaultRules #-}
class ToString a where
data M = forall a. (ToString a) => B a

在GHCI中，我可以做以下操作，没有问题:

let bs = [B, B]

但是如果我尝试在编译文件中这样做，我会得到以下错误:

No instance for (ToString a0) arising from a use of ‘B’
The type variable ‘a0’ is ambiguous
Relevant bindings include
  bs :: [a0 -> M] (bound at src/My/Module:7:1)

我错过了哪个扩展名(s)，这将允许我创建B的列表，如所示?或者我错过了GHCI正在添加的什么?

Answer 1

这是因为GHCi不启用单态限制，而GHC(默认情况下)启用了。为了证明，下面的文件类型检查:

{-# LANGUAGE ExistentialQuantification #-}
{-# LANGUAGE NoMonomorphismRestriction #-}
class ToString a where
data M = forall a. (ToString a) => B a
bs = [B, B]

bs的推断类型当然是相当无用的:

*Main> :t bs
bs :: ToString a => [a -> M]

如果您不想关闭单态限制，您可以在bs的定义中添加类型签名:

{-# LANGUAGE ExistentialQuantification #-}
class ToString a where
data M = forall a. (ToString a) => B a
bs :: (ToString a) => [a -> M]
bs = [B, B]