polyTypeOf 很神秘

Question

PolyTypeable 是 Typeable 的模拟多态类型。但它的工作原理相当不可预测：

ghci> :t show
show :: Show a => a -> String
ghci> polyTypeOf show
a1 -> [Char]
ghci> :t fromEnum 
fromEnum :: Enum a => a -> Int
ghci> polyTypeOf fromEnum 

<interactive>:1:12:
    Ambiguous type variable `a0' in the constraint:
      (Enum a0) arising from a use of `fromEnum'
    Probable fix: add a type signature that fixes these type variable(s)
    In the first argument of `polyTypeOf', namely `fromEnum'
    In the expression: polyTypeOf fromEnum
    In an equation for `it': it = polyTypeOf fromEnum

库源代码很难理解，您能解释一下为什么 polyTypeOf 接受某些参数集而无法接受其他参数，甚至非常相似？

Answer 1

原因与此相同

Prelude> show undefined
"*** Exception: Prelude.undefined
Prelude> fromEnum undefined

<interactive>:0:1:
    Ambiguous type variable `a0' in the constraint:
      (Enum a0) arising from a use of `fromEnum'
    Probable fix: add a type signature that fixes these type variable(s)
    In the expression: fromEnum undefined
    In an equation for `it': it = fromEnum undefined

，即 ghci 的扩展默认规则允许它解决 Show 约束的歧义性，但不能解决 Enum 约束的歧义性。如果您尝试使用 foo = polyTypeOf show 编译源文件，您还会收到不明确的类型变量错误（除非您使用 {-# LANGUAGE ExtendedDefaultRules #-}）。