Haskell - 模棱两可的类型变量，为什么？

Question

为什么以下编译:

{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE OverlappingInstances #-}

class IsList a where
  isList :: a -> Bool

instance IsList a where
  isList x = False

instance IsList [a] where
  isList x = True

main = print (isList 'a') >> print (isList ['a'])  

但改变 main至 this :

main = print (isList 42) >> print (isList [42])  

给出以下错误:

Ambiguous type variable `a0' in the constraints:
  (Num a0) arising from the literal `42' at prog.hs:13:22-23
  (IsList a0) arising from a use of `isList' at prog.hs:13:15-20
Probable fix: add a type signature that fixes these type variable(s)
In the first argument of `isList', namely `42'
In the first argument of `print', namely `(isList 42)'
In the first argument of `(>>)', namely `print (isList 42)'

isList肯定不在 Num上课是吗？如果不是，为什么会模棱两可？

Answer 1

问题不在于 isList，而在于常量 42。常量 'a' 有一个具体的 Char 类型。常数 42 没有具体类型。

ghci> :t 42
42 :: Num a => a

编译器需要一个具体的类型。如果您将 main 更改为以下内容，它将起作用:

main = print (isList (42 :: Int)) >> print (isList [42 :: Int])