在haskell中将浮动类型转换为浮动类型

Question

doubles :: Floating -> Floating
doubles  =  (0.1 + 1.1) - 1.2

我在Haskell中收到此错误

• Expecting one more argument to ‘Floating’
      Expected a type, but ‘Floating’ has kind ‘* -> Constraint’
    • In the type signature: doubles :: Floating -> Floating
   |
32 | doubles :: Floating -> Floating 
   |                        ^^^^^^^^

Answer 1

Floating不是类型，Floating是类型类，类型的“组”，例如Float和Double是此类型类的“成员”。因此，您应该添加类型约束：

doubles :: Floating a => a
doubles = (0.1 + 1.1) - 1.2

因此，这意味着对于每种类型a，其中a是Floating类型类的成员，都定义了此函数。

类型为Floating a => a而不是Floating => a -> a，因为没有输入：doubles是常数。