Haskell Enum比较

Question

我已经定义了一个枚举:

data Direction = Clockwise | CounterClockwise deriving (Enum)

变量'direction'的类型为'Direction'.在进行以下比较时:

direction == Clockwise

我收到这个错误:

  No instance for (Eq OrbitDirection) arising from a use of `=='
  In the expression: direction == Clockwise

Answer 1

这很简单:将Eq实例添加到派生实例列表:

data Direction = Clockwise | CounterClockwise deriving (Enum, Eq)