我试图定义：
square :: Integer -> Integer
square = round . (** 2)
我得到：
<interactive>:9:9: error:
• No instance for (RealFrac Integer) arising from a use of ‘round’
• In the first argument of ‘(.)’, namely ‘round’
In the expression: round . (** 2)
In an equation for ‘square’: square = round . (** 2)
<interactive>:9:18: error:
• No instance for (Floating Integer)
arising from an operator section
• In the second argument of ‘(.)’, namely ‘(** 2)’
In the expression: round . (** 2)
In an equation for ‘square’: square = round . (** 2)
我对这种语言还是新手，我似乎无法将 Floating 实例转换为 Integer。有人知道我该怎么做吗？
Haskell 有多个不同类型的指数函数：
- (^) :: (Num a, Integral b) => a -> b -> a http://hackage.haskell.org/package/base-4.9.0.0/docs/Prelude.html#v:-94-
- (^^) :: (Fractional a, Integral b) => a -> b -> a http://hackage.haskell.org/package/base-4.9.0.0/docs/Prelude.html#v:-94--94-
- (**) :: Floating a => a -> a -> a http://hackage.haskell.org/package/base-4.9.0.0/docs/Prelude.html#v:-42--42-
您正在寻找的只是(^)。有了它，你甚至不需要round:
square :: Integer -> Integer
square = (^ 2)
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