Question

有没有办法询问GHCi类型是否实例化类型类？我在和^^运算符混在一起，然后输入：

Prelude> :type (^^)
(^^) :: (Integral b, Fractional a) => a -> b -> a

非常有用。然后我想知道Rational是否实例化小数。我在Hoogle上找到了Rational，并取消引用了Ratio，最后发现：

Integral a => Fractional (Ratio a)

有没有更简单的方法？像这样：

:listypeclasses Rational

或

:listinstances Fractional

Answer 1

您要:info，也可以缩写为:i。使用:info <type>列出该类型的实例，或使用:info <class>列出该类的实例。

对于Rational，由于它是类型别名，因此:info不会直接列出实例。但是，它会会告诉您这是一个类型别名，您可以通过对别名使用:info来跟踪它：

ghci> :info Ratio
data Ratio a = !a :% !a     -- Defined in ‘GHC.Real’
instance Eq a => Eq (Ratio a) -- Defined in ‘GHC.Real’
instance Integral a => Ord (Ratio a) -- Defined in ‘GHC.Real’
instance Show a => Show (Ratio a) -- Defined in ‘GHC.Real’
instance (Integral a, Read a) => Read (Ratio a)
  -- Defined in ‘GHC.Read’
instance Integral a => Enum (Ratio a) -- Defined in ‘GHC.Real’
instance Integral a => Fractional (Ratio a)
  -- Defined in ‘GHC.Real’
instance Integral a => Num (Ratio a) -- Defined in ‘GHC.Real’
instance Integral a => Real (Ratio a) -- Defined in ‘GHC.Real’
instance Integral a => RealFrac (Ratio a) -- Defined in ‘GHC.Real’

Answer 2

这是一种方法：

> :set -XFlexibleContexts
> :t undefined :: (Fractional Rational) => Int

<interactive>:1:14: warning: [-Wsimplifiable-class-constraints]
    • The constraint ‘Fractional Rational’
        matches an instance declaration
      instance Integral a => Fractional (GHC.Real.Ratio a)
        -- Defined in ‘GHC.Real’
      This makes type inference for inner bindings fragile;
        either use MonoLocalBinds, or simplify it using the instance
    • In an expression type signature: (Fractional Rational) => Int
      In the expression: undefined :: (Fractional Rational) => Int
undefined :: (Fractional Rational) => Int :: Int

这只是一个警告，因此该实例存在。

> :t undefined :: (Fractional Bool) => Int

<interactive>:1:1: error:
    No instance for (Fractional Bool)
      arising from an expression type signature

这是一个错误，因此没有实例。

（上面的Int是任意的，您可以使用任何其他类型。）

检查类型是否实例化GHCI中的类型类

2 个答案: