Question

我试图更好地理解手动指定Haskell函数签名，并且在这种情况下我不理解=>运算符。

这是我的例子：

Prelude> add a b = a + b 
Prelude> add 1 2
3

Haskell推断出此功能签名：

Prelude> :info add
add :: Num a => a -> a -> a

这看起来很棒，因为如果我要手动定义它，我会做的：

add :: Int -> Int -> Int
add a b = a + b

因此，我知道推断类型更好，因为它允许：

Prelude> add 1.0 2.0
3.0
Prelude> add 1.0 2
3.0

但是我应该如何阅读

Num a => a -> a -> a

？

为什么不这样写，例如：

add :: Num -> Num -> Num

或

add :: Num a -> Num a -> Num a

？

这两个均失败：

add :: Num -> Num -> Num


[1 of 1] Compiling Main             ( my.hs, interpreted )

my.hs:4:8: error:
    • Expecting one more argument to ‘Num’
      Expected a type, but ‘Num’ has kind ‘* -> Constraint’
    • In the type signature: add :: Num -> Num -> Num
  |
4 | add :: Num -> Num -> Num
  |        ^^^

my.hs:4:15: error:
    • Expecting one more argument to ‘Num’
      Expected a type, but ‘Num’ has kind ‘* -> Constraint’
    • In the type signature: add :: Num -> Num -> Num
  |
4 | add :: Num -> Num -> Num
  |               ^^^

my.hs:4:22: error:
    • Expecting one more argument to ‘Num’
      Expected a type, but ‘Num’ has kind ‘* -> Constraint’
    • In the type signature: add :: Num -> Num -> Num
  |
4 | add :: Num -> Num -> Num
  |                      ^^^
Failed, no modules loaded.

和

add :: Num a -> Num a -> Num a

my.hs:4:8: error:
    • Expected a type, but ‘Num a’ has kind ‘Constraint’
    • In the type signature: add :: Num a -> Num a -> Num a
  |
4 | add :: Num a -> Num a -> Num a
  |        ^^^^^

my.hs:4:17: error:
    • Expected a type, but ‘Num a’ has kind ‘Constraint’
    • In the type signature: add :: Num a -> Num a -> Num a
  |
4 | add :: Num a -> Num a -> Num a
  |                 ^^^^^

my.hs:4:26: error:
    • Expected a type, but ‘Num a’ has kind ‘Constraint’
    • In the type signature: add :: Num a -> Num a -> Num a
  |
4 | add :: Num a -> Num a -> Num a
  |                          ^^^^^
Failed, no modules loaded.

所以我知道这是正确的语法。但是这个运算符“ =>”叫什么意思，即我将如何用英语阅读/发音上面的签名，以及如何在函数定义和其他地方查找其语法？

Answer 1

如果我们输入：

add :: Num a => a -> a -> a

=>是约束部分和签名本身之间的分隔符。

Num a意味着a必须属于Num类型类。

您可以具有多个约束(Num a, Show a) => a或不同类型的约束(Num a, Eq b) => a -> b -> c

Haskell函数签名中的“等于”箭头

1 个答案: