[英]Haskell Generic Type
So I just started learning Haskell and I have been stuck on this for quite some time already. 因此,我才刚刚开始学习Haskell,并且在此方面已经停留了很长时间。 So I have a function that calculates a number after an offset has been minus (minimum value is 0). 所以我有一个函数,可以在偏移量为负(最小值为0)之后计算数字。 I managed to do this function with the types explicitly shown. 我设法使用显式显示的类型执行此功能。
offSetter :: Int -> Int -> Int
offSetter number offset
| number - offset >= 0 = number - offset
| otherwise = 0
But when I tried to change it to use generic types as below, it keeps giving me an error. 但是,当我尝试将其更改为使用如下所示的泛型类型时,它总是给我一个错误。 Am I doing it wrong? 我做错了吗?
offSetter :: Num a => a -> a -> a
offSetter number offset
| number - offset >= 0 = number - offset
| otherwise = 0
The error I'm getting: 我得到的错误:
* Could not deduce (Ord a) arising from a use of '>='
from the context: Num a
bound by the type signature for:
offSetter :: forall a. Num a => a -> a -> a
at src\test.hs:57:1-33
Possible fix:
add (Ord a) to the context of
the type signature for:
offSetter :: forall a. Num a => a -> a -> a
* In the expression: number - offset >= 0
In a stmt of a pattern guard for
an equation for `offSetter':
number - offset >= 0
Solved it by adding Ord a: 通过添加Ord来解决它:
offSetter :: (Num a, Ord a) => a -> a -> a
offSetter number1 offSet
| number1 - offSet >= 0 = number1 - offSet
| otherwise = 0
As you discovered, you need to add the type class Ord
as a constraint to the type a
with the following type signature: 如您所见,您需要将类型类Ord
作为约束添加到具有以下类型签名的类型a
:
offSetter :: (Num a, Ord a) => a -> a -> a
This is because Ord
is the typeclass with comparison operators like (>=)
. 这是因为Ord
是具有比较运算符(>=)
如(>=)
的类型类。
So Ord is used because there are elements like Strings that is not applicable to Num? 之所以使用Ord,是因为存在像Strings这样的元素不适用于Num?
No, since String
is not a member of the Num
typeclass, the original declaration already excludes it as a possible candidate for the type a
. 不可以,因为String
不是Num
类型类的成员,所以原始声明已将其排除为a类型a
可能候选者。 As I stated earlier, you need to use Ord
in order to guarantee that the type a
has the operator (>=)
available. 如前所述,您需要使用Ord
来确保类型a
具有可用的运算符(>=)
。
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