[英]JSON output with Haskell
I am trying to use the show function and get back output that looks like JSON The type I have to work with is 我正在尝试使用show函数并获取类似JSON的输出,我必须使用的类型是
data JSON = JNum Double
| JStr String
I am looking for 我在寻找
JNum 12, JStr"bye", JNum 9, JStr"hi" to return JNum 12,JStr“ bye”,JNum 9,JStr“ hi”返回
[12, "bye", 9, "hi"]
I have attempted: 我尝试过:
instance Show JSON where
show ans = "[" ++ ans ++ "]"
but fails with a compile error. 但由于编译错误而失败。 I have also tried
我也尝试过
instance Show JSON where
show ans = "[" ++ ans ++ intercalate ", " ++ "]"
but failed with "Not in scope: data constructor 'JSON' Not sure how to use "ans" to represent whatever type JSON receives as input in the ouput, be it a string, double..etc... Not very good with Haskell so any hints would be great. 但由于“不在范围内:数据构造器'JSON'而失败”不知道如何使用“ ans”来表示JSON在输出中作为输入接收的任何类型,可以是字符串,也可以是double..etc ...对于Haskell不太好所以任何提示都会很棒。
Thx for reading 阅读Thx
You can have GHC automatically derive a show function for you by adding deriving (Show)
to your data declaration, eg: 您可以通过在数据声明中添加
deriving (Show)
来使GHC为您自动派生一个show函数,例如:
data JSON = ... deriving (Show)
As for your code, in order for show ans = "[" ++ ans ++ "]"
to type check ans
needs to be a String, but ans
has type JSON. 至于您的代码,为了使
show ans = "[" ++ ans ++ "]"
键入check, ans
必须是String,但是ans
具有JSON类型。
To write your own show function you have to write something like: 要编写自己的show函数,您必须编写如下内容:
instance Show JSON where
show (JNum d) = ... code for the JNum constructor ...
show (JObj pairs) = ... code for the JObj constructor ...
show (JArr arr) = ... code for the JArr constructor ...
...
Here d
will have type Double, so for the first case you might write: 此处
d
类型为Double,因此对于第一种情况,您可以编写:
show (JNum d) = "JNum " ++ show d
or however you want to represent a JSON number. 或者您想代表一个JSON数字。
If you want to write your own instance, you can do something like this: 如果要编写自己的实例,可以执行以下操作:
instance Show JSON where
show (JNum x) = show x
show (JStr x) = x
show (JObj xs) = show xs
show (JArr xs) = show xs
Note that for JObj
and JArr
data constructor, the show will use the instance defined for JObj
and JArr
. 请注意,对于
JObj
和JArr
数据构造函数,该节目将使用为JObj
和JArr
定义的实例。
Demo: 演示:
λ> JArr[JNum 12, JStr"bye", JNum 9, JStr"hi"]
[12.0,bye,9.0,hi]
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