I am trying to use the show function and get back output that looks like JSON The type I have to work with is
data JSON = JNum Double | JStr String
I am looking for
JNum 12, JStr"bye", JNum 9, JStr"hi" to return
[12, "bye", 9, "hi"]
I have attempted:
instance Show JSON where show ans = "[" ++ ans ++ "]"
but fails with a compile error. I have also tried
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.
Thx for reading
You can have GHC automatically derive a show function for you by adding
deriving (Show) to your data declaration, e.g.:
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.
To write your own show function you have to write something like:
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 ... ...
d will have type Double, so for the first case you might write:
show (JNum d) = "JNum " ++ show d
or however you want to represent a JSON number.
If you want to write your own instance, you can do something like this:
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
JArr data constructor, the show will use the instance defined for
λ> JArr[JNum 12, JStr"bye", JNum 9, JStr"hi"] [12.0,bye,9.0,hi]