F＃JSON连接的任务

Question

Suppose I have the following code for my custom union:

type JSON =
| Object of (string * JSON) list
| Array of JSON list
| Number of double
| Boolean of bool
| Null
| String of string

How can I implement the function 'find the sum of all numbers in JSON object'? let summarizer (tree : JSON) : int

It should recursively traverse the JSON tree and return as a result the sum of all numbers in JSON object.

Answer 1

You should start by considering each individual case of the JSON type, and deciding how that case would contribute to the sum. I'll go from bottom to top since the top cases are the harder ones:

• String - would add 0.0 to the sum.
• Null - would add 0.0 to the sum.
• Boolean - would add 0.0 to the sum.
• Number - would add itself to the sum.
• Array - Go through the individual JSON items in the array, and calculate each of their individual sums. All those sums added together get added to the running total sum.
• Object - Go through the pairs, and in each pair, the second item is a JSON object that would have a sum. Sum all those up, and you get a number to add to the sum.

And now we have the function almost written for us already!

let rec sumOfJson = function
    | Object list -> List.sumBy (snd >> sumOfJson) list
    | Array list -> List.sumBy sumOfJson list
    | Number n -> n
    | Boolean _
    | Null
    | String _ -> 0.0

If you don't understand the snd >> sumOfJson part, that's exactly equivalent to fun x -> x |> snd |> sumOfJson . Here, x is a string * JSON pair, so that's equivalent to fun (_, json) -> sumOfJson json .