Fable: BigInt does not have toJSON implementation

[Zaid-Ajaj]

Description

Stringifying bigint to JSON using the following function:

let stringify (value: 'a) : string =
    JS.JSON.stringify(value, (fun _ v ->
        match v with
        | :? string -> v
        | :? System.Collections.IEnumerable ->
            if JS.Array.isArray(v) then v
            else JS.Array.from(v :?> JS.Iterable<obj>) |> box
        | _ -> v
    ), 0)

for example, stringify 5I will return this:

{"signInt":1,"v":{"bound":1,"digits":[5]}}

instead of

"5"

Related information

• Fable version: fable-compiler 2.1.8
• Operating system: Windows 10

[alfonsogarciacaro]

Yes, for the fable-library types defined in JS/TS, we add toJson to the prototype, but we haven't done it yet for types defined in F#. I guess we should have a IJsonSerializable interface with a toJson method (lowercase) and make the types implement it so we are sure the member is attached to the prototype.

[Zaid-Ajaj]

@alfonsogarciacaro I think I might have found a workaround but it is pretty ugly, testing for the existence of internal properties of the compiled BigInt, same for DateTimeOffset (workaround for #1710):

[<AutoOpen>]
module InteropUtil =    
    [<Emit("$1[$0]")>]
    let get<'a> (key: string) (x: obj) : 'a = jsNative
    [<Emit("$0 instanceof Date")>]
    let isDate (x: obj) = jsNative
    [<Emit("$0 in $1")>]
    let hasKey (key: string) (x: 'a) = jsNative
    let isDateOffset (x: obj) = isDate x && hasKey "offset" x
    let isObjectLiteral (x: obj) = getTypeOf x = "object" 

    let isBigInt (x: obj) = 
        not (isNull x)
        && isObjectLiteral x
        && hasKey "signInt" x 
        && hasKey "v" x 
        && hasKey "digits" (get "v" x)
        && hasKey "bound" (get "v" x)

    let stringify (value: 'a) : string =
        JS.JSON.stringify(value, (fun key v ->
            if isDateOffset (get key jsThis) then 
                let dateOffset : DateTimeOffset = get key jsThis
                box (dateOffset.ToString("O"))
            elif isBigInt (get key jsThis) then 
                let bigInt : bigint = get key jsThis
                box (string (decimal bigInt))
            else 
            match v with
            | :? string -> v
            | :? System.Collections.IEnumerable ->
                if JS.Array.isArray(v) then v
                else JS.Array.from(v :?> JS.Iterable<obj>) |> box
            | _ when isBigInt v -> box (string (decimal (unbox<bigint> v)))
            | _ when isDateOffset v -> box ((unbox<DateTimeOffset> v).ToString("O"))
            | _ -> v
        ), 0)

Another little problem is that calling string or ToString() on bigint result in "[object Object]" (that's why I am converting to decimal first, probably needs a separate issue)

[alfonsogarciacaro]

Hmm, actually converting the bigint to string should be the best way to serialize it to JSON. ToString with bigints is working fine in the REPL, either boxed or unboxed. I don't understand why you're getting "[object Object]" 😕

[Zaid-Ajaj]

I might not had latest fable when I ran the tests, I will check :)

[alfonsogarciacaro]

@Zaid-Ajaj Can this be closed?

[Zaid-Ajaj]

I have fixed the problem by using reflection, so for me it is fixed in SimpleJson but if someone was to call JSON.stringify on big integers, they would get unexpected results. I think it is still a good idea to implement toJSON out of the box for these integers

[alfonsogarciacaro]

Hopefully fixed by https://github.com/fable-compiler/Fable/commit/20a6d4555caceb9fa89718402c6d99d4787cc9ad. Will be released with 3.1.12, please reopen if it's still an issue.