Unable to decode struct with Sumtype. "unknown sum type value"

[ismyhc]

V version: V 0.3.0 89a757a, timestamp: 2022-07-14 11:10:15 +0300
OS: macOS Monterey 12.4, Apple M1 Max

What did you do?

import json

pub type GetBlockResult = string | GetBlockResultB

pub struct GetBlockResponse {
	result GetBlockResult
}

pub struct GetBlockResultB {
	hash               string
	confirmations      u64
}

fn main() {
	a := '{ "result": "hello" }'
	res_a := run<GetBlockResponse>(a) or { panic(err) return }
	println(res_a)

	b := '{ "result": { "hash": "00000000c937983704a73af28acdec37b049d214adbda81d7e2a3dd146f6ed09", "confirmations": 743970 } }'
	res_b := run<GetBlockResponse>(b) or { panic(err) return }
	println(res_b)
}

fn run<T>(s string) ?T {
	res := json.decode(T, s) or { return err }
	return res
}

What did you expect to see?

GetBlockResponse{
    result: GetBlockResult('hello')
}
GetBlockResponse{
    result: GetBlockResult{
        hash: '00000000c937983704a73af28acdec37b049d214adbda81d7e2a3dd146f6ed09'
        confirmations: 743970
    }
}

What did you see instead?

GetBlockResponse{
    result: GetBlockResult('hello')
}
GetBlockResponse{
    result: unknown sum type value
}

[ken0x0a]

It's complicated, how to determine with type to use from sumtype.

Rust has lots of options for this kind of task.
https://serde.rs/enum-representations.html

[medvednikov]

So this is a json decoding/encoding of sum types issue.

Right now it's not supported by V's json lib.

[medvednikov]

Related: #14058

[yuyi98]

I thinks it can work without _type, we can handle this by matching variants.

[Dletta]

Is there a status or recommendation on a workaround?

I have a few complex objects with similar parameter names, but different types on values. In Go I could decode into an interface and then access them with type checking.

I also tried the 'Smart Casting' Approach with json2.decode -> match Type flow, but this failed.

[enghitalo]

module main

import x.json2 as json

pub type GetBlockResult = string | GetBlockResultB

pub struct GetBlockResponse {
	result GetBlockResult
}

pub struct GetBlockResultB {
	hash               string
	confirmations      u64
}

fn main() {
	a := '{ "result": "hello" }'
	res_a := run<GetBlockResponse>(a) or { panic(err) return }
	println(res_a)

	b := '{ "result": { "hash": "00000000c937983704a73af28acdec37b049d214adbda81d7e2a3dd146f6ed09", "confirmations": 743970 } }'
	res_b := run<GetBlockResponse>(b) or { panic(err) return }
	println(res_b)
}

fn run<T>(s string) ?T {
	res := json.decode[T](s) or { return err }
	return res
}

V panic: The type of `result` can't be decoded. Please open an issue at https://github.com/vlang/v/issues/new/choose
v hash: 1ba1f99
/tmp/v_1000/v_bug.14826479504152638481.tmp.c:8139: at _v_panic: Backtrace
/tmp/v_1000/v_bug.14826479504152638481.tmp.c:17540: by main__main
/tmp/v_1000/v_bug.14826479504152638481.tmp.c:18008: by main

[JalonSolov]

Try using the x.json2 as_map function, then iterate the map to find what you need.

[enghitalo]

@enghitalo try fix

[JalonSolov]

If you want to try to fix it, just assign yourself to it.

[enghitalo]

If you want to try to fix it, just assign yourself to it.

Thank @JalonSolov . But I think that is need a special rule for it. https://docs.github.com/pt/issues/tracking-your-work-with-issues/assigning-issues-and-pull-requests-to-other-github-users