Serde deserialising tuple containing varying types

Question

I have some JSON of this format:

[ 123, {“some struct”: “is always here”}, {“optional struct”: ”is not always here”}, “String1”, “String2” ]

Note that the second struct is sometimes not present - the array will sometimes contain 4 elements, other times 5. This means that seq[1] is always the struct, but seq[2] may either be the struct or String1.

I’ve written a custom Deserialize using the SeqAccess visitor, looking something like this:

let number: usize = v.next_element()?; let struct: SomeStruct = v.next_element()?; let maybe_struct: OtherStruct = v.next_element()?; // …

The problem is that calling v.next_element() where the struct is NOT present, it wants to deserialise immediately into OtherStruct, which fails because the next element is String1. If I call v.next_element() again, the visitor has already moved onto the next element, and I’ll get String2.

I think I’m looking at this wrong and probably need to write a visitor which receives a method call for each element in the sequence, I’ve not been able to locate any examples or documentation that explains how to do this.

I think you'd probably need to deserialize the third next_element into some sort of intermediate representation - e.g. a StructOrString enum. Then you could match on that to determine what to expect for the remaining elements. — Joe Clay
– Joe Clay, Commented Nov 23, 2022 at 11:47

Holloway · Accepted Answer · 2022-11-23 12:12:29Z

You should be able to do this without using a custom deserializer if you wrap the combinations in an enum and use serde's untagged attribute.

Something like this should work

use serde::Deserialize; use serde_json::from_str; #[derive(Debug, Deserialize)] #[serde(untagged)] enum Either { Four(i32, SomeStruct, String, String), Five(i32, SomeStruct, OtherStruct, String, String), } pub fn main() { let src_with_four = todo!(); let src_with_five = todo!(); let f1 = from_str::<Either>(src_with_four); let f2 = from_str::<Either>(src_with_five); println!("{f1:?}"); println!("{f2:?}"); }

Updated to use tuple variants directly based on @cafce23's comment

Previous struct version in playground

@cafce25's example in the playground

The tuple variant is probably cleaner, I will update my answer
This works wonderfully, thanks! It's very close to what I initially attempted, using struct syntax instead of tuples, which doesn't work.

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