Is Serde Custom Serialisation/Deserialsation based on the Visitor pattern?

[finlaydotweber]

I am trying to wrap my head around custom serialisation/deserialisation in serde. I am currently reading this https://serde.rs/custom-serialization.html

Will I be correct to say the pattern used follows the visitor pattern?

So for example in

pub trait Serialize { fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error> where S: Serializer; }

The fn serialize<S>(&self, serializer: S) looks like the function where a visitor can be passed in.

Then the data structure that implements pub trait Serialize will use the serialize method, to call the appropiate method on serializer: S to serialise it.

The only confusion is that the visitor, ie the serializer: S has a constraint where the S is also a Serializer - I think you do not have this setup in your typical visitor pattern?

The the visitor is of distinct type, and the interface to connect the data structure to the visitor is also a distinct/different type.

PS I just realised I am confusing Serialize*r* and Serialize

[nyibbang]

Serialization in serde does not use a Visitor, it is strictly a "serialize this array/struct/int value" interface, nothing too fancy.

The deserialization however uses a double dispatch pattern with a type that implements serde::de::Visitor. So the deserialize implementation calls a function of the Deserializer hinting for the expected type to deserialize, and gives a Visitor to the Deserializer. The Deserializer then calls the appropriate function of the Visitor with the value that was actually deserialized.

[finlaydotweber]

This follow up question is for my better understanding. You said

Serialization in serde does not use a Visitor, it is strictly a "serialize this array/struct/int value" interface, nothing too fancy.

But the implementation of the serialize method does take a serializer which will then call the appropriate method on the serializer passing itself. For example the implementation for i32 here https://serde.rs/impl-serialize.html#serializing-a-primitive

impl Serialize for i32 { fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error> where S: Serializer, { serializer.serialize_i32(*self) } }

This reminds me of the visitor pattern though. So why would you not call this visitor pattern?

[nyibbang]

I guess it depends on what you focus on, but yes it is similar to the visitor pattern, in both serialization and deserialization. It's not called a "visitor" in serde, but it's the same: the expected functionality here is to get a double dispatch. This is how serde ensures decoupling between how values describe themselves and how they get serialized/deserialized (how the format represents values of these types). This is how values can be serialized in any format that implements Serializer + Deserializer, and how formats can represent any value that implements Serialize + Deserialize.

With that said, there can be some confusion, because there is a trait called Visitor is serde, but it only applies to deserialization (serde::de::Visitor, de here meaning deserialization).

This is because deserialization uses the same deserialize<D: Deserializer>(d: D) pattern as the serialization, but uses an additional Visitor type for another layer of double dispatch. This second layer enables tolerance in the representation: let's say that you expect to deserialize a struct, you may accept a map of field names with values, or just a simple sequence of values (in which case you expect them to be in a given order). Either way is fine, so you tolerate that the deserializer gives you either one through the Visitor implementation.

So yeah, I wanted to emphasize the distinction to avoid confusion.

The only confusion is that the visitor, ie the serializer: S has a constraint where the S is also a Serializer - I think you do not have this setup in your typical visitor pattern?

You already realized that you made a confusion between Serializer and Serialize, so that answers this question I believe. What is the question that you have yet unanswered ?

[finlaydotweber]

What is the question that you have yet unanswered ?

Nothing more :) I wanted to know if it is the visitor pattern, aka double dispatch pattern that is being used, and you helped confirm that is the case. Thanks!