Missing `SerializeValue` impl for `MaybeEmptiable<T>`.

[wprzytula]

Background

Empty CQL values are those that have length equal to 0. It's a legacy from old Cassandra versions.

MaybeEmpty is type-safe representation of any type that can be inserted into the DB as empty (some types are emptiable and others are not).
It was introduced to support typed deserialization of a value that is possibly empty in the DB; therefore, it implements DeserializeValue.

There's also CqlValue::Empty variant, so it's possible to insert empty values to the DB using this type-erased method (i.e., CqlValue umbrella type).

Problem

MaybeEmpty does not implement SerializeValue, so we're missing the proper way to insert empty types into the DB. Let's add this missing implementation.

[Lorak-mmk]

Did you mean MaybeEmpty? I see it:

pub enum MaybeEmpty<T: Emptiable> {
    Empty,
    Value(T),
}

but I don't see MaybeEmptiable.

One issue with it is that it is defined in deserialize/value.rs. Should move it into more neutral place (and re-export in value for backwards compatibility).

One other idea: MaybeEmpty is generic, so requires specifying a type. This ofc makes sense for deserialization, but no always for serialization - you may want to just serialize an empty value. Currently this requires CqlValue::Empty, but requiring its use seems quite weird. Maybe we could add Empty type? It would serialize to empty value (for types that support it). Optionally, we could impl DeserializeValue for it (failing for non-emptiable column types and non-empty values), but its less important.

[wprzytula]

Did you mean MaybeEmpty? I see it:

pub enum MaybeEmpty<T: Emptiable> {
    Empty,
    Value(T),
}

but I don't see MaybeEmptiable.

Yes, redacted.

One issue with it is that it is defined in deserialize/value.rs. Should move it into more neutral place (and re-export in value for backwards compatibility).

Agreed.

One other idea: MaybeEmpty is generic, so requires specifying a type. This ofc makes sense for deserialization, but no always for serialization - you may want to just serialize an empty value. Currently this requires CqlValue::Empty, but requiring its use seems quite weird. Maybe we could add Empty type? It would serialize to empty value (for types that support it). Optionally, we could impl DeserializeValue for it (failing for non-emptiable column types and non-empty values), but its less important.

Note that we have no Null rust type. Instead, we use strongly typed Option<T>, so null is always parametrised by the Rust type it corresponds to. This increases type safety. I'd prefer to have exactly the same for empty values, because we're in Rust and strong typing is the way to live.

One thing I do not understand:

you may want to just serialize an empty value. Currently this requires CqlValue::Empty, but requiring its use seems quite weird

Once we add the missing SerializeValue impl, you will be able to "just" serialize MaybeEmpty::Empty.

[Lorak-mmk]

Ok, you convinced me,

So, todo:

• impl SerializeValue for MaybeEmpty
• Move MaybeEmpty to some other place
• Add re-export in deserialize/value.rs, with a note to remove in 2.0