Use MaybeNullableValue in other postgres function

[guissalustiano]

In postgres to_json function @weiznich comes with a magic MaybeNullableValue implementation (he provides an explanation here)
This trait allows "overload" the to_json function to work as

• to_json(e: SqlType) -> Json
• to_json(e: Nullable<SqlType>) -> Nullable<Json>
  Which makes their use much cleaner, without the need to call assume_not_nullable() and just trust in the type system.

There are multiple other functions with this same behavior listed below: if you pass null, Postgres returns null, if you don't pass null it will never return not null

• isempty
• lower_inc
• upper_inc
• lower_inf
• upper_inf
• cardinality
• array_length
• array_lower
• array_position
• array_position_with_subscript
• array_positions
• array_ndims
• array_upper

Would it be possible to generalize the MaybeNullableValue so we can use it in the other cases? Or would it be better to make a helper type for each one?

Thanks in advance!

[weiznich]

Got catch. I would say the correct solution is to use MaybeNullableValue<> for these functions as well. As far as I can see it should already be possible to just use the existing trait for that, so we shouldn't need new helper traits for that. Essentially you would transform fn foo<T: AnyOtherTrait>(t: T) -> ReturnType into fn foo<T: AnyOtherTrait + MaybeNullableValue<ReturnType>(t: T) -> T::Out and it should work.

[guissalustiano]

Awesome, I didn't understand only looking at the type signature. Thanks!

[guissalustiano]

I implemented that on #4260