Scalar API

Design choices:

Scalar is trait object

There are three reasons:

• a scalar should have a small memory footprint, which an enum would not ensure given the different physical types available.
• forward-compatibility: a new entry on an enum is backward-incompatible
• do not expose implementation details to users (reduce the surface of the public API)

Scalar MUST contain nullability information

This is to be aligned with the general notion of arrow's Array.

This API is a companion to the Array, and follows the same design as Array. Specifically, a Scalar is a trait object that can be downcasted to concrete implementations.

Like Array, Scalar implements

• dtype, which is used to perform the correct downcast
• is_valid, to tell whether the scalar is null or not

There is one implementation per arrows' physical type

• Reduces the number of match that users need to write
• Allows casting of logical types without changing the underlying physical type