Module scalar 

We can enforce the invariants (see MapArray) in too many places (both MapValue, MapRef and MapArray).

So we define the types and constructors in a separated mod to prevent direct construction. We only check the invariants in the constructors. After they are constructed, we assume the invariants holds.

Structs

MapRef

A map is just a slice of the underlying struct array.

MapValue

Refer to MapArray for the invariants of a map value.