Join

This typeclass with its implicit instances decides how two containers should be joined.

Supported containers are

• Id for scalar values
• Option for optional values
• Seq for a vector of values

Those container types form an ordering from most specific to most abstract:

• Id contains always one value
• Option contains always zero or one value
• Seq can contain any number of values

The rule to determine what the result type of joining two container types is that the result is as generic as the more generic of both of the input types.

The implicit definitions in the companion object of join form evidence for this ordering.