Let you create any joinable type easily.

This is the api used for Dict.OneToOne and Dict.ManyToOne implementations. A good other container implementation would be a database table with a forward key in addition to the primary key.

To make your container "joinable", you just need to provide a way to convert it into a Dict. Then add a from, innerJoin and (optionally) outerLeftJoin method like this:

from =
    Dict.Joinable.from convertMyContainerToDict


innerJoin =
    Dict.Joinable.innerJoin convertMyContainerToDict


outerLeftJoin =
    Dict.Joinable.outerLeftJoin convertMyContainerToDict


convertMyContainerToDict : MyContainer a -> Dict comparable a
convertMyContainerToDict =
    ...