Rel

N-ary relation (where N is determined by L). Loosely speaking, it represents the signature of a database table, i.e. the datatypes of the columns.

An implementation MyRel[A, B] extends Rel[F[A, B]] must ensure that

• if (rel1: MyRel[T, U]) == (rel2: MyRel[V, W]), i.e. two values of MyRel with different types are equal, then
  • for all row1: F[T, U], row2: F[V, W]
    • row1 == row2 ⇒ F[T, U] =:= F[V, W]

In other words, the table signature can be somewhat parametric, as long as the actual row values cannot be mistakenly thought to be of a different type.

Moreover, queries have to specify values of enough columns, so that the types of the remaining columns are uniquely determined by the types of the given columns.