. . .

New problem expressed in terms of old problem.

In addition to the SQL way of framing this, we could also do this with a Julia struct, though admittedly that would not capture the foreign key requirements. On the right, I show how we declare this data structure in AlgebraicJulia, which is declaring the database schema. (…)

For our first concrete example of abstraction reuse: consider this big triangle. We could always just code this up again from scratch, but what if we recognize that the big triangle is just three different triangles that have been glued together? It’s a mark of a good abstraction for our triangle function that it permits us to do this.