shapes

A catalogue of shapes behind one trait: the dynamic half of the trait system.

The axis: forgetting the type. Every other program in the set knows the type of everything it computes with. This one does not, and that is the whole exercise — a &Shape is a fat pointer that has forgotten what it points at, and the only thing left to do with it is ask the trait’s questions and believe the answers.

Why it needs two families

The shapes come in two kinds, and the second is what makes the problem worth writing.

Four of them are leaves — a circle, a rectangle, a triangle, a polygon — and each answers out of its own fields. The other three are combinators: a group is a shape made of shapes, and a move and a scale each wrap one shape and adjust what it says.

A combinator holds &Shape, so it holds something it cannot name, and a group of groups is dynamic dispatch calling itself. A trait-object design that only ever holds leaves has not been tested; one that holds arbitrary other objects of the same trait has.

What it exercises

Erasure is a coercion, applied per position. Each concrete shape is constructed, boxed because a &Shape was expected, and then erased — the ordinary “write the construction and it is allocated” rule with one more step. Because the coercion applies per branch, an if or a match whose arms are different concrete types meets at one trait object, which is the point of having them.

Object safety is what makes the trait usable this way. A trait can be made into an object when every member has a receiver and mentions Self nowhere but there. Shape‘s members answer questions about the shape and never hand another one back at its own type, so it qualifies — where the whole operator catalogue does not, add(self, rhs: Self) -> Self being the clearest case. That exclusion is the right answer rather than a limitation: an operator over two values of one type is a question about types known while compiling.

A bounding box is a plain struct rather than a shape. It is the answer to a question about a shape and not another thing to dispatch on — a distinction that is easy to lose when everything in sight is a trait object, and the reason the file says so out loud.

Source · Next: scheduler — OS shapes, and reference graphs mutated through references.