Struct Properties vs Generics in Racket

Question

Racket seems to have two mechanisms for adding per-type information to structs: generics and properties. Unfortunately, the documentation doesn't seem to indicate when one is preferred over the other. The docs do say:

Generic Interfaces provide a high-level API on top of structure type properties.

But that doesn't seem to provide a good intuition when I should use one over the other. It does seem pretty clear that define-generic provides a much higher level interface than make-struct-type-property. But many struct types still only use properties, which seems to indicate that there are still cases where the low-level API is preferred.

So the question is, when is using the struct properties system better than using the generics one, or does the properties library only exist as a historic relic?

Answer 1

The struct property system is the implementation strategy for the generic interface library, so it's not deprecated. It, or something like it, is necessary to make generic interfaces work. Not all uses of struct properties make sense as generic interfaces either.

That said, for many typical use cases the define-generic form is preferred. As the #:methods form for structs suggests, it is useful for code that is organized in an object-oriented fashion with interface-based dispatch. Examples of this include sequences (gen:sequence from data/collection) and dictionaries (gen:dict).

Plain struct properties in the Racket codebase are typically used when some data just needs to be stored in a struct as metadata, or when there is only one "method" and it's needlessly complicated to use define-generic, or when the interface is more complicated than "just put a procedure in here". Examples include prop:procedure or prop:evt.