with-slots vs. with-accessors

Most CLOS objects are implemented as standard-instances. A standard-instance is a collection of storage cells called slots, and the slots are addressed by name. You could imagine an alternative implementation where an instance is a vector that is addressed by an integer, but named slots are more flexible and abstract.

Many object systems map the named fields of an instance into lexically scoped variables. Within a method body, you can just refer to the slot as if it were a variable. Assignment to the variable effectively updates the slot. There are pros and cons to this. On the plus side, it is very convenient to refer to slots as if they were variables. On the minus side, it is difficult to rename a slot, because you have to rename all the references to it, and slot names can collide with lexical variables. It can make the code brittle with regard to slot naming. But CLOS lets you choose if you want to do this or not. The with-slots macro installs a set of symbol macros that let you refer to each slot as if it were a variable.

But the slots of an instance are an implementation detail. You really want an abstract API for your objects. You want logical fields to be accessed by getter and setter functions. The logical field will typically be backed by a slot, but it could be a computed value. Logical fields are more flexible and abstract than slots.

When you define a slot, you can specify a :reader and :accessor function for that slot. This covers the very common use case of a getter/setter pair that is backed by a slot in the instance.

You can also map the logical fields of an instance into lexical variables. The with-accessors macro installs a set of symbol macros that let you refer to each logical field as if it were a lexical varible.

I often see with-slots used where with-accessors would be more appropriate. If you find yourself wanting to use with-slots, consider if you should be using with-accessors instead.

Personally, I prefer to avoid both with-slots and with-accessors. This makes CLOS objects act more like structs. Structs are easier for me to understand than magic lexical variables.

Tip

The accessors for slots are generic. You therefore want them to have generic names. For example, suppose you have a point class with an x and y slot. You don't want to call your accessors point-x and point-y because the names would be inappropriate for subclasses. You want to have names something like get-x and get-y. These functions would naturally work on subclasses of points, but because get-x and get-y are generic, you could also extend them to work on any class that has a meaningful x and y.