Common Lisp the Language 2nd Edition

19.4. Defstruct Slot-Options

Each slot-description in a defstruct form may specify one or more slot-options. A slot-option consists of a pair of a keyword and a value (which is not a form to be evaluated but the value itself). For example:

(defstruct ship
(x-position 0.0 :type short-float)
(y-position 0.0 :type short-float)
(x-velocity 0.0 :type short-float)
(y-velocity 0.0 :type short-float)
(mass *default-ship-mass* :type short-float :read-only t))

This specifies that each slot will always contain a short-format floating-point number and that the last slot may not be altered once a ship is constructed.

The available slot-options are as follows.

:type

The option :type type specifies that the contents of the slot will always be of the specified data type. This is entirely analogous to the declaration of a variable or function; indeed it effectively declares the result type of the access function. An implementation may or may not choose to check the type of the new object when initializing or assigning to a slot. Note that the argument form type is not evaluated; it must be a valid type specifier.

:read-only

The option :read-only x where x is not nil specifies that this slot may not be altered; it will always contain the value specified at construction time. setf will not accept the access function for this slot. If x is nil this slot-option has no effect. Note that the argument form x is not evaluated.

Note that it is impossible to specify a slot-option unless a default value is specified first.