How to force slot's type to be checked during make-instance?

Question

Let\'s say I have the following class declaration:

(defclass foo-class ()
  ((bar :initarg :bar
        :type list)))

When I create an instance

Answer 1

Whether slot types are being checked or not is undefined for both structures and CLOS instances.

Many implementations will do it for structures - but not all.

Few implementations will do it for CLOS instances - Clozure CL actually does it for example.

SBCL also can check CLOS slot types - when safety is high:

* (declaim (optimize safety))

NIL
* (progn
(defclass foo-class ()
  ((bar :initarg :bar
        :type list)))
(make-instance 'foo-class :bar 'some-symb))

debugger invoked on a TYPE-ERROR: The value SOME-SYMB is not of type LIST.

Type HELP for debugger help, or (SB-EXT:EXIT) to exit from SBCL.

restarts (invokable by number or by possibly-abbreviated name):
  0: [ABORT] Exit debugger, returning to top level.

((SB-PCL::SLOT-TYPECHECK LIST) SOME-SYMB)
0] 

How to do it otherwise?

This is kind of an advanced subject which probably needs some CLOS meta-object-protocol hackery. Two variants:

• define a method for SHARED-INITALIZE which checks the init arguments.

• define a metaclass for your class and a method on SET-SLOT-VALUE-USING-CLASS . But then you need to be sure that your implementation actually provides AND uses SET-SLOT-VALUE-USING-CLASS. This is a generic function, which is part of the MOP. Some implementations provide it, but some are only using it when requested (otherwise setting a slot may get a speed penalty).

For the latter here is self-built SBCL version to check types for writing slots:

First the metaclass:

; first a metaclass for classes which checks slot writes
(defclass checked-class (standard-class)
  ())

; this is a MOP method, probably use CLOSER-MOP for a portable version
(defmethod sb-mop:validate-superclass
           ((class checked-class)
            (superclass standard-class))
   t)

Now we check all slot writes for that metaclass:

; this is a MOP method, probably use CLOSER-MOP for a portable version    
(defmethod (setf sb-mop:slot-value-using-class) :before
              (new-value (class checked-class) object slot)
  (assert (typep new-value (sb-mop:slot-definition-type slot))
      ()
    "new value ~a is not of type ~a in object ~a slot ~a"
    new-value (sb-mop:slot-definition-type slot) object slot))

Our example class uses that metaclass:

(defclass foo-class ()
  ((bar :initarg :bar :type list))
  (:metaclass checked-class))

Using it:

* (make-instance 'foo-class :bar 42)

debugger invoked on a SIMPLE-ERROR in thread
#:
  new value 42 is not of type LIST
  in object #
  slot #

Type HELP for debugger help, or (SB-EXT:EXIT) to exit from SBCL.

restarts (invokable by number or by possibly-abbreviated name):
  0: [CONTINUE] Retry assertion.
  1: [ABORT   ] Exit debugger, returning to top level.