functioncommon-lispsbclclos

Are lambda functions CLOS objects?

When I do this:

(defparameter thing #'(lambda () (+ 1 1)))

The returned value for thing is

#<FUNCTION (LAMBDA ()) {53A11BEB}>

This print out looks a lot like the print-object for a CLOS.

So, I thought id like to see the actual function detail in the print-object.

I cannot seem to find out the name of the function object to use in my list-class-slots function:

(defun list-class-slots (class)
  (mapcar #'sb-mop::slot-definition-name
     (sb-mop::class-direct-slots (class-of (make-instance class)))))

I tried sb-mop::funcallable-standard-class but this returns nil. I also tried function and the error told me to use the funcallable-standard-class.

Am I wrong here? is the original lambda printout NOT related to CLOS at all? if not, what is the source of the function printout?

Solution

  • A function created with lambda is an object, yes. It's an instance of the system class function:

    * (class-of thing)
#<SB-PCL:SYSTEM-CLASS COMMON-LISP:FUNCTION>

    Functions are printed in an implementation-dependent manner, in practice often in the convention for an unreadable value (#<...>). It appears that SBCL has a specialization of the print-object generic method for functions.

    * (find-method #'print-object '() (mapcar #'find-class '(function t)))
#<STANDARD-METHOD COMMON-LISP:PRINT-OBJECT #'T {100042B4C3}>

    Indeed, here it is. You can poke around in it to see how it works.