Multithreading in sbcl

Question

I am newbie at programming in lisp.I am using sbcl.In one part of program thread is created as-

(sb-thread:make-thread
  (lambda ()
    (progn
      (sleep 0)
      (setf c (+ a b))
      (print "ADDITION:")
      (print c))))

I am not getting what lambda() and progn(sleep 0) does, and why it is written inside sb-thread bracket?

Answer 1

sb-thread:make-thread takes a function to call in a newly created thread.

(lambda ()
   (progn
      (sleep 0)
      (setf c (+ a b))
      (print "ADDITION")
      (print c)))

is an anonymous function. In turn progn creates a program block (un-necessarily in this case, as a lambda body is an implicit progn) and (sleep 0) is probably used as a "please invoke the scheduler here" (it's essentially a no-op, sleeping for 0 seconds).