A placeholder (e.g. the symbol `_' which should normally be unbound)
stands for an argument that is used from the args provided in the actual
call:
#+begin_src emacs-lisp
(defmacro applying-partially (call)
(let ((args (make-symbol "args")))
`(lambda (&rest ,args)
(apply #',(car call)
,@(mapcar (lambda (arg)
(if (eq arg '_)
`(pop ,args)
arg))
(cdr call))
,args))))
(defalias 'minus-10 (applying-partially (- _ 10)))
(minus-10 120) ;; ==> 110
(defalias 'my-list-with-some-elts
(applying-partially (list 0 _ 2 _ 4)))
(my-list-with-some-elts 'a 'b 'c 'd) ; => (0 a 2 b 4 c d)
(symbol-function 'my-list-with-some-elts)
;; => (closure (t) (&rest args)
;; (apply #'list 0 (pop args) 2 (pop args) 4 args))
#+end_src
--
Daanturo.