collapse.lisp

;;; Lisp 1 vs. Common Lisp vs. Scheme vs. Emacs Lisp

;;; Rainer Joswig, joswig@lisp.de, 2012

;;; The original code is from
;;;  THE LISP 1 PROGRAMMER'S MANUAL FROM 1960, PAGE 99FF

; the Lisp 1 Programmer's manual is the first manual for the first Lisp implementation.
; DEFINE then was the operator to define new function.s
; With minor rewriting the code runs largely unchanged in Common Lisp.
; That's what makes Common Lisp one of the main Lisp dialects:
;
;  it still contains at its core the original functionality of LISP from 1960.
;
; This makes it possible to understand code from decades back to the 1960s.
;
; One can see below that both Emacs Lisp and Scheme are two other languages
; which can express the same program also with only minor changes.


#|

; THE LISP 1 PROGRAMMER'S MANUAL FROM 1960, PAGE 99FF:
; http://bitsavers.org/pdf/mit/rle_lisp/LISP_I_Programmers_Manual_Mar60.pdf

; Lisp 1 used DEFINE to define functions. The comma character was whitespace
; between the list elements.

DEFINE
(((COLLAPSE,(LAMBDA,(L),(COND,
   ((ATOM,L),(CONS,L,NIL))
   ((NULL,(CDR,L)),
     (COND,((ATOM,(CAR,L)),L),(T,(COLLAPSE,(CAR,L)))))
   (T,(APPEND,(COLLAPSE,(CAR,L)),(COLLAPSE,(CDR,L)))))
))))))

|#


; THE SAME, JUST REFORMATTED, IN COMMON LISP:

(DEFUN COLLAPSE (L)
  (COND 
   ((ATOM L) (CONS L NIL))
   ((NULL (CDR L))
    (COND ((ATOM (CAR L)) L)
          (T (COLLAPSE (CAR L)))))
   (T (APPEND (COLLAPSE (CAR L))
              (COLLAPSE (CDR L))))))

#|

CL-USER > (COLLAPSE '(((A B) ((C))) ((D (E F)) (G) ((H)))))
(A B C D E F G H)

|#

; Scheme

(define collapse
  (lambda (l)
    (cond 
     ((atom? l) (cons l '()))
     ((null? (cdr l))
      (cond ((atom? (car l)) l)
            (else (collapse (car l)))))
     (else (append (collapse (car l))
                   (collapse (cdr l)))))))

; Emacs Lisp

(defun collapse (l)
  (cond 
   ((atom l) (cons l nil))
   ((null (cdr l))
    (cond ((atom (car l)) l)
          (t (collapse (car l)))))
   (t (append (collapse (car l))
              (collapse (cdr l))))))