scheme,lisp,common-lisp , Simulate scheme define in common lisp


Simulate scheme define in common lisp

Question:

Tag: scheme,lisp,common-lisp

I wonder how to simulate the scheme define in common lisp,and want to write a macro for simulate the define. So what's the different between cl's defun deparameter defvar and scheme's define ,and how i can do this?


Answer:

define in Scheme is not easily implemented in Common Lisp. It's not easily implemented in Scheme either. It's transformation is different in different scopes:

(define test value)              ; defines test, value can be anything even (lambda ..) which makes a procedur object
(define (test arg ...) body ...) ; defines the procedure test

(define (somefun)
  (define test ...)
  (define (test2 x) ...)
  ...)           

Is a fancey way of writing:

(define (somefun)
  (letrec ((test ...)
           (test2 (lambda (x) ...))
    ...))

So what are the eqvivalents in Common Lisp:

(define myplus otherfun)             ; (setf (symbol-function 'myplus) otherfun)
(define myplus (lambda args . body)) ; (defun myplus args . body)
(define (myplus . args) . body )     ; (defun myplus args . body)
(define value 10)                    ; (defparameter value 10)

Here is my take on the macro. Still this won't work for internal defines:

(defmacro define (name-or-ll &body expressions)
  (flet ((dotted-to-rest (lst)
           (let ((last (last lst)))
             (if (null (cdr last))
                 lst
                 (append (butlast lst)
                         (list (car last) '&rest (cdr last)))))))

    (cond ((consp name-or-ll)             ; (define (fun a b) ...)
           `(progn 
              (defun ,(car name-or-ll) ,(dotted-to-rest (cdr name-or-ll)) ,@expressions)
              (defparameter ,(car name-or-ll) #',(car name-or-ll))))
          ((and (consp (car expressions)) ; (define fun (lambda (a b) ...)) 
                (eq (caar expressions) 'lambda))
            `(define (,name-or-ll ,@(cadar expressions)) ,@(cddar expressions)))
          (t `(let ((value ,(cons 'progn expressions)))                
                (when (functionp value)
                  (setf (symbol-function ',name-or-ll) value))
                (defparameter ,name-or-ll value))))))

(define proc (lambda (x) (* x x)))
(define myproc proc)
(define myplus #'+)
(define test 'test)
(define (testproc a b) (+ a b))
(define testproc2 (lambda (a . b) (apply myplus a b)))
(list (proc 10) (myproc 10) 
      (myplus 2 3) (testproc 2 3) 
      (testproc2 2 2 1) (funcall testproc2 2 2 1) 
      test) ; => (100 100 5 5 5 5 TEST)

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