Recursive 'compose' in common lisp (translating from scheme)

Question

I'm studying Common Lisp. Don't know Scheme yet.

I have no problem with following compose in Common Lisp

(defun compose (&rest fns)  
  (destructuring-bind (fn1 . rest) (reverse fns)
    #'(lambda (&rest args)                       
       (reduce #'(lambda (v f) (funcall f v))    
               rest                              
               :initial-value (apply fn1 args)))))

(it's interesting the deep similarity between compose & reduce :) )

Then, reading around, I found an example of compose in Scheme, but done recursively, like so:

(define (compose . fs)
  (if (null? fs) (lambda (x) x)
      (lambda (x) ((car fs) ((apply compose (cdr fs)) x)))))

My attempt to translate the above to Common Lisp (rather in the dark here):

(defun compose-r (&rest fs)
  (if (null fs)
      #'(lambda (x) x)                   ; base case
      #'(lambda (x) (funcall (car fs)    ; first given fn, applied to...
                     (compose-r (cdr fs) x)))))

Interestingly the above produces:

COMPOSE-R
[179]> (mapcar (compose-r #'list #'abs #'round #'sqrt) '(4 9 16 25 130))
((#<FUNCTION :LAMBDA (X) (FUNCALL (CAR FS) (COMPOSE-R (CDR FS) X))>)
 (#<FUNCTION :LAMBDA (X) (FUNCALL (CAR FS) (COMPOSE-R (CDR FS) X))>)
 (#<FUNCTION :LAMBDA (X) (FUNCALL (CAR FS) (COMPOSE-R (CDR FS) X))>)
 (#<FUNCTION :LAMBDA (X) (FUNCALL (CAR FS) (COMPOSE-R (CDR FS) X))>)
 (#<FUNCTION :LAMBDA (X) (FUNCALL (CAR FS) (COMPOSE-R (CDR FS) X))>))
[180]>

So something's working but I missed a function call.

It would be great perhaps if someone could use this example to point out some key differences between Common Lisp and Scheme and to outline the key things one must know in translating.

I already noticed that in scheme it seems possible to evaluate an expression in position zero of an expression, whereas Common Lisp complains that "should be a lambda expression".

Thanks in advance.

Rainer Joswig Rainer Joswig · Accepted Answer · 2019-04-26T14:34:12

(define (compose . fs)
  (if (null? fs)
      (lambda (x) x)
      (lambda (x)
        ((car fs)
         ((apply compose (cdr fs)) x)))))

is

(defun compose (&rest fs)
  (if (null fs)
      (lambda (x) x)
      (lambda (x)
        (funcall (car fs)
                 (funcall (apply #'compose (cdr fs)) x)))))

Then:

CL-USER 53 > (mapcar (compose #'list #'abs #'round #'sqrt) '(4 9 16 25 130))
((2) (3) (4) (5) (11))

The main differences in Common Lisp:

call function objects with FUNCALL
retrieve a function object with #'some-fun or (function some-fun)

Recursive 'compose' in common lisp (translating from scheme)

1 Answers