CODEWITHSUNDEEP
common-lispsbclcdr
Bandoos
Bandoos

Reputation: 93

Write a function that behaves like car, cadr, caddr etc

I'm new to lisp (i'm experimenting with sbcl and ccl), i came across the use of car and cdr that can be chained arbitrarily within a single function call like (caddr).

I was wandering how one would write functions that behave like this... Say for example i'd like my-eval to eval the input s-exp 3 times if i invoke it like (my-evaaal '(+ 2 1))

I've hacked my way around with a macro like (my-ev $$$$ '(...)) where the behavior is dictated by the number of '$' in the first argument by transforming it into char sequence (coerce (symbol-name x) 'list) and the evaluate and recurse until the list is nil...

basic need:

;; if 
(defvar *foo* 1)
(eval '*foo*) ;; => 1
(eval ''*foo*) ;; => *foo*
(eval '''*foo*) ;; => '*foo*

;; then
(eval (eval (eval '''*foo*))) ;; => 1

desired syntax

(my-eval '''*foo*) ;; => '*foo*
(my-evaal '''*foo*) ;; => *foo*
(my-evaaal '''foo) ;; => 1

Upvotes: 1

Views: 468

Answers (1)

coredump
coredump

Reputation: 38809

Functions like CAAR, CADR are just regular functions; you can define a macro to help you define them easily if you want to.

 Macros

 (defpackage :so (:use :cl :ppcre))
 (in-package :so)

 (defmacro eval%% (count form)
   (case count
     (0  form)
     (1 `(eval ,form))
     (t (check-type count (integer 2))
        `(eval%% ,(1- count) (eval ,form)))))

For example, the following :

(eval%% 3 '''most-positive-fixnum)

expands successively as:

(EVAL%% 2 (EVAL '''MOST-POSITIVE-FIXNUM))
(EVAL%% 1 (EVAL (EVAL '''MOST-POSITIVE-FIXNUM)))
(EVAL (EVAL (EVAL '''MOST-POSITIVE-FIXNUM)))

Then, you can define custom eval functions as follows, or even with another macro:

(defun evaal (x) (eval%% 2 x))
(defun evaaal (x) (eval%% 3 x))

Handler and restarts

Alternatively, note that you can catch calls to undefined functions:

(block nil
  (handler-bind ((undefined-function
                  (lambda (e)
                    (return
                      (values (cell-error-name e)
                              (compute-restarts e))))))
    (evaaaaaal 'a)))

=> EVAAAAAAL
  (#<RESTART CONTINUE {7FD5F5F8CE43}> #<RESTART USE-VALUE {7FD5F5F8CE03}>
   #<RESTART SB-KERNEL::RETURN-VALUE {7FD5F5F8CDC3}>
   #<RESTART SB-KERNEL::RETURN-NOTHING {7FD5F5F8CD83}>
   #<RESTART SWANK::RETRY {7FD5F5F8DA13}> #<RESTART ABORT {7FD5F5F8DEC3}>
   #<RESTART ABORT {7FD5F5F8EB03}>)

You can also use the standard USE-VALUE restart to provide a different function to call:

(defun multi-eval-handler (condition)
  (let ((name (cell-error-name condition)))
    (when (eq (symbol-package name) (find-package :so))
      (register-groups-bind ((#'length count)) ("EV\(A+\)L" (string name))
        (invoke-restart 'use-value (make-repeated-evaluator count))))))

You need an auxiliary function that computes an evaluation N times:

(defun make-repeated-evaluator (count)
  (case count
    (0 #'identity)
    (1 #'eval)
    (t (check-type count (integer 2))
       (lambda (form)
         (loop
            for value = form then (eval value)
            repeat count
            finally (return value))))))

For example:

(funcall (make-repeated-evaluator 3)
         '''most-positive-fixnum)
=> 4611686018427387903

And then, you can have arbitrarily long eval functions:

 (handler-bind ((undefined-function #'multi-eval-handler))
     (evaaaaaaaaaaaaaal '''''''''''''0))

Now, if you compile the code, you'll have warnings at compile-time about the unknown function, when then you can muffle warnings.

Upvotes: 4

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