Write a function that behaves like car, cadr, caddr etc - common-lisp

Question

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

Answer 1

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.