Neither a function nor a macro would do

Question

Consider this question. Here the basic problem is the code:

(progv '(op arg) '(1+ 1)
 (eval '(op arg)))

The problem here is that progv binds the value to the variable as variable's symbol-value not symbol-function. But, that's obvious because we didn't explicitly suggest which values are functions.

The Plan

So, in order to solve this problem, I thought of manually dynamically binding the variables, to their values based on the type of values. If the values are fboundp then they should be bound to the symbol-function of the variable. A restriction, is that match-if can't be a macro. It has to be a function, because it is called by a funcall.

Macro : functioner:

(defmacro functioner (var val)
  `(if (and (symbolp ',val)
            (fboundp ',val))
       (setf (symbol-function ',var) #',val)
       (setf ,var ,val)))

Function: match-if:

(defun match-if (pattern input bindings)
  (eval `(and (let ,(mapcar #'(lambda (x) (list (car x))) bindings)
                (declare (special ,@ (mapcar #'car bindings)))
                (loop for i in ',bindings
                      do (eval `(functioner ,(first i) ,(rest i))))
                (eval (second (first ,pattern))))
              (pat-match (rest ,pattern) ,input ,bindings))))

Here, the let part declares all the variables lexically (supposedly). Then declare declares them special. Then functioner binds the variables and their values aptly. Then the code in the pattern is evaluated. If the code part is true, then only the pattern-matcher function pat-match is invoked.

The Problem

The problem is that in the function, all it's arguments are evaluated. Thus bindings in the let and declare parts will be replaced by something like :
((v1 . val1)(v2 . val2)(v3 . val3))
not
'((v1 . val1)(v2 . val2)(v3 . val3))
So, it's treated as code, not a list.
So, I'm stuck here. And macros won't help me on this one.
Any help appreciated.

coredump · Accepted Answer · 2017-06-23T21:15:01.390

Not the answer you are looking for, but PROGV is a special operator; it is granted the ability to modify the dynamic bindings of variables at runtime; AFAIK, you can't simply hack it to operate on "dynamic function bindings". The point of progv is to use list of symbols and values that are evaluated, meaning that you can generate symbols at runtime and bind them dynamically to the corresponding values.

You might be able to find a solution with eval but note that if you macroexpand into (eval ...), then you loose the surrounding lexical context, which is generally not what you want ("eval" operates on the null lexical environment). I speculate that you could also have a custom code walker which works on top-level forms but reorganizes them, when it finds your special operator, to bring the context back in, producing something like (eval '(let (...) ...)).

http://christophe.rhodes.io/notes/blog/posts/2014/naive_vs_proper_code-walking/ is a good start — coredump, Jun 24 '17 at 06:21
[This](https://stackoverflow.com/a/44691747/5345646) version of `match-if` seems to work. All I had to do was add a quote before `,pattern` ,`,input` and `,bindings`. — Mooncrater, Jun 29 '17 at 15:41

Neither a function nor a macro would do

The Plan

The Problem

1 Answers1