Unquoted Let

Update: The basic idea is unsound as is my implementation below. Besides this, the cl-letf macro can already do this sort of thing. At the time I obviously did not fully understand Lisp macros nor language implementation in general. Using eval is usually a stupid idea, and it's even more stupid to try to use it inside of a macro where it will most likely be used, causing an error, at compile time. I doubt this would work well with byte-compiled code. I'll leave this post here for historical purposes.

The lisp macro/form let quotes variable's symbols. This is almost always more useful than not quoting it. Otherwise, simple use would look like,

(let (('var 100))

instead of,

(let ((var 100))

In elisp, this is analogous to setq, which quotes its first argument, and set, which doesn't. It could be used like so in elisp,

(setq foo 'bar)
(setq bar 25)

(uq-let ((foo 10))
  (+ 10 bar))

The unquoted let form evalues to 20, not 35, because foo evaluates to the symbol bar, which is bound to 10. The unquoted version is used to select variable names dynamically.

This isn't very useful in a lexically scoped lisp because the variables that can access it are decided, well, lexically. With dynamic scoping we can use it to temporarily mask variables and select what variables to mask dynamically. I found a use for it in one of my projects.

I had a recursive function that searched a graph made of symbols. The symbols globally stored the state of the node. The current node was passed into the function, which would change the state of that node, and recurse. Here's how I was doing it,

(defun search (graph node)
  (set node 'visited)
  (set node 'unvisited))

That looks a lot like it is emulating a let form. If we use an unquoted let,

(defun search (graph node)
  (uq-let ((node 'visited))

There, that's a lot more lispy. Here is an elisp macro for uq-let,

(defmacro uq-let (vars &rest body)
  "Unquoted let."
  (declare (indent defun))
  `(let ,(mapcar
          (lambda (a)
            (cons (eval (car a)) (cdr a))) vars)

From this, making a uq-let* is trivial. The documentation string could use some work too.

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null program

Chris Wellons