Faking local dynamic function bindings

[Joost Kremers <joost.m.kremers@gmail.com>]

Hi all,

I sorta trialed and errored my way to the following code (simplified[1]):

```
(defun foo (sexpr)
  (eval
   `(cl-macrolet ((contains (regexp)
                            `(string-match ,regexp string)))
      (sort (delq nil (mapcar #'(lambda (item)
                                  (let ((string (assq item strings-alist)))
                                    (when ,sexpr)
                                    string))
                              items-alist))
            'string<))))
```

Assume that `items-alist` is an alist with (SYMBOL . STRING) items.

`sexpr` is an interactively built expression that is used to filter
items-alist. It is a single s-expression and is runnable Elisp code,
provided that a macro `contains` is defined. Because I don't want to
define this macro globally (I don't want to have to put a "package"
prefix on it), I tried to define it locally.

That didn't work at first, because the expression `sexpr` is generated
at run-time, which means it cannot be in the lexical scope of
cl-macrolet. So I ended up creating a larger expression containing the
cl-macrolet and eval'ing that.

I have two questions:

- Is there a better ways of doing this? Of course, flet creates dynamic
  function bindings, but it's deprecated. The suggested alternative,
  `(cl-letf (symbol-value 'bar) ...)` doesn't work, because it requires
  that `bar' has a (global) symbol-value, which `contains` in this
  example does not.

- Can't this function be defined as a macro? It seems to me that a
  backquoted expression that is subsequently eval'ed is essentially the
  same as a macro, but my attempts to write foo as a macro have failed.

TIA

Joost




[1] The actual code is here:

https://github.com/joostkremers/ebib/blob/master/ebib.el#L3478



-- 
Joost Kremers                                   joostkremers@fastmail.fm
Selbst in die Unterwelt dringt durch Spalten Licht
EN:SiS(9)