Re: Writing a function/macro in Elisp that generates a function at runtime

[Barry Margolin <barmar@alum.mit.edu>]

In article <mailman.2216.1225168079.25473.help-gnu-emacs@gnu.org>,
 Toby Cubitt <tsc25@cantab.net> wrote:

> Hi,
> 
> I'm trying to write a function in Elisp that generates a function at
> runtime, but hitting the limits of my Lisp abilities. Here's a first
> attempt at the kind of thing I'm trying to do:
> 
> (defun wrap-insert-function (insfun)
>   `(lambda (new-data old-cell)
>      (setf (cell-data old-cell)
>            (funcall ,insfun new-data (cell-data old-cell)))
>      old-cell))
> 
> I want to use this to wrap functions that don't know anything about the
> "cell" data structures, so they can operate on them:
> 
> (setq wrapped-insert-function
>   (wrap-insert-function
>    (some-complicated-code-that-returns-a-suitable-function)))
> 
> (funcall wrapped-insert-function data cell)
> 
> 
> The problem is, because it's quoted, the `setf' macro never gets
> expanded by the byte-compiler. And that's no good, because the cl
> package where `setf' is defined shouldn't be loaded at runtime, and

To do that you'd need lexical closures, but Elisp doesn't have them.  
Since you're not generating the function until runtime, there's nothing 
to compile at compile time.

> anyway we want it to be expanded at compile-time, not at run-time as
> that would be ugly and inefficient.

You don't need to generate a function on the fly for this.  Do something 
like this:

(defun insert-with-function (insfun new-data old-cell)
  (setf (cell-data old-cell)
        (funcall insfun new-data (cell-data old-cell)))
  old-cell)

-- 
Barry Margolin, barmar@alum.mit.edu
Arlington, MA
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