Locally overriding a function throughout a dynamic extent

A horribly dangerous but occasionally useful Lisp technique.

My use case is as follows. ebib has a command to copy a formatted reference to the kill ring, using citar-citeproc-format-reference to actually do the formatting. This means it’s easy to change the style of the formatted reference. However, citar-citeproc-format-reference itself uses citar-render-bib with a plain-text formatter. This is a sensible default, but since I’m almost always copying references into org-more documents, it loses a lot of information: it’d be better to use the org formatter, but there’s no argument to specify it.

Clearly the correct solution is to change citar-citeproc-format-reference to take a key or optional argument to specify the formatter, but that involves changing someone else’s code. The hacker solution is to change the call (citeproc-render-bib proc 'plain) to (citeproc-render-bib proc 'org), but without re-writing the entire surrounding function to keep the change just to the case where I need it.

One way to do this would be to define a variant citeproc-render-bib that ignores its second argument (the formatter) and always uses 'org instead, and then substitute this variant for the original – but only in the dynamic extent of a particular call to citar-citeproc-format-reference. In most languages this would be impossible – but not in Emacs Lisp.

The solution is to use cl-letf, which overrides the values of general places for the duration of its body forms and restores the original value on exit (normal or otherwise). The important point is that the change occurs across the extent of the body – the body and all the code called from the body – and not merely in the scope of the body, which would only affect calls made there directly.

For example, consider in the following:

(defun f(a b)
  (+ a b))

(defun first (a)
  (f a 10))

Which when called gives:

(first 26)

If we want to override the default value (10) that’s passed to f and instead use 25, we can define a new version that ignores the second argument and uses our preferred default, and then temporarily override the definition of f in the calling environment. If we want to use the original in the overriding definition we need to grab it first. This gives:

(let ((origf (symbol-function 'f)))
  (cl-letf (((symbol-function 'f) (lambda (a b)
				    (funcall origf a 25))))
    (first 26)))

What’s going on? The cl-letf macro is like let but works with general places (as in setf). It sets the places in its argument list for the duration of its body, and then restores them on exit, regardless of whether that exit is normal or via a condition.

The (symbol-function 'f) form returns the place that stores the function associated with symbol f. We use it twice: once to capture this function so we can use it later, and once to identify the place where we store our new variant function. This new binding is then used for all calls made from the body of the cl-letf, regardless of depth, so the call to first makes use of our variant definition of f rather than the original – but with the original then being used in the variant in our case!

If we’d used let or cl-flet instead of cl-letf we wouldn’t have got the behaviour we’re looking for:

(let ((origf (symbol-function 'f)))
  (cl-flet ((f (a b)
	      (funcall origf a 25))))
    (first 26))

Why? Because let and cl-flet work over the scope of the body, so only calls to f directly from the body of the assignment are affected – not calls from calls. This is a great illustration of the difference between the closely-related concepts of (static, lexical) scope and (dynamic, run-time) extent, incidentally.

I did say it was horrible :-). It’s basically like adding temporary :around advice, and could probably benefit from a macro to wrap it up. It’s also inconceivable that it’s thread- or co-routine-safe, although I haven’t checked.

Part of the horribleness comes from the fact that the redefinition is made for the entire dynamic extent of the body forms, which means all instances of the overridden function will use the overridden value. There might be more than you think! But for well-understood code it’s sometimes useful, avoiding duplicating code to make tiny changes.