How to replace a running function in Common Lisp?

Suppose we use SBCL's #'save-lisp-and-die to create an server applicatioon App1, which works very well. Now we want to replace a function #'func1 with a new version without stopping App1. How can we do it in Common Lisp ?

Any suggestion is appreciated !

回答1:

You need to load the new function definition. Then new function will be available immediately; code will call newly loaded function.

New function definition may be loaded in many ways:

(load (compile-file "file.lisp")) where file.lisp is a source code for function
(load "file.fasl") where file.fasl is compiled source code
(eval (defun ...))

Of course, there are exceptions and complications:

This will not replace already running calls of previous functions; for example, an infinite event loop can not be changed this way - it will have to support some kind of stopping and calling a new function. However, such long-running functions are rare. It can be worked around by using recursion instead of looping (but not all compilers do tail-call optimization).
If you somewhere grabbed a pointer to function (for example, by (function FOO) where FOO is the name of a function), it will retain its old value. To avoid this, use symbols instead of function pointers (symbols are funcallable).
The code of a function is a subject to garbage collection. You should be careful not to leave references to old versions of a function lying around. Also, if some functions become not needed, you should not forget to fmakunbound their symbols.
If the function was used at compile time, all of the affected code should also be reloaded
If you had high levels of optimization (which is not by default), compiler might have inlined the function into others. CLHS discriminates the cases when redefining a function becomes «undefined behavior».

But in practice, code reloading works well in most Common Lisp implementations.

回答2:

I personally ensure that SWANK (the server part of SLIME) is running, so that I can connect to the image at any time with Emacs+SLIME and redefine whatever I want.

(ql:quickload "swank")
(swank:start-server :port 1234) ;; listen for SLIME connections on port 1234

Then in Emacs, you can M-x slime-connect, and follow the prompts.

If you don’t want to do this for whatever reason, your implementation might offer something specific.