difference between (defalias 'A (symbol-function 'B)) and (defalias 'A 'B)

alias, elisp, emacs, function

Solution

The two `defalias` usages are slightly different. The first links the function cell for `'backward-delete-char` to that of `'delete-backward-char`. The second links the `'search-forward-regexp` to the function that is currently called by `'re-search-forward`.

The difference is that if you later change the definition of '`delete-backward-char`, `'backward-delete-char` will now have the new behavior. Whereas in the second case, changing the function for `'re-search-forward` has no effect on the behavior of `'search-forward-regexp`.

Perhaps some ascii art can help:

+-------------------------+     +-----------------+
|#<subr re-search-forward>| <-- |re-search-forward|
+-------------------------+     +-----------------+
                        ^       +---------------------+
                        \------ |search-forward-regexp|
                                +---------------------+

+----------------------------+     +--------------------+     +--------------------+
|#<subr delete-backward-char>| <-- |delete-backward-char| <-- |backward-delete-char|
+----------------------------+     +--------------------+     +--------------------+

This documentation might help clear things up.

Problem

I was reading subr.el and saw this code: ``` (defalias 'backward-delete-char 'delete-backward-char) (defalias 'search-forward-regexp (symbol-function 're-search-forward)) ``` Interestingly, the first line doesn't use symbol-function while the second line does. The only difference I know of these two ways of using defalias is that the help for backward-delete-char (the first one) displays that it is an alias for delete-backward-char while the help for search-forward-regexp doesn't. Is there a case when the second way is better than the first way?

Original source