There is no Emacs Lisp syntax for optional arguments with defaults other than nil. Which idiom is preferred when providing defaults for those arguments?
Here's an excessively explicit implementation to show what I mean.
(defun perform (entry &optional given-extra)
(let ((extra (or given-extra fallback-extra)))
(perform-body entry extra)))Is there a generally preferred way to write this?
With cl-defun, optional arguments can be given their own default expressions:
(cl-defun perform (entry &optional (extra fallback-extra))
(perform-body entry extra))Here the default expression, fallback-extra, gets evaluated on every invocation of the function.
Without the Common Lisp additions @tobin suggested, you have to test whether an optional argument was provided. Note that a let isn't actually necessary in this situation. I'd find this version more idiomatic:
(defun perform (entry &optional extra)
(or extra (setq extra fallback))
(perform-body entry extra))The comments suggest that unless could be a better choice than or:
(defun perform (entry &optional extra)
(unless extra (setq extra fallback))
(perform-body entry extra))Another point from the comments: a more purely functional approach uses let, like the question does, without needing two different names:
(defun my-perform (entry &optional extra)
(let ((extra (or extra fallback)))
(perform-body entry extra)))Naturally, when the optional argument occurs only once, you can simply write:
(defun my-perform (entry &optional extra)
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