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Daniel Janus’s blog

Keyword arguments

4 May 2010

There’s been an ongoing debate about how to pass optional named arguments to Clojure functions. One way to do this is the defnk macro from clojure.contrib.def; I hesitate to call it canonical, since apparently not everyone uses it, but I’ve found it useful a number of times. Here’s a sample:

user> (use 'clojure.contrib.def)
user> (defnk f [:b 43] (inc b))
user> (f)
user> (f :b 100)

This is an example of keyword arguments in action. Keyword arguments are a core feature of some languages, notably Common Lisp and Objective Caml. Clojure doesn’t have them, but it’s pretty easy to emulate their basic usage with macros, as defnk does.

But there’s more to Common Lisp’s keyword arguments than defnk provides. In CL, the default value of a keyword argument can be an expression referring to other arguments of the same function. For example:

CL-USER> (defun f (&key (a 1) (b a))
           (+ a b))
CL-USER> (f)
CL-USER> (f :a 45)
CL-USER> (f :b 101)

I wish defnk had this feature. Or is there some better way that I don’t know of?