Named Parameters for Clojure

Clojure can simulate named parameters, or a mix of positional and named parameters. This is really old news, dating back to Clojure 1.2 if evidence serves, but I always have trouble finding this exact syntax, and it is not fully obvious.

Say you want to accept a certain number of optional parameters with names, and perhaps, defaults. It turns out you can combine rest parameters (the ones that come after a &) with map destructuring.

In the simple case, you don't care what the possible options are, and you don't have any defaults.

	(defn named-parameters
	[& {:as params}]
	params)

view raw named params.clj hosted with ❤ by GitHub

The keys and values you pass to this function, say (named-parameters :foo 1 :bar 2), are collected together as symbol params.

If you don't provide an even number of values (that is, the same number of keys and values), you'll get a reasonable exception, such as java.lang.IllegalArgumentException: No value supplied for key: :bar

Easy-peasey ... but you need to extract values from the params map to use them inside the function, e.g.: (:foo params). It would be nicer to have them as symbols, just like with normal positional parameters. This is also easy, by leveraging more of the features of map destructuring:

	(defn named-params
	[& {:keys [foo bar]
	:or {foo "foo-default" bar "bar-default"}}]
	{:output-foo foo :output-bar bar})

view raw named-params.clj hosted with ❤ by GitHub

The :keys identifies the keywords expected in the map; it works backwards from the symbol name, foo, to the expected keyword, :foo.

There's also a :syms (for when the keys are expected to be symbols) and :strs (for when the keys are expected to be strings).

The :or identifies default values for each symbol. The end result is that we can rely on defaults from :or or provide our own values when invoking the function:

	(named-params :bar "override-bar")
	==> {:output-foo "foo-default", :output-bar "override-bar"}

view raw named-params.clj hosted with ❤ by GitHub

And since this is Clojure, you can combine all of these things together quite easily ... some positional parameters, some named, some identified by keywords, others identified by symbols.