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Named Parameters for Clojure

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Named Parameters for Clojure

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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)
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})
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"}
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.

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