Fallback on maps lookup on Clojure (or a gentle introduction to macros)

Rodrigo Flores — Mon, 16 Oct 2017 23:14:49 +0000

So, you have to deal with a map, but you can't be sure whether or not you have a value for a given key.

The easiest, and probably the most known is to use the get function (that accepts optionally a third argument which is the fallback value).

(def a-qb {:name "Josh"
           :surname "McCown"
           :team "New York Jets"})

(def another-qb {:name "Robert"
                 :surname "Griffin III"})

(get a-qb :team) ; => "New York Jets"
(get another-qb :team) ; => nil 
(get another-qb :team "Free Agent") ; => "Free Agent"

Another way is to use the keyword as a function or even the map as a function.

(:team a-qb) ; => "New York Jets"
(:team another-qb) ; => nil 

(a-qb :team) ; => "New York Jets"
(another-qb :team) ; => nil

I've learned that you can also specify a second argument to these cases, and if the value is not found, it will return this second argument.

(:team a-qb "Free Agent") ; => "New York Jets"
(:team another-qb "Free Agent") ; => "Free Agent" 

(a-qb :team "Free Agent") ; => "New York Jets"
(another-qb :team "Free Agent") ; => "Free Agent"

But what if you want to raise an exception if it does not exists?

(a-qb :team (throw (Exception. )))
(a-qb :team (throw (Exception. )))

If you call it this way, it will evaluate all parameters before the function is called (because it is a function), raising an exception regardless if the key-value pair exists or not on the map. A way to workaround this it is to use an or.

(or (a-qb :team) 
    (throw (Exception. ))

Because the or is implemented as a macro, it will only evaluate the second sexp (throw ...) unless the first sexp is falsy (nil or false) and thus raising an exception only on this case.

DEV Community: Rodrigo Flores

Fallback on maps lookup on Clojure (or a gentle introduction to macros)