A Map Function

A map contains pairs, where the first element is the key and the second element is the attached value: you can set one up with curly brackets, for example:

user=> (def partner {:laurel :hardy, :hardy :laurel, :chaplin nil})
#'user/partner

Now you can look up, say, Laurel's partner:

user=> (get partner :laurel)
:hardy

In our map, Chaplin does not have a partner, so you get this:

user=> (get partner :chaplin)
nil

Of course - we made the value nil when we created the map.  However, if we look up someone who is not in the map, we also get nil:

user=> (get partner :abbott)
nil

The problem (A nil key) today is to write the code that will check for the case where the key really is in the map but the value is nil - - as opposed to the case where the key is not in the map.

We need the function contains?, which asks whether a key is in a map:

user=> (contains? partner :abbott)
false
user=> (contains? partner :chaplin)
true

And we also need the useful fact that nil equals nil.  Therefore the code we want looks like this:

(fn [k m]
  (and
    (contains? m k)
    (= (get m k) nil)))