Theory: Accessing hash-maps

The request is a Clojure hash-map made up of key / value pairs, referred to as the request map. The keys are Clojure keywords. The values are typically strings or Clojure collections (vectors, hash-maps).

Here is an example of a request map

{:request-params {:name "John"}}

Using the get function to return the value for a particular keyword in the request-map

(get request-map :keyword)

Using hash-map as a function

A hash-map can be evaluated as a function call to the map with the key as an argument. Any type of key can be used in this expression.

(request-map :keyword)
(request-map "key as string")

Nested hash-maps

Two get expressions could be used to return a particular value when accessing a nested hash-map. The inner get expression returns a hash-map and the outer get expression returns the value.

(get (get outer-map :outer-keyword) :inner-keyword)

With many nested maps, the get function can lead to code that is harder to read. Using the get-in function provides a simpler syntax for traversing nested maps

get-in walks through the nested hash-map along the path defined by the vector of keys.

(get-in request-map [:outer-keyword :inner-keyword])

Using keywords and hash-maps

Keywords can be evaluated as a function call with a hash-map as an argument and return their associated value in that hash-map.

(def response-map {:name "john" :path "/hello"}

You can get the value from this map using the keyword

(response-map :name)

=> "john"

(response-map :path)

=> "/hello"

Other types of keys do not work as function calls. Either use the map as a function with the key as an argument or use the get and get-in functions as appropriate.