011: Maps - conj

A simple look at joining collections to create a map.  Includes looking as some
of the potential errors that may arise and what they mean.

src/four_clojure/011-maps-conj.clj

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+;; # 011 - Maps: conj
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+;; Difficulty:	Elementary
+;; Topics:
+
+;; When operating on a map, the conj function returns a new map with one or more key-value pairs "added".
+
+;; (= {:a 1, :b 2, :c 3} (conj {:a 1} __ [:c 3]))
+
+
+;; Deconstruct the problem
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+;; Simply fill in a collection that contains the right values.
+
+;; Maps need to include values in pairs, a key and a value that the key is associated with.
+;; Maps can conj from values in a vector, so long as the vector contains key/value pairs
+
+;; So this expression works
+
+(conj {:a 1} [:b 2] [:c 3])
+;; => {:a 1, :b 2, :c 3}
+
+;; and of course using a map with a key value pair would also work
+
+(conj {:a 1} {:b 2} [:c 3])
+
+
+;; Using a vector that contains an a key without a value will cause and error.
+
+(conj {:a 1} [:b 2 :d] [:c 3])
+;; java.lang.IllegalArgumentException
+;; Vector arg to map conj must be a pair
+
+;; The same expression with a map this time does return something, which is surprising.
+;; Although evaluating it also throws a reader exception.
+;; This is because second map in the expression has a syntax error,
+;; {:b 2 :d} is not valid syntax.
+
+(conj {:a 1} {:b 2 :d} [:c 3])
+;; => [:c 3]
+;; clojure.lang.LispReader$ReaderException
+;; java.lang.RuntimeException: Unmatched delimiter: )
+
+
+;; Trying to construct a map using the hash-map function and an incorrect set of key value pairs
+;; also creates an error.  However, this time its much clearer as to the error.
+(conj {:a 1} (hash-map :b 2 :d) [:c 3])
+
+(hash-map :b 2 :d)
+;; java.lang.IllegalArgumentException
+;; No value supplied for key: :d
+
+;; NOTE: The above code has been evaluated with Clojure 1.9.  Version 1.10 may have improved error messages when using ill-formed maps.
+
+;; Answers summary
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+;; Simplest answers
+
+[:b 2]
+{:b 2}
+(hash-map :b 2)
+
+;; Overthought answers
+
+;; Least valuable answer