Clojure Koans 27 Multimethods Notebook

{:hello (clojure-version)}

(defmulti multimethod-without-args

    (fn [keyword-arg] keyword-arg))

;; note: This appears to be some sort of identity function...

;; question 27011: What exactly is going on here?

(defmethod multimethod-without-args :first [_]

    (str "Hello, World!"))

(defmethod multimethod-without-args :second [_]

  (str "Hello there"))

;; note: The two defmethods above seem to ignore passed arguments..

;; question 27012: What exactly is going on here?

(defmulti multimethod-with-args

  (fn [opt-one opt-two] opt-one))

;; note: This multimethod seems to always return a function that always returns the first of two arguments...

;; question 27013: What exactly is going on here?

(defmethod multimethod-with-args :path-one [_ opts]

    (:first-opt opts))

;; question 27014: What exactly is going on here?

(defmethod multimethod-with-args :path-two [_ opts]

    (let [numbers (:second-opt opts)]

        (->> numbers

             (map inc)

          (reduce +))))

;; question 27015: What exactly is going on here?

(defmethod multimethod-with-args :path-three [_])

;; question 27016: What exactly is going on here?

;; "A multimethod takes one or more arguments to dispatch on"

(= "Hello, World!"

   (multimethod-without-args :first))

;; question 27001: What is the function/role/term for the keyword following a defmethod's identifier/name?

;; question 27002: Why is it called "multimethod-without-args"? It seems to take arguments, at least, sometimes (see the underscore on this line `(defmethod multimethod-without-args :first [_]`)?

;; question 27003: What exactly does it mean for a multimethod "to dispatch on" arguments? Please show simple, clear examples, ideally in context. 

;; question 27004: What is a "multimethod" in Clojure? 

;; question 27005: What are the similarities and differences between multimethods and multi-arity functions?

;; "Though it can be ignored and represented by _ in defmethods"

(= "Hello there"

   (multimethod-without-args :second))

;; question 27006: What is the significance/function/role of the underscore character/symbol "_" (?) in defmethods?

;; question 27007: What exactly can be ignored and represented by "_"? (What is "it" referring to above?)

;; "Alternatively, we can use the arguments in defmethods"

(= 1

   (multimethod-with-args :path-one {:first-opt 1

                                     :second-opt 2}))

;; question 27008: What do "opt" and "opts" stand for, "options"?

;; "This allows us to do something different in each method implementation"

(= 6

   (multimethod-with-args :path-two {:first-opt 1

                                     :second-opt [0 1 2]}))

;; question 27009: What are some helpful/practical use cases for multimethods?

;; question 27010: What are the pros/cons/tradeoffs of multimethods?