Clojure Koans 16 Refs Notebook

{:hello (clojure-version)}

(def the-world (ref "hello"))

(def bizarro-world (ref {}))

;; skill 16001: Define the binding of a reference to a symbol (?)

;; "In the beginning, there was a word"

(= "hello" (deref the-world))

;; question 16001: What does `ref` stand for in Clojure?

;; answer 16001a: "Refs" (short for "references") are Clojure's way of dealing with immutability.

;; question 16002: What is a reference in Clojure and what does it do specifically?

;; question 16003: How does the `ref` function/macro work in Clojure?

;; question 16004: Is there any differences between a reference binding and a non-reference binding, and if so, what are they?

;; question 16005: What is the word "word" refering to?

;; question 16006: What is dereferencing explained in plain words?

;; skill 16002: Dereference a binding/reference (variable?) by using the `deref` function/macro (?)

;; "You can get the word more succinctly, but it's the same"

(= "hello" @the-world)

;; skill 16003: Dereference a binding/reference (variable?) by using the `@` prefix (macro?)

;; "You can be the change you wish to see in the world."

(= "better" (do

                (dosync (ref-set the-world "better"))

                @the-world))

;; skill 16004: Update/mutate/replace (?) a reference (?)

;; question 16007: Why is this wrapped in a `dosync` and `do` combo?  

;; skill 16005: Use the `do` macro/function to exercute multiple forms/statements (?) and return the result of the final form/statement (?)

;; "Alter where you need not replace"

(= "better!!!" (let [exclamator (fn [x] (str x "!"))]

                   (dosync

                       (alter the-world exclamator)

                       (alter the-world exclamator)

                       (alter the-world exclamator))

                   @the-world))

;; question 16008: What is the difference between `ref-set` and `alter`?

;; question 16009: What is the difference between replacing and altering?

;; skill 16006: Change the value bound to / held by (?) a ref (?) by using `alter`

;; knowledge: Asychronous work such as `ref-set`, `ref`, and `deref` needs to happen within a `dosync` to keep the work within a single "transaction" (?) becuase ... (?)

;; "Don't forget to do your work in a transaction!"

(= 0 (do (dosync (ref-set the-world 0))

         @the-world))

;; question 16010: What is a "transaction" in Clojure?

;; answer 16010a: ~~Anytime the value of a reference or object is changed.~~ (?)

;;                Interactions with mutable containers are considered "transactions".

;;                Transactions are `do` blocks where references to mutable state are contained within.

;;  question 16011: What is the role of reification in transactions?

;;  question 16012: Why are transactions (dosync) necessary/helpful?

;;  answer 16012a: Transactions enable/allow "roll-back"

;; 

;;  skill 16007: Create a transaction by using the `dosync` macro/function (?)

;;  question 16013: What does `do` do?

;;  answer 16013a: The `do` makes the evaluation of conatined `do-sync`s eager.

;;  question 16014: What does "eager" mean in the context of transations?

;;  question 16015: What is the difference between "eager" and "lazy"?

;;  answer 16015a: Conceptually speaking, lazy means to evaluate/execute only as needed, versus eager which means to evaluate/execute things as soon as they are encountered.

;;  question 16016: What are concrete, concise examples of "eager" vs "lazy"

;;  question 16017: What is the relevant context of "eager" and "lazy"?

;;  answer 16017a: Data structures can be implemented themselves (intrinsic to the data structure) in an eager or lazy way.

;; "Functions passed to alter may depend on the data in the ref"

(= 20 (do

          (dosync (alter the-world #(+ % 20)))))

;; question 16018: My clj-kondo linter tells me that the `do` is redundant. Is this indeed true and if yes why?

;; question 16019: Why might a Clojure programmer prefer `partial + 20` rather than `#(+ % 20)` for the above solution?

;; question 16020: What are the differences between the current solution and this `alter the-world + 20` ?

;; "Two worlds are better than one"

(= ["Real Jerry" "Bizarro Jerry"]

   (do

       (dosync

           (ref-set the-world {})

           (alter the-world assoc :jerry "Real Jerry")

           (alter bizarro-world assoc :jerry "Bizarro Jerry")

           [(the-world :jerry) (bizarro-world :jerry)])))

;; question 16021: Why are `[(@the-world :jerry) (@bizarro-world :jerry)]` and the same minus the `@` deref prefixes functionally equivalent here? Or are they not but just appear to be?

;; question 16022: Where and when is it critical to deref?

;; question 16023: What are typical/common gotcha's in Clojure referencing/dereferencing?

;; question 16024: What are some more useful/helpful/clear/concrete examples of `do` and `dosync` used in context?