Clojure Koans 26 Transducers Notebook

{:hello (clojure-version)}

(def example-transducer

  (map inc))

;; skill 26001: Create a simple transducer

(def transforms

  (comp (map inc)

     (filter even?)))

;; skill 26002: Combine sequence operations together using `comp` function to be composed as a transducer

;; question 26015: Is `transforms` above a transducer and if yes why? If it is not a transducer, then what is `transforms`?

;; "A sequence operation with only one argument often returns a transducer"

(= [2 3 4]

   (sequence example-transducer [1 2 3]))

;; question 26001: What exactly is happening here?

;; question 26002: Where is the transducer being returned here?

;; question 26003: What are the differences between `(sequence example-transducer [1 2 3])` and `(map inc [1 2 3])`?

;; question 26004: Why does the text above say "sequence operation with only one argument" when the above `sequence` operation seems to take two arugments (`example-transducer` and the vector [1 2 3])?

;; skill 26003: Use a simple transducer via the `sequence` function (?)

;; question 26005: What does the `sequence` function do?

;; answer 26005a: "Coerces coll to a (possibly empty) sequence, if it is not already

;;    one. Will not force a lazy seq. (sequence nil) yields (), When a

;;    transducer is supplied, returns a lazy sequence of applications of

;;    the transform to the items in coll(s), i.e. to the set of first

;;    items of each coll, followed by the set of second

;;    items in each coll, until any one of the colls is exhausted.  Any

;;    remaining items in other colls are ignored. The transform should accept

;;    number-of-colls arguments"

;; question 26006: What is an "xform"? See docs: https://clojuredocs.org/clojure.core/sequence

;; answer 26006a: "A transducer (sometimes referred to as xform or xf) is a transformation from one reducing function to another" source: https://clojure.org/reference/transducers

;; question 26007: What are the differences between the `seq` and `sequence` functions? See  https://clojuredocs.org/clojure.core/sequence and https://clojuredocs.org/clojure.core/seq

;; "Consider that sequence operations can be composed as transducers"

(= [2 4]

   (transduce transforms conj [1 2 3]))

;; description: TBD...

;; question 26008: What exactly does the `transduce` function do?

;; question 26009: What exactly is the `conj` function doing here?

;; question 26010: What exactly is a "transducer" in plain words (eli5)?

;; question 26011: What does it mean to compose sequence operations as transducers?

;; skill 26004: Compose sequence operations as a transducer & then use the transducer with the `transduce` function (?)

;; "We can do this eagerly"

(= [2 4]

   (into [] transforms [1 2 3]))

;; question 26012: What exactly is happening here?

;; skill 26005: Eagerly apply (?) a transducer over a sequence into a new seuence

;; "Or lazily"

(= [2 4]

   (sequence transforms [1 2 3]))

;; question 26013: What exactly is happening here?

;; skill 26006: Lazily apply (?) a transducer over a sequence into a new seuence

;; "The transduce function can combine mapping and reduction"

(= 6

   (transduce transforms + [1 2 3]))

;; question 26014: What exacty is happening here?

;; skill 26007: Apply both a mapping over a sequence as well as a reduction via a transducer composition of a map function and a reduce function

;; reference: https://clojure.org/reference/transducers

(filter odd?) ;; returns a transducer that filters odd

(map inc)     ;; returns a mapping transducer for incrementing

(take 5)      ;; returns a transducer that will take the first 5 values

;; reference: https://clojure.org/reference/transducers

;; The transducer xf is a transformation stack that will be applied by a process to a series of input elements.

(def xf

  (comp

    (filter odd?)

    (map inc)

    (take 5)))

;; The transformation above is equivalent to the sequence transformation:

(defn seq-transform [col]

  (->> coll

    (filter odd?)

    (map inc)

    (take 5)))