clojure_notes.clj

(+ 1 2)

; KEYWORDS
; used to map values in hashes

(def person {:name "Andre"
             :city "Florianopolis"})

; Keywords are functions that look themselves up in collections passed to them.

(:city person)

; Double-colon before a keyword creates it in the global namespace.

(def pizza {:name "Baggio"
            :location "Florianopolis"
            ::location "-43 +12"})

pizza

(:user/location pizza)
(:location pizza)

;
; Keywords are named values. Means they have a name:

(name :user/location)

(namespace :user/location)

(namespace :location)

; SYMBOLS
; Are identifiers that evaluate to values (held by vars).

(defn average [numbers]
  (/ (apply + numbers) (count numbers)))

(average [10, 20, 30])

; ^ average is a symbol. refers to the function held in var named average.

; Commas are optional
(= [1, 2, 3] [1 2 3])

; def defines variables in the current namespace.

(def x 1)
x

; *ns* shows the current namespace
*ns*

; ns function changes the current namespace
(ns user)


; CLOJURE SPECIAL FORMS

; 1. quote: suppresses evaluation of expressions. Symbols evaluate to themselves.

x

(quote x)

(symbol? (quote x))

; ' (aposthrofe) is the same as quote.

x
'x

; anything can be quoted.
; quoting lists supress evaluation, returning the list itself.

(+ x x)
'(+ x x)
(list? '(+ x x))

(list '+ 'x 'x)

; 2. do: evaluates a list of expressions. Last expression is the return value.

(do
  (println "hi")
  (apply * [4 5 6]))

; 3. def: define vars.

(def p "foo")

p

; 4. let: create local bindings.

(defn hypot
  [x y]
  (let [x2 (* x x)
        y2 (* y y)]
    (Math/sqrt (+ x2 y2))))

(hypot 3.0 4.0)

; all local bindings created by let are immutable

; 5. destructuring: pulls apart collections

; - sequential destructuring: works with sequential collections.

(def v [42 "foo" 99.2 [5 12]])

v

(let [[a b c] v]
  (+ a c))

; destructuring can be nested.

(let [[a _ _ [b c]] v]
  (+ a b c))

; & gets the rest.

(let [[a & rest] v]
  rest)

; :as makes the next symbol hold the original values

(let [[x _ y :as original] v]
  (conj original (+ x y)))

; - map destructuring: works with maps.

(def m {:a 5 :b 6
        :c [7 8 9]
        :d {:e 10 :f 11}
        "foo" 88
        42 false})

(let [{a :a b :b} m]
  (+ a b))

(let [{f "foo"
       v 42} m]
  (+ f (if v 0 1)))

; map destructuring syntax can be used with vectors if indexes are provided.

(let [{a 1 b 4} [1 2 3 4 5]]
  (+ a b))

; can be nested.

(let [{{a :e} :d} m]
  (* 2 a))

; can be mixed with sequential destructuring

(let [{[x _ y] :c} m]
  (+ x y))

; default values

(def m2 {:a 1 :b 2})

(let [{a :a b :x
       :or {b 10}} m]

  (+ a b))

; DRY, or mapping keys to locals with same name

(def chas {:name "Chas"
           :age  31
           :location "Massachusetts"})

(let [{:keys [name age location]} chas]
  (format "%s is %s years old and lives in %s" name age location))


; Destructuring a vector of key-value pairs into a map: (using the rest - & - notation).

(def user-info [:username "andre" :name "Andre" :city "floripa"])

(let [[& {:keys [username name city]}] user-info]
  (format "%s (%s) is from %s" username name city))

; 6. FN: create functions

(fn [x] (+ 10 x))

; calling functions with parentheses:

((fn [x] (+ 10 x)) 8)

; put a function in a var:

(def add-10 (fn [x] (+ 10 x)))

(add-10 8)

; defn is short for this (create and put it in a var)

(defn add-10 [x]
  (+ 10 x))

(add-10 9)

; multi-arity functions:

(defn add-default
  ([x] (+ x 1))
  ([x y] (+ x y)))

(add-default 1)

(add-default 1 2)

; variadic functions: using destructuring in arguments

(defn concat-rest
  [x & rest]
  (apply str rest))

(concat-rest 1 2 3 4)


; zero-arity functions

(defn adder
  [& attrs]
  (apply + (or attrs [0])))

(adder)
(adder 1 2 3 5 7 9 11)


; keyword arguments, also using destructuring.

(defn now [] (java.util.Date.))


(defn make-user
  [username & {:keys [email join-date]
               :or {join-date (now)}}]
  {:username username
   :join-date join-date
   :email email})

(make-user "andre"
           :email "abernardes@gmail.com")

; literals for anonymous functions

(#(+ %1 %2) 2 1)

(read-string "#(+ %1 %2)")


; 7. IF conditional

(if true "andre" "bernardes")

; 8. Loop and Recur

(loop [x 5]             ; implicit let form
  (if (neg? x)
    x                   ; final expression, returns its value
    (recur (dec x))))   ; or back to the loop, rebinds x to (dec x)

; functions are a loop head:

(defn countdown
  [x]
  (if (zero? x)
    :blastoff!
    (do (println x)
        (recur (dec x)))))


; ******************************************************************
; Functional programming in Clojure:

;   - Preference for working with immutable values
;     - Immutable data structures
;     - Functions as values > higher order functions
;   - Declarative processing of data
;   - Incremental composition for higher level abstractions

; FIRST CLASS AND HIGHER ORDER FUNCTIONS

(defn call-twice [f x]
  (f x)
  (f x))

; Higher order functions: functions that receive other functions as
; arguments or return a function as a result. Examples:

; map

(map clojure.string/lower-case ["Andre" "Bernardes" "Oliveira"])

(map + [1 2 3] [4 5 6])

; reduce : reduce a collection to a single value

(reduce max [0 10 8 2])

(reduce + 1 [2 3 4])

(reduce
  (fn [collection value]
    (assoc collection value (* value value)))
  {}
  [1 2 3 4])

; Partial Apply

; apply: send an array as arguments to a function

(apply hash-map [:a 5 :b 6])

(apply + 1 2 [3 4 5])

; partial apply: define a function with preset arguments

(def only-strings (partial filter string?))
(only-strings [1 "a" 2 "b"])

; Function Composition

; comp: compose functions.

(defn negated-sum-str ; regular function
  [& numbers]
  (str (- (apply + numbers))))
(negated-sum-str 10 100 5 12)


(def negated-mult-str (comp str - *)) ;pipeline with comp
(negated-mult-str 1 2 5)


(defn split-camel [s]
  (str/split s #"(?<=[a-z])(?=[A-Z])"))

(def camel->keyword
  (comp keyword
        str/join
        (partial interpose \-)
        (partial map str/lower-case)
        #(str/split % #"(?<=[a-z])(?=[A-Z])")))

(camel->keyword "FooBar")
(camel->keyword "fooBar")

(def camel-pairs->map
  (comp (partial apply hash-map)
        (partial map-indexed (fn [i x]
                               (if (odd? i)
                                 x
                                 (camel->keyword x))))))

(camel-pairs->map ["FooBar" 3 "lowFooBar" 5])

; Writing Higher-order functions

; a function that returns a function

(defn adder
  [n]
  (fn [x] (+ n x)))
((adder 5) 18)

(defn doubler
  [f]
  (fn [& args]
    (* 2 (apply f args))))
((doubler +) 1 2 3)

; example: logging system

; print-logger produces a function that logs messages to a writer
(defn print-logger
  [writer]
  #(binding [*out* writer]
     (println %)))

; file-logger produces a function that uses print-logger to
; print to a file.
(require 'clojure.java.io)
(defn file-logger
  [file]
  #(with-open [f (clojure.java.io/writer file :append true)]
     ((print-logger f) %)))

; multi-logger returns a function that writes to multiple loggers
; at once
(defn multi-logger
  [& logger-fns]
  #(doseq [f logger-fns]
     (f %)))

(def log (multi-logger
           (print-logger *out*)
           (file-logger "messages.log")))

; adds timestamps to the logger
(defn timestamped-logger
  [logger]
  #(logger (format "[%1$tY-%1$tm-%1$te %1$tH:%1$tM:%1$tS] %2$s" (java.util.Date.) %)))

(def log-timestamped
  (timestamped-logger
    (multi-logger
      (print-logger *out*)
      (file-logger "messages.log"))))

; PURE FUNCTIONS

; memoization

(defn prime?
  [n]
  (cond
    (== 1 n) false
    (== 2 n) true
    (even? n) false
    :else (->> (range 3 (inc (Math/sqrt n)) 2)
               (filter #(zero? (rem n %)))
               empty?)))

(time (prime? 1125899906842679))

(let [m-prime? (memoize prime?)]      ; memoized function
  (time (m-prime? 1125899906842679))
  (time (m-prime? 1125899906842679))) ; second time returns cached result

; * Important: always use memoize in local scopes


; CHAPTER 3 - COLLECTIONS AND DATA STRUCTURES

; 1. Used in terms of abstractions
; 2. Immutable and Persistent

; COLLECTIONS

(conj [1 2] 3)

(seq #{1 2 3})

(count {:a 1 :b 2})

(empty '(1 2 3))

(= [1 2] [1 2])

; SEQUENCES

(seq "Clojure")

(seq {:a 5 :b 6})

(first "Clojure")
(rest "clojure")
(next "clojure")

(rest [1]) ; Returns empty
(next [1]) ; Returns nil

; seqs are only "realised" when needed
(let [s (range 1e6)]
  (time (count s)))  ; takes a long time bc has to realise the seq before counting

(let [s (apply list (range 1e6))]
  (time (count s)))  ; lists are realized and track their size

; Creating Seqs

; cons - prepend value to collection, returns a seq
(cons 0 (range 1 5))

; list* - sequential cons.
; don't do this:
(cons 1 (cons 2 (range 3 5)))

; do this instead:
(list* 1 2 (range 3 5))

; Lazy Evaluation
(defn random-ints
  "Returns a lazy seq of random integers in the range [0, limit)."
  [limit]
  (lazy-seq
    (cons (rand-int limit)
          (random-ints limit))))

(take 10 (random-ints 50))

; Fibonnaci sequence using a lazy seq
; I actually made this myself!
(defn fib
  ([]
   (lazy-seq
    (list* 0 1 1 (fib 1 1))))
  ([x1 x2]
   (let [sum (+ x1 x2)]
     (lazy-seq
       (cons sum (fib x2 sum))))))

(take 10 (fib))