teach/learnclj.clj

;; ============================================
;; learnclj.clj — Clojure Learning Examples
;; ============================================
(ns learnclj
  (:require [clojure.string :as str]))

;; --- 1. Basics ---

;; Literals
123          ; => 123 (Integer)
3.14         ; => 3.14 (Double)
22/7         ; => 22/7 (Ratio/rational)
:keyword     ; => :keyword (Keyword)
"string"     ; => "string" (String)
\c           ; => \c (Character)
true         ; => true (Boolean)
nil          ; => nil (Null/Nothing)

;; === Function Basics ===

;; defn — defines a named function
(defn greet [name]
  (str "Hello, " name "!"))
(greet "Sam")           ; => "Hello, Sam!"

;; Anonymous function — fn form
(fn [x y] (+ x y))     ; => #object[clojure.core$fn__... 0x...]

;; Anonymous function — sharp-syntax shorthand
(#(+ %1 %2) 3 4)       ; => 7

;; Function with multiple parameters
(defn add [x y]
  (+ x y))
(add 3 4)               ; => 7

;; Default arguments via destructuring in parameters
(defn greet-default [name {:keys [formal] :or {formal false}}]
  (if formal
    (str "Good day, " name)
    (str "Hey, " name)))
(greet-default "Sam" {})                      ; => "Hey, Sam"
(greet-default "Sam" {:formal true})          ; => "Good day, Sam"

;; Variadic function — & collects extra args into a seq
(defn sum [& numbers]
  (reduce + 0 numbers))
(sum 1 2 3 4)         ; => 10

;; --- 3. Data Structures ---

;; Lists — linked lists, quoted for data
'(1 2 3)          ; => (1 2 3) (quoted list)
(list 1 2 3)       ; => (1 2 3)

;; Vectors — indexed, sequential
[1 2 3]            ; => [1 2 3]
(vector "a" "b" "c") ; => ["a" "b" "c"]
(get [10 20 30] 1)     ; => 20

;; Maps — key-value pairs
{:name "Alice" :age 30}   ; => {:name "Alice", :age 30}
{"x" 1 "y" 2}            ; => {"x" 1, "y" 2}
(get {:a 1 :b 2} :a)   ; => 1
(:name {:name "Alice"}) ; => "Alice" (keyword as accessor)

;; Sets — unique values
#{1 2 3 4 5}        ; => #{1 2 3 4 5} (order not guaranteed)
(set [1 2 2 3 3 3])    ; => #{1 2 3} (duplicates removed, order not guaranteed)

;; --- 4. Control Flow ---

;; if
(if (> 5 3)
  "yes"
  "no")           ; => "yes"

;; when — like if but for side effects (no else branch)
(when true
  (print "Side effect here")
  :result)         ; => :result

;; cond — multiple conditions
(cond
  (= 1 1) "one"
  (= 2 2) "two"
  :else "other")   ; => "one"

;; case — exact match dispatch
(case 2
  1 "one"
  2 "two"
  3 "three"
  "other")         ; => "two"

;; and / or — short-circuiting
(and true false nil)   ; => false (returns first falsy value)
(or false true 42)     ; => true

;; --- 5. Sequences ---

;; map — apply function to each element
(map inc [1 2 3])           ; => (2 3 4)
(map str ["a" "b"] [1 2])  ; => ("a1" "b2")

;; filter
(filter even? [1 2 3 4 5]) ; => (2 4)

;; reduce
(reduce + [1 2 3 4])       ; => 10
(reduce str ["a" "b" "c"]) ; => "abc"

;; into — combine collections
(into [] (map inc [1 2 3]))   ; => [2 3 4]
(into {} (map #(vector %1 %2) [:a :b] [1 2])) ; => {:a 1, :b 2}

;; sequence processing
(sequence (comp
            (filter odd?)
            (map #(* % %)))
          [1 2 3 4 5 6 7])  ; => (1 9 25 49)

;; === Destructuring ===

;; Vector destructuring — bind positions to names
(let [[first second & rest] [1 2 3 4 5]]
  [first second rest])
; => [1 2 (3 4 5)]

;; Map destructuring — bind keys to names
(let [{name :name age :age} {:name "Bob" :age 25}]
  [name age])
; => ["Bob" 25]

;; :keys shorthand — automatic key-to-name binding
(let [{:keys [name age]} {:name "Bob" :age 25}]
  [name age])
; => ["Bob" 25]

;; :or — default values when key is missing or nil
(let [{:keys [name] :or {name "Anonymous"}} {:age 25}]
  name)
; => "Anonymous"

;; Nested destructuring
(let [{:keys [x y] :as point} {:point {:x 10 :y 20}}
      {:keys [x y]} (:point point)]
  [x y])
; => [10 20]

;; Destructuring in function parameters (same patterns as let)
(defn show-point [{:keys [x y] :or {x 0 y 0}}]
  (str "Point: (" x ", " y ")"))
(show-point {:x 10 :y 20})  ; => "Point: (10, 20)"
(show-point {})              ; => "Point: (0, 0)"

;; Destructuring in loop bindings
(loop [[first & rest] [1 2 3]]
  (if first
    (do
      (println "Got:" first)
      (recur rest))
    :done))
; => Got: 1, Got: 2, Got: 3, :done

;; --- 7. Namespaces ---

;; require — load a namespace
;; (require '[clojure.string :as str])
;; (str/upper-case "hello")

;; use — import functions directly
;; (use '[clojure.string :only [upper-case]])
;; (upper-case "hello")

;; import — Java classes
;; (import java.util.Date)
;; (Date.)

;; --- 8. Recursion & Tail Recursion ---

;; Regular recursion
(defn factorial [n]
  (if (<= n 1)
    1
    (* n (factorial (dec n)))))

;; Tail-recursive with loop/recur
(defn factorial-tail [n]
  (loop [n n acc 1]
    (if (<= n 1)
      acc
      (recur (dec n) (* acc n)))))

;; --- 9. Conditionals with when-let / if-let ---

;; when-let — bind and condition in one
(when-let [val (get {:a 1} :b)]
  val)  ; => nil (doesn't evaluate body)

(if-let [val (get {:a 1} :a)]
  val
  "not found")  ; => 1

;; --- 10. Multiline strings & syntax quoting ---

;; Multiline string
(def multiline
  "This is a
   multiline
   string")

;; Syntax quoting (backquote) — auto-qualifies vars
(defn make-list []
  `(~"a" ~"b" "c"))  ; => ("a" "b" "c")

;; --- 11. Protocols (OOP-like) ---

(defprotocol Shape
  (area [this])
  (perimeter [this]))

(deftype Rectangle [width height]
  Shape
  (area [this] (* width height))
  (perimeter [this] (* 2 (+ width height))))

(defn make-rectangle [w h]
  (Rectangle. w h))

(make-rectangle 4 5)       ; => #object[learnclj.Rectangle ...]
(area (make-rectangle 4 5))      ; => 20
(perimeter (make-rectangle 4 5)) ; => 18

;; --- 12. Records ---

(defrecord Person [name age email])

(def alice (->Person "Alice" 30 "alice@example.com"))
(:name alice)           ; => "Alice"
(->Person "Alice" 30 "alice@example.com")  ; => #learnclj.Person{:name "Alice", :age 30, :email "alice@example.com"}
(map->Person {:name "Bob" :age 25 :email "bob@example.com"}) ; => #learnclj.Person{:name "Bob", :age 25, :email "bob@example.com"}

;; --- 12b. Type & Protocol Extension ---

;; Protocol to define
(defprotocol Formattable
  (format-for-display [this] "Returns a formatted string representation"))

;; extend-type — add protocol methods to an existing type
(extend-type Number
  Formattable
  (format-for-display [this]
    (if (integer? this)
      (str "Int: " this)
      (str "Num: " this))))

(format-for-display 42)      ; => "Int: 42"
(format-for-display 3.14)    ; => "Num: 3.14"

;; extend-protocol — add protocol methods to multiple types at once
(extend-protocol Formattable
  java.util.Date
  (format-for-display [this]
    (str "[Date] " (.toString this)))

  java.time.LocalDateTime
  (format-for-display [this]
    (str "[LocalDateTime] " (.toString this)))

  java.util.UUID
  (format-for-display [this]
    (str "[UUID] " (.toString this))))

(format-for-display (java.util.Date.))        ; => "[Date] ..."
(format-for-display (java.util.UUID/randomUUID)) ; => "[UUID] ..."
(format-for-display (java.time.LocalDateTime/now)) ; => "[LocalDateTime] ..."

;; --- 13. Atoms & State ---

(def counter (atom 0))

(swap! counter inc)     ; => 1
(swap! counter + 5)     ; => 6
(reset! counter 0)      ; => 0

;; --- 14. Java Interop ---

;; === Method Calls ===

;; Direct form: (. object methodName args…)
(.toUpperCase "hello")   ; => "HELLO"
(.substring "hello" 1 3) ; => "el"

;; Field form: (. object fieldName) — for getters (no args)
(. "hello" length)       ; => 5
(.age alice)              ; => 30

;; Dot-dot (..): chained field access + method call
(.. alice email toUpperCase) ; => "ALICE@EXAMPLE.COM"
;; Equivalent to: (. (. alice email) toUpperCase)

;; === Static Methods ===

;; (Class/fieldName) or (Class/staticMethod args…)
(System/getProperty "user.name")  ; => "ajet" (static field/property)
(System/getProperty "java.version") ; => "21.0.x" (static method call)

;; === Constructors ===

;; (Class.) or (Class. arg1 arg2…)
(java.util.Date.)  ; => #inst "2026-09-14T..." (current date/time)
(java.util.Date. 2024 0 1) ; => #inst "2024-01-01T..."

;; --- 15. Threading Macros ---

;; Thread-first (->) — passes result as first arg
;; Note: (.method args) inside -> is a method call on the threaded value
(-> "hello"
    (.toUpperCase)              ; method call: .toUpperCase on "hello"
    (.replace "H" "J")         ; method call: .replace on "HELLO"
    (.toLowerCase))             ; method call: .toLowerCase on "JELLO"
  ; => "jello"

;; Thread-last (->>) — passes result as last arg
(->> [1 2 3 4 5]
     (filter odd?)
     (map #(* % %))
     (reduce +))   ; => 35

;; --- 16. Error Handling ---

;; try/catch
(try
  (/ 1 0)
  (catch ArithmeticException e
    (str "Math error: " (.getMessage e))))  ; method call: (.getMessage e) => "Math error: Divide by zero"

;; assert — throws AssertionError on failure
(assert (= 2 (+ 1 1)))  ; => nil (assert returns nil on success)
;; (assert (= 1 2) "custom message") ; would throw with custom message

;; --- 17. Macros (simplified) ---

;; Define a simple macro
(defmacro unless [pred & body]
  `(if (not ~pred)
     ~@body))

(unless true
  (print "Won't print")
  "result")  ; => "result"

;; --- 18. Common Patterns ---

;; Compose functions — comp reads right-to-left (like math)
;; Example: slugify — find word chunks, lowercase, join with dashes
(def slugify (comp #(str/join "-" %) #(mapv str/lower-case %) #(re-seq #"\S+" %)))

;; Usage
(slugify "  Hello World  ")  ; => "hello-world"
(slugify "Foo Bar Baz")      ; => "foo-bar-baz"
(slugify "  Hello  World  ") ; => "hello-world"

;; Another: trim then uppercase
(def trim->upper (comp str/upper-case str/trim))

;; Usage
(trim->upper "  hello  ")  ; => "HELLO"
(trim->upper "  Clojure is fun  ")  ; => "CLOJURE IS FUN"

;; Partial application

;; Partial application
(def add-5 (partial + 5))
(add-5 3)  ; => 8

;; Complement — negate a predicate
(every-pred (complement nil?) (complement zero?))  ; => #object[clojure.core$every_pred ...]

;; Juxt — create multiple functions
(def j (juxt :x :y))   ; => #object[clojure.core$juxt ...]
(j {:x 1 :y 2})  ; => [1 2]