Clojure Cheat Sheet

Basics

(f arg1 arg2): Call a function (prefix notation)
(def rate 0.2): Name a value in the namespace
(let [x 1 y (+ x 1)] (* x y)): Local bindings
42 3.14 1/3 42N 3.14M: Long, double, ratio, BigInt, BigDecimal
"text" \c :keyword 'symbol nil true: String, char, keyword, symbol, nil, boolean
(str "a" 1) / (format "%.2f" x): Build and format strings

[1 2 3] / '(1 2 3): Vector / list
{:name "Ada" :age 36} / #{:a :b}: Map / set
(conj coll x) / (assoc m :k v) / (dissoc m :k): Add / set key / remove key (returns new)
(get m :k default) / (:k m): Look up a key
(update m :count inc): Apply a function to a value
(get-in m [:a :b]) / (assoc-in m [:a :b] v): Nested read / write
(merge m1 m2) / (select-keys m [:a :b]): Combine / pick keys

(if test then else) / (when test ...): Only false and nil are falsy
(cond (neg? n) "neg" (zero? n) "zero" :else "pos"): Multi-branch conditional
(case x 1 "one" (2 3) "few" "many"): Match constants
(or x default) / (and a b): Defaults and short-circuiting
(loop [i 0] (when (< i 3) (recur (inc i)))): Loop with recur (no stack growth)
(doseq [x xs] (println x)) / (dotimes [i 3] ...): Side-effecting loops

(defn greet "doc" [name] (str "Hi " name)): Define a function with a docstring
(fn [x] (* x x)) / #(* % %): Anonymous function / shorthand
(defn f ([] (f 1)) ([x] x)): Multiple arities
(defn f [x & more] ...): Variadic arguments
(partial f a) / (comp f g) / (juxt f g): Build functions from functions
(apply f args): Call with a collection as arguments

(map f xs) / (filter pred xs) / (remove pred xs): Transform and select (lazy)
(reduce + 0 xs): Combine into one value
(->> xs (filter odd?) (map inc) (reduce +)): Thread-last pipeline
(-> m (assoc :a 1) (update :n inc)): Thread-first pipeline
(group-by :team people) / (frequencies xs): Group / count
(sort-by :age > people) / (take 5 (range)): Sort by key / take from infinite seq
(into [] (comp (filter odd?) (map inc)) xs): Transducer pipeline

(def a (atom 0)) / @a: Create and read an atom
(swap! a inc) / (reset! a 10): Update with a function / set
(dosync (alter r1 - 5) (alter r2 + 5)): Coordinated change with refs (STM)
(future (work)) / @f / (deref f 100 :timeout): Run on another thread
(go (>! ch v)) / (<!! ch): core.async put in a go block / blocking take

(ns app.core (:require [clojure.string :as str])): Declare a namespace and require
(.toUpperCase s) / (Math/abs x) / (java.util.Date.): Instance method, static method, constructor
(defprotocol Shape (area [s])): Protocol (polymorphism on type)
(defrecord Point [x y]) / (->Point 1 2): Record (a map with a type)
(defmulti area :shape) / (defmethod area :circle [s] ...): Multimethod (dispatch on any value)
(try ... (catch Exception e (ex-message e))): Handle exceptions
(throw (ex-info "msg" {:type :not-found})): Throw an error with data