Clojure Programming/Examples/API Examples

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This page gives example usage for Clojure API function calls. For a more general introduction to Clojure by example please see http://en.wikibooks.org/wiki/Clojure_Programming/By_Example

Contents 1 TO DO:CLASSIFY 1.1 -> 1.2 cond 1.3 condp 1.4 str 2 Variable Definition 3 Sequence Building 4 MultiMethod 5 Reference Tools 6 do Macros 7 Hash-map tools 8 Sequence Operators 9 Predicate functions 10 Recursion Tools 11 Function Tools 12 Mapping Operators 13 Java Interaction 14 Namespaces 14.1 ns 14.2 require 15 Math Functions 15.1 rem 16 Structure Tools 17 test 18 Sets 18.1 union 19 Zip Utility 19.1 zippers 19.2 zipmap

[edit] TO DO:CLASSIFY

[edit] ->

This is the "threading" operator.

(macroexpand-1 '(-> true (if "clojure")))
=>(if true "clojure")

this:

user=> (-> "abc" .toUpperCase ,, (.replace ,, "B" "-"))
"A-C"

is equivalent to:

user=> (.replace (.toUpperCase "abc") "B" "-")
"A-C"

NOTE: you do not need to insert ',,', since clojure treats them like whitespace, but marking the forms second position this way enhances readability.

However -> works on everything

user=> (-> true (if ,, inc dec) (map ,, [1 2 3]))
(2 3 4)

or expanded

  (map (if true inc dec) [1 2 3])

So one can also use macros and normal functions in ->, ie. non-methods.

Here is an example that should make every devoted Programmer fall in love with lisp, if he compares it with equivalent Java code. Reading the name of the first entry in a zip file:

user=> (import '(java.net URL) '(java.util.zip ZipInputStream))
nil
user=> (-> "http://clojure.googlecode.com/files/clojure_20081217.zip" URL. .openStream ZipInputStream. .getNextEntry bean :name)
"clojure/"

[edit] cond

(defn fib [n] 
    (cond 
        (== 0 n) 0 
        (== 1 n) 1 
        (< 1 n) (+ 
            (fib (- n 1)) 
            (fib (- n 2))))) 

[edit] condp

(let [a 5]
  (condp = a
    1 "hi"
    5 "fun"))
-> "fun"

condp throws an java.lang.IllegalArgumentException if no clauses are matched in the above example. condp also allows to add a single default expression after the clauses whose value will be returned if no clauses matches.

(let [a 7]
  (condp = a
    1 "hi"
    5 "fun"
    "no match"))
-> "no match"

[edit] str

user=> (str [1 2])
"[1 2]"
user=> (str 1 2)
"12"

[edit] Variable Definition

This page provides examples for the following functions.

• def
• binding

[edit] Sequence Building

This page provides examples for the following functions.

• conj
• concat
• merge

[edit] MultiMethod

This page defines the following functions

• defmulti
• defmethod

[edit] Reference Tools

This page defines the following functions.

• ref
• ref-set
• deref
• @
• delay
• force
• commute

[edit] do Macros

This page provides examples for the following functions.

• doseq
• doall
• dorun
• doto

[edit] Hash-map tools

This page provides examples for the following functions.

• get
• assoc-in

[edit] Sequence Operators

This page provides examples for the following functions.

• interpose
• interleave
• reverse
• butlast
• replace

[edit] Predicate functions

This page provides examples for the following functions.

• filter
• remove
• every?
• some

[edit] Recursion Tools

This page defines the following functions.

• loop
• recur
• lazy-cons

[edit] Function Tools

This page provides examples for the following functions.

• fn
• #()
• "lambda"
• %

[edit] Mapping Operators

This page provides examples for the following functions.

• map
• reduce
• apply

[edit] Java Interaction

This page provides examples for the following functions.

• memfn
• proxy
• into-array
• double-array
• genclass

[edit] Namespaces

[edit] ns

(ns test.test 
  (:refer-clojure :exclude [+ -])) 
(defn + 
  [a b] 
  33) 
(+ 1 2) ;= 33 

[edit] require

(require '[clojure.zip :as zip]) 
(require ['clojure.contrib.sql :as 'sql])

[edit] Math Functions

[edit] rem

user=> (rem 5 2) 
1 

[edit] Structure Tools

This page provides examples for the following functions.

• accessor
• assoc
• defstruct
• struct
• struct-map

[edit] test

user=> (defn
  #^{:test (fn []
    (assert (= 4 (myadd 2 2))))}
  myadd [a b]
  (+ a b))
#'user/myadd
user=> (test #'myadd)
:ok

[edit] Sets

[edit] union

user=> (clojure.set/union #{1 2 3} #{1 4 7})
#{1 2 3 4 7}

[edit] Zip Utility

[edit] zippers

user=> (-> (zip/vector-zip [[1 2] 3 [[4 5] 7 8]])
  zip/down
  zip/right
  zip/right
  zip/down
  zip/down
  zip/right
  (zip/edit inc)
  zip/root)
[[1 2] 3 [[4 6] 7 8]] 

A zipper is a way to modify a tree in a functional way- To do this conveniently, there needs to be a way for you to "drill down" a branch in the tree and perform a local edit, without having to tediously rebuild the upper areas of the tree again. In this example, the vector-zip function converts nested arrays into a zipper. Then we "drill down" the zipper with "down" and "right". Once we've reached the node we want to edit, we do so with "edit", in this case, incrementing the number. Then we can call "root" to convert the zipper back into nested arrays. Note that we were able to efficiently make a pinpoint change in the tree in a very elegant manner.

[edit] zipmap

user=> (let [ks [1 3 4]] (zipmap ks (map inc ks)))
{4 5, 3 4, 1 2}