Clojure: Simple By Design

Simple by Design:
Clojure
@stuarthalloway
stu@cognitect.com
Copyright Cognitect, Inc.
This presentation is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License.
See http://creativecommons.org/licenses/by-nc-sa/3.0/us/

Thoughtworks Radar
Oct 2012:
Adopt Clojure
http://www.thoughtworks.com/radar

Redmonk Top 20
Clojure
http://redmonk.com/sogrady/2014/06/13/language-rankings-6-14/

Design
!
specification of an artifact
using components to meet
goals subject to constraints

Valuable But Not Simple
convenience
beginner-friendliness
ease
familiarity
smaller size
lesser count

Agenda
examples of simplicity in Clojure
benefits of simplicity in Clojure
producing Clojure

Examples of Simplicity
syntax
protocols
values and references

"Hello World"
that is the program

type examples
string "foo"
character f
integer 42, 42N
floating point 3.14, 3.14M
boolean true
nil nil
symbol foo, +
keyword :foo, ::foo

type properties examples
list sequential (1 2 3)
vector sequential and
random access
[1 2 3]
map associative
{:a 100!
:b 90}
set membership #{:a :b}

Function Call
semantics: fn call args
(+ 2 2)
structure: symbol longs
list

Function Definition
define a fn fn name
docstring
(defn greet!
"Returns a friendly greeting"!
[your-name]!
(str "Hello, " your-name))
arguments
fn body

…Still Just Data
symbol symbol
string
(defn greet!
"Returns a friendly greeting"!
[your-name]!
(str "Hello, " your-name))
vector
list

Complexities Avoided
lots of syntax to learn
ordering dependencies
operator precedence
code over data bias
tedious metaprogramming

Reversible
Jacket Belt String

Reversible
interface
implementation
Jacket Belt String

Reversible
interface
implementation
extension
Jacket Belt String

(defprotocol Reversible!
(reverse [_]))!
!
!
!
(defrecord ReversibleTie [a b])!
!
!
!
(extend-protocol Reversible!
ReversibleTie!
(reverse [tie] (->ReversibleTie (:b tie) (:a tie)))!
String!
(reverse [s] (-> s!
StringBuilder.!
.reverse!
.toString)))
interface
implementation
extension

adapter pattern
wrapper pattern
translator pattern
monkey patching
StringUtils
note that these are all combinatorial

Watch OO Flounder
Me 182
nachos
184
instance?
field?
method?
??

If you have more things
than names, your design
is broken.

Clojure’s Simplicity
Me 182
nachos
184
reference
value
pure
function
succession function new value

Values and References
(defprotocol Nachos!
(yum [_] "eat some nachos"))!
!
(defrecord Person [name lbs]!
Nachos!
(yum [person]!
(update-in person [:lbs] + 2)))!
!
(def me (atom (->Person "Stu" 182)))!
!
(def me-before @me)!
!
(swap! me yum)!
!
(def me-after @me)

!
Nachos!
(yum [person]!
functional
!
!
!
(swap! me yum)!
!
(def me-after @me)

!
Nachos!
(yum [person]!
!
!
!
(swap! me yum)!
!
(def me-after @me)
update
semantics

!
Nachos!
(yum [person]!
!
!
!
(swap! me yum)!
!
(def me-after @me)
multiple
point-in-time
values

incidental complexity temporal reasoning
single-threading
locking
defensive copying
setter methods
String vs. StringBuilder vs. StringBuffer
note (again!) that these are all combinatorial

Benefits of Clojure
concision
generality
robustness
agility

StringUtils
indexOfAny
https://commons.apache.org/proper/commons-lang/javadocs/api-2.6/org/apache/commons/lang/StringUtils.html

indexOfAny Spec
StringUtils.indexOfAny(null, *) = -1!
StringUtils.indexOfAny("", *) = -1!
StringUtils.indexOfAny(*, null) = -1!
StringUtils.indexOfAny(*, []) = -1!
StringUtils.indexOfAny("zzabyycdxx",['z','a']) = 0!
StringUtils.indexOfAny("zzabyycdxx",['b','y']) = 3!
StringUtils.indexOfAny("aba", ['z']) = -1

// From Apache Commons Lang, http://commons.apache.org/lang/!
public static int indexOfAny(String str, char[] searchChars) {!
if (isEmpty(str) || ArrayUtils.isEmpty(searchChars)) {!
return -1;!
}!
for (int i = 0; i < str.length(); i++) {!
char ch = str.charAt(i);!
for (int j = 0; j < searchChars.length; j++) {!
if (searchChars[j] == ch) {!
return i;!
}!
}!
}!
return -1;!
}
indexOfAny Impl

public static int indexOfAny(String str, char[] searchChars) {!
when (searchChars)!
for (int i = 0; i < str.length(); i++) {!
char ch = str.charAt(i);!
for (int j = 0; j < searchChars.length; j++) {!
return i;!
}!
}!
}!
}!
}
- Corner Cases

indexOfAny(str, searchChars) {!
when (searchChars)!
for (i = 0; i < str.length(); i++) {!
ch = str.charAt(i);!
for (j = 0; j < searchChars.length; j++) {!
return i;!
}!
}!
}!
}!
}
- Type Decls

when (searchChars)!
for (i = 0; i < str.length(); i++) {!
ch = str.charAt(i); !
when searchChars(ch) i;!
}!
}!
}
+ When Clause

when (searchChars)!
for ([i, ch] in indexed(str)) {!
when searchChars(ch) i;!
}!
}!
}
+ Comprehension

Lispify
(defn index-filter [pred coll]!
(when pred !
(for [[idx elt] (indexed coll) :when (pred elt)] idx)))

imperative functional
searches strings searches any sequence
matches characters matches any predicate
returns first match returns lazy seq of all
matches

; idxs of heads in stream of coin flips!
(index-filter #{:h}
[:t :t :h :t :h :t :t :t :h :h]) !
-> (2 4 8 9)!
!
; Fibonaccis pass 1000 at n=17!
(first !
(index-filter #(> % 1000) (fibo)))!
-> 17
+ Generality

Clojure
programs can have fewer
lines of code than OO
programs have files

imperative functional
functions 1 1
classes 1 0
internal exit points 2 0
variables 3 0
branches 4 0
boolean ops 1 0
function calls* 6 3
total 18 4

Plain Immutable
Collection Objects
(PICOs)

PICOS Everywhere
collections
directories
files
XML
JSON
result sets
web requests
web responses
sessions
configuration
metrics
logs

Consuming JSON
What actors are in more than one movie
currently topping the box office charts?
http://developer.rottentomatoes.com/docs/
read/json/v10/Box_Office_Movies

Consuming JSON
find the JSON input!
download it!
parse json!
walk the movies!
accumulating cast!
extract actor name!
get frequencies!
sort by highest frequency

Consuming JSON
(->> box-office-uri!
slurp!
json/read-json!
:movies!
(mapcat :abridged_cast)!
(map :name)!
frequencies!
(sort-by (comp - second)))

Consuming JSON
["Shiloh Fernandez" 2] !
["Ray Liotta" 2] !
["Isla Fisher" 2] !
["Bradley Cooper" 2] !
["Dwayne "The Rock" Johnson" 2] !
["Morgan Freeman" 2] !
["Michael Shannon" 2] !
["Joel Edgerton" 2] !
["Susan Sarandon" 2] !
["Leonardo DiCaprio" 2]

PICOs for Big Data
(defn my-data-2 []
(->>
(pig/load-tsv "input.tsv")
(pig/map (fn [[a b c]]
{:sum (+ (Integer/valueOf a) (Integer/valueOf b))
:name c}))
(pig/filter (fn [{:keys [sum]}]
(< sum 5)))))
!
=> (pig/dump (my-data-2))
[{:sum 3, :name "foo"}]
https://github.com/Netflix/PigPen

Me 182
nachos
184
reference
value
pure
function
new value
succession
function

Me t-1
transact
t-2
connection
db value
pure
function
new db value
ACID

ACID data of record
persistent data structures: “scm for business data”
distributed, componentized, read scalable & elastic
information and logic as PICOs in any peer process

Connect and Query
Connection conn = !
connect("datomic:ddb://us-east-1/mb/mbrainz");!
!
!
Database db = conn.db();!
!
!
Set results = q(..., db);!
!
!
Set crossDbResults = q(..., db1, db2);!
!
!
Entity e = db.entity(42);

Connect and Query
Connection conn = !
connect("datomic:ddb://us-east-1/mb/mbrainz");!
!
!
Database db = conn.db();!
!
!
Set results = q(..., db);!
!
!
Set crossDbResults = q(..., db1, db2);!
!
!
Entity e = db.entity(42);
database is a lazily
realized value, available
to all peers equally

Goal
!
give skilled devs superpowers
to build business software
systems

Constraints
for wide adoption
open source
target established platforms
for viability
performance
stability

Open Source
licensed under EPL
contributor agreement
artifacts in Maven Central
not just language: bunch of libs too

Might Surprise You
we take patches, not pull requests
we prefer designs over patches
we prefer problem statements over designs

Server Performance
Clojure
note:
log
scale!
http://benchmarksgame.alioth.debian.org/u64q/which-programs-are-fastest.php

Maintaining Programming Clojure
release date breakage*
1.0 05/2009 -
1.1 12/2009 None
1.2 08/2010 None
1.3 09/2011 Small
1.4 04/2012 None
1.5 03/2013 None
1.6 03/2014 None
1.7 TBD None

Examples of Simplicity
syntax
protocols
values and references
PICOs!

Clojure
http://clojure.com. The Clojure language.
http://cognitect.com/. The company behind Clojure, ClojureScript, & Datomic.
http://blog.cognitect.com/cognicast/. The Cognicast.
http://bit.ly/clojure-bookshelf. 40 recommendations from Rich.
http://clojure.in/. Planet Clojure.
!
@stuarthalloway
https://github.com/stuarthalloway/presentations/wiki. Presentations.
https://github.com/stuarthalloway/exploring-clojure. Sample Code.
http://pragprog.com/book/shcloj2/programming-clojure. Programming Clojure.
mailto:stu@cognitect.com

Clojure: Simple By Design

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