Clojure A Dynamic Programming Language for the JVM

Clojure
A Dynamic Programming Language for the JVM

Rich Hickey

Clojure Fundamentals
• 3 years in development, released 10/2007
• A new Lisp, not Common Lisp or Scheme
• Functional
• emphasis on immutability
• Supporting Concurrency
• language-level coordination of state
• Designed for the JVM
• exposes and embraces platform

Clojure is a Lisp
• Dynamically typed, dynamically compiled
• Interactive - REPL
• Load/change code in running program
• Code as data - Reader
• Small core
• Sequences
• Syntactic abstraction - macros
• Not Object-oriented

Atomic Data Types
• Arbitrary precision integers -12345678987654

• Doubles , BigDecimals
1.234 1.234M

• Ratios -22/7

• Strings -“fred”, Characters -a b c

• Symbols - fred ethel , Keywords - :fred :ethel

• Booleans - true false , Null -
nil

• Regex patterns #“a*b”

Data Structures
• Lists - singly linked, grow at front
• (1 2 3 4 5), (fred ethel lucy), (list 1 2 3)

• Vectors - indexed access, grow at end
• [1 2 3 4 5], [fred ethel lucy]

• Maps - key/value associations
• {:a 1, :b 2, :c 3}, {1 “ethel” 2 “fred”}

• Sets #{fred ethel lucy}

• Everything Nests

Syntax
• You’ve just seen it
• Data structures are the code
• Not text-based syntax
• Syntax is in the interpretation of data
structures
• Things that would be declarations, control
structures, function calls, operators, are all
just lists with op at front
• Everything is an expression

# Norvig’s Spelling Corrector in Python
# http://norvig.com/spell-correct.html

def words(text): return re.findall('[a-z]+', text.lower())

def train(features):
model = collections.defaultdict(lambda: 1)
for f in features:
model[f] += 1
return model

NWORDS = train(words(file('big.txt').read()))
alphabet = 'abcdefghijklmnopqrstuvwxyz'

def edits1(word):
n = len(word)
return set([word[0:i]+word[i+1:] for i in range(n)] +
[word[0:i]+word[i+1]+word[i]+word[i+2:] for i in range(n-1)] +
[word[0:i]+c+word[i+1:] for i in range(n) for c in alphabet] +
[word[0:i]+c+word[i:] for i in range(n+1) for c in alphabet])

def known_edits2(word):
return set(e2 for e1 in edits1(word) for e2 in edits1(e1) if e2 in NWORDS)

def known(words): return set(w for w in words if w in NWORDS)

def correct(word):
candidates = known([word]) or known(edits1(word)) or known_edits2(word) or [word]
return max(candidates, key=lambda w: NWORDS[w])

; Norvig’s Spelling Corrector in Clojure
; http://en.wikibooks.org/wiki/Clojure_Programming#Examples

(defn words [text] (re-seq #"[a-z]+" (.toLowerCase text)))

(defn train [features]
(reduce (fn [model f] (assoc model f (inc (get model f 1))))
{} features))

(def *nwords* (train (words (slurp "big.txt"))))

(defn edits1 [word]
(let [alphabet "abcdefghijklmnopqrstuvwxyz", n (count word)]
(distinct (concat
(for [i (range n)] (str (subs word 0 i) (subs word (inc i))))
(for [i (range (dec n))]
(str (subs word 0 i) (nth word (inc i)) (nth word i) (subs word (+ 2 i))))
(for [i (range n) c alphabet] (str (subs word 0 i) c (subs word (inc i))))
(for [i (range (inc n)) c alphabet] (str (subs word 0 i) c (subs word i)))))))

(defn known [words nwords] (for [w words :when (nwords w)] w))

(defn known-edits2 [word nwords]
(for [e1 (edits1 word) e2 (edits1 e1) :when (nwords e2)] e2))

(defn correct [word nwords]
(let [candidates (or (known [word] nwords) (known (edits1 word) nwords)
(known-edits2 word nwords) [word])]
(apply max-key #(get nwords % 1) candidates)))

Java Interop
Math/PI
3.141592653589793

(.. System getProperties (get "java.version"))
"1.5.0_13"

(new java.util.Date)
Thu Jun 05 12:37:32 EDT 2008

(doto (JFrame.) (add (JLabel. "Hello World")) pack show)

;expands into:
(let [x (JFrame.)]
(do (. x (add (JLabel. "Hello World")))
(. x pack)
(. x show))
x)

Clojure is Functional
• All data structures immutable
• Core library functions have no side effects
• Easier to reason about, test
• Essential for concurrency
• Functional by convention insufﬁcient
• let-bound locals are immutable
• loop/recur functional looping construct
• Higher-order functions

Sequences
(drop 2 [1 2 3 4 5]) -> (3 4 5)

(take 9 (cycle [1 2 3 4]))
-> (1 2 3 4 1 2 3 4 1)

(interleave [:a :b :c :d :e] [1 2 3 4 5])
-> (:a 1 :b 2 :c 3 :d 4 :e 5)

(partition 3 [1 2 3 4 5 6 7 8 9])
-> ((1 2 3) (4 5 6) (7 8 9))

(map vector [:a :b :c :d :e] [1 2 3 4 5])
-> ([:a 1] [:b 2] [:c 3] [:d 4] [:e 5])

(apply str (interpose , "asdf"))
-> "a,s,d,f"

(reduce + (range 100)) -> 4950

Maps and Sets
(def m {:a 1 :b 2 :c 3})

(m :b) -> 2 ;also (:b m)

(keys m) -> (:a :b :c)

(assoc m :d 4 :c 42) -> {:d 4, :a 1, :b 2, :c 42}

(merge-with + m {:a 2 :b 3}) -> {:a 3, :b 5, :c 3}

(union #{:a :b :c} #{:c :d :e}) -> #{:d :a :b :c :e}

(join #{{:a 1 :b 2 :c 3} {:a 1 :b 21 :c 42}}
#{{:a 1 :b 2 :e 5} {:a 1 :b 21 :d 4}})

-> #{{:d 4, :a 1, :b 21, :c 42}
{:a 1, :b 2, :c 3, :e 5}}

Persistent Data Structures
• Immutable, + old version of the collection is still
available after 'changes'
• Collection maintains its performance guarantees
• Therefore new versions are not full copies
• Structural sharing - thread safe, iteration safe
• All Clojure data structures are persistent
• Hash map/set and vector based upon array
mapped hash tries (Bagwell)
• Practical - much faster than O(logN)

Concurrency
• Conventional way:
• Direct references to mutable objects
• Lock and worry (manual/convention)
• Clojure way:
• Indirect references to immutable persistent
data structures (inspired by SML’s ref)

• Concurrency semantics for references
• Automatic/enforced
• No locks in user code!

Typical OO - Direct
references to Mutable Objects
foo

:a ?
:b ?
:c 42
:d ?
:e 6

• Uniﬁes identity and value
• Anything can change at any time
• Consistency is a user problem

Clojure - Indirect references
to Immutable Objects
foo :a "fred"
:b "ethel"
@foo :c 42
:d 17
:e 6

• Separates identity and value
• Obtaining value requires explicit
dereference
• Values can never change
• Never an inconsistent value

Persistent ‘Edit’
foo :a "fred"
:b "ethel"
@foo :c 42
:d 17
:e 6

Structural sharing
:a "lucy"
:b "ethel"

•
:c 42
New value is function of old :d 17

•
:e 6
Shares immutable structure
• Doesn’t impede readers
• Not impeded by readers

Atomic Update
foo :a "fred"
:b "ethel"
:c 42
:d 17
:e 6

@foo
Structural sharing
:a "lucy"
:b "ethel"

• Always coordinated
:c 42
:d 17

• Multiple semantics
:e 6

• Next dereference sees new value
• Consumers of values unaffected

Clojure References
• The only things that mutate are references
themselves, in a controlled way
• 3 types of mutable references, with different
semantics:
• Refs - Share synchronous coordinated
changes between threads
• Agents - Share asynchronous autonomous
changes between threads
• Vars - Isolate changes within threads

Refs and Transactions
• Software transactional memory system (STM)
• Refs can only be changed within a transaction
• All changes are Atomic, Consistent and Isolated
• Every change to Refs made within a
transaction occurs or none do
• No transaction sees the effects of any other
transaction while it is running
• Transactions are speculative
• Will be retried automatically if conﬂict
• User must avoid side-effects!

The Clojure STM
• Surround code with (dosync ...)

• Uses Multiversion Concurrency Control (MVCC)

• All reads of Refs will see a consistent snapshot of
the 'Ref world' as of the starting point of the
transaction, + any changes it has made.

• All changes made to Refs during a transaction
will appear to occur at a single point in the
timeline.

• Readers never impede writers/readers, writers
never impede readers, supports commute

Refs in action
(def foo (ref {:a "fred" :b "ethel" :c 42 :d 17 :e 6}))

@foo -> {:d 17, :a "fred", :b "ethel", :c 42, :e 6}

(assoc @foo :a "lucy")
-> {:d 17, :a "lucy", :b "ethel", :c 42, :e 6}


(commute foo assoc :a "lucy")
-> IllegalStateException: No transaction running

(dosync (commute foo assoc :a "lucy"))
@foo -> {:d 17, :a "lucy", :b "ethel", :c 42, :e 6}

Agents
• Manage independent state
• State changes through actions, which are
ordinary functions (state=>new-state)
• Actions are dispatched using send or send-off,
which return immediately
• Actions occur asynchronously on thread-pool
threads
• Only one action per agent happens at a time

Agents
• Agent state always accessible, via deref/@, but
may not reﬂect all actions
• Can coordinate with actions using await
• Any dispatches made during an action are held
until after the state of the agent has changed
• Agents coordinate with transactions - any
dispatches made during a transaction are held
until it commits
• Agents are not Actors (Erlang/Scala)

Agents in Action
(def foo (agent {:a "fred" :b "ethel" :c 42 :d 17 :e 6}))


(send foo assoc :a "lucy")


(await foo)

@foo -> {:d 17, :a "lucy", :b "ethel", :c 42, :e 6}

Java Integration
• Clojure strings are Java Strings, numbers are
Numbers, collections implement Collection,
fns implement Callable and Runnable etc.
• Core abstractions, like seq, are Java interfaces
• Clojure seq library works on Java Iterables,
Strings and arrays.
• Implement and extend Java interfaces and
classes
• Primitive arithmetic support equals Java’s
speed.

Implementation - Functions
• Dynamically compiles to bytecode in memory
• Uses ASM
• No AOT compilation at present
• Every function is new Class
• Implements IFn interface
• Set of invoke methods, overloaded on arity
• All signatures take/return Objects
• Variadics based on sequences

Implementation - Calls
• Function calls are straight Java method calls
• No alternate type system, thunks etc
• No special extra args
• Calls to Java are either direct or via reflection
• No wrappers, caches or dynamic thunks
• Type hints + inference allow direct calls
• Very few hints needed to avoid reflection
• compiler flag can generate warnings

Implementation - Primitives
• Locals can be primitives, arrays of primitives
• Math calls inlined to primitive-arg static methods
• HotSpot ﬁnishes inlining to primitive math
• Result is same speed as Java
(defn foo [n] (defn foo2 [n]
(loop [i 1] (let [n (int n)]
(if (< i n) (loop [i (int 0)]
(recur (inc i)) (if (< i n)
i))) (recur (inc i))
i)))))
(time (foo 100000))
"Elapsed time: 1.428 msecs" (time (foo2 100000))
100000 "Elapsed time: 0.032 msecs"
100000

Implementation - STM
• Not a lock-free spinning optimistic design
• Uses locks, wait/notify to avoid churn
• Deadlock detection + barging
• One timestamp CAS is only global resource
• No read tracking
• Coarse-grained orientation
• Refs + persistent data structures
• java.util.concurrent is still right tool for
caches/queues

Pain Points
• No tail call optimization
• Important for some functional idioms
• Major point of criticism for choice of JVM
from functional circles
• Use Java’s boxed Numbers + own Ratio
• Integer, Long, BigInteger etc
• Slow generic math, numbers on heap
• Would love tagged ﬁxnums and/or standard
high performance boxed math lib

Conclusion
• Very happy with the JVM
• Good performance, facilities, tools, libraries
• Clojure ﬁlls a niche
• Dynamic + functional + JVM
• Lots of interest in ﬁrst 11 months:
• 500+ user mailing list, 500+ messages/month
• 10,000+ SVN reads/month
• Active community

Thanks for listening!

http://clojure.org

Clojure A Dynamic Programming Language for the JVM

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