7li7w devcon5

What I learned from
Seven Languages
in Seven Weeks
Kerry Buckley (@kerryb) – DevCon5 25/5/12

RubyIoProlog
ScalaErlang
ClojureHaskell

What is the typing model?
What is the programming model?
How will you interact with it?
What are the decision constructs
and core data structures?
What are the core features that
make the language unique?

Java was like having a rich lawyer as a brother. He was
fun when he was younger, but now he’s a black hole
that sucks away all the joy in a 100-mile radius.

Meet Ruby, one of my favorites. She’s sometimes quirky,
always beautiful, a little mysterious, and absolutely magical.

The irb console
>> puts "Hello world"
Hello world
=> nil

>> name = "Kerry"
=> "Kerry"

>> puts "Hello #{name}"
Hello Kerry
=> nil

Everything’s an object
>> 42.class
=> Fixnum

>> nil.class
=> NilClass

>> String.class
=> Class

> 42.methods
=> [:to_s, :+, :-, :*, :/, :abs, ...

Duck typing
def double(x)
x * 2
end

>> double(2)
=> 4

>> double(2.5)
=> 5.0

>> double("foo")
=> "foofoo"

>> double(false)
NoMethodError: undefined method `*' for false:FalseClass
from (irb):2:in `double'

Arrays, ranges & hashes
>> array = [1, 2.0, "three"]

>> array[0]
=> 1

>> array.reverse
=> ["three", 2.0, 1]

>> range = (1..10)

>> range.include? 5
=> true

>> hash = {:foo => 123, "bar" => "", 456 => [1, 2]}

>> hash[456]
=> [1, 2]

Blocks & yield
>> 3.times { puts "hello" }
hello
hello
hello

>> File.open "some-file", "w" do |f|
?> f.write "some-data"
>> end

>> def delayed_by seconds
>> sleep seconds
>> yield
>> end

>> delayed_by 5 do
?> puts "At last!"
>> end
At last!

Open classes
class NilClass
def blank?
true
end
end

class String
def blank?
empty?
end
end

>> [nil, "", "foo"].map &:blank?
=> [true, true, false]

Modules & mixins
module Debug
def info
"#{self.class}[#{object_id}]: #{to_s}"
end
end

class Object
include Debug
end

>> 2.info
=> "Fixnum[5]: 2"

>> "foo".info
=> "String[70107061928320]: foo"

Collections & Enumerable
class MyCollection
include Enumerable
def each
...
end
end

>> MyCollection.instance_methods.sort
=> [:all?, :any?, :chunk, :collect, :collect_concat,
:count, :cycle, :detect, :drop, :drop_while, :each,
:each_cons, :each_entry, :each_slice, :each_with_index,
:each_with_object, :entries, :find, :find_all, :find_index,
:first, :flat_map, :grep, :group_by, :include?, :inject,
:map, :max, :max_by, :member?, :min, :min_by, :minmax,
:minmax_by, :none?, :one?, :partition, :reduce, :reject,
:reverse_each, :select, :slice_before, :sort, :sort_by,
:take, :take_while, :to_a, :zip]

method_missing magic
class MyStruct
def initialize values
@values = values
end

def method_missing name, *args
@values[name]
end
end

>> struct = MyStruct.new foo: 123, bar: 456
=> #<MyStruct:0x007fc2c21433b8 @values={:foo=>123, :bar=>456}>

>> struct.foo
=> 123

>> struct.bar
=> 456

Dynamic definition
class MyStruct
def initialize values
@values = values
end

def method_missing name, *args
puts "Defining method #{name}"
self.class.class_eval do
define_method(name) { @values[name] }
end
send(name, *args)
end
end

>> struct.foo
Defining method foo
=> 123

>> struct.foo
=> 123

Pros

Power and ﬂexibility
Developer productivity and fun
Raw execution speed
Limited concurrency support
Type-aware tool support
Cons

Io is a rule bender. He’s young, wicked smart, and easy to
understand but hard to predict. He might give you the ride of
your life, wreck your dad’s car, or both.

Message passing
Io> "oI olleH" reverse
==> Hello Io

Io> list("iH", "oI") map(reverse)
==> list(Hi, Io)

Io> list(1, 2, 3) map(** 2) sum
==> 14

Objects & Slots
Io> Person := Object clone
==> Person_0x7f922c06ad00:
type = "Person"

Io> Person firstName := "John"
==> John
Io> Person lastName := "Doe"
==> Doe
Io> Person firstName
==> John

Defining methods
Io> Person name := method (
firstName .. " " .. lastName
)

Io> Person name
==> John Doe

Prototypal inheritance
Io> Arthur := Person clone
==> Arthur_0x7fd5406cd860:
type = "Arthur"

Io> Arthur name
==> John Doe

Io> Arthur firstName = "Arthur"
Io> Arthur lastName = "Dent"
Io> Arthur name
==> Arthur Dent

Io> Arthur proto
==> Person_0x7fd540693ed0:
firstName = "John"
lastName = "Doe"
name = method(...)
type = "Person"

Control structures
Io> for(i, 1, 3, i println)
1
2
3
==> 3

Io> if(true,
"Yes" println,
"No" println)
Yes
==> Yes

Reflection
Io> foo := method(
call sender println
call message arguments println
)

Io> foo("bar", 42)
Object_0x7fcf78418920:
Lobby = Object_0x7fcf78418920
Protos = Object_0x7fcf78417c00
...

list("bar", 42)

Coroutines
Io> Ping := Object clone
Io> Pong := Object clone

Io> Ping ping := method (
2 repeat("Ping!" println; yield))

Io> Pong pong := method (
2 repeat(yield; "Pong!" println))

Io> Ping @@ping; Pong @@pong
Io> Coroutine currentCoroutine pause
Ping!
Pong!
Ping!
Pong!

Actors
Io> Slow := Object clone
Io> Fast := Object clone

Io> Slow go := method(
wait(1)
"Slow" println
)

Io> Fast go := method(
"Fast" println
)

Io> Slow @@go; Fast @@go; wait(2)
Fast
Slow

Futures
Io> page := URL with(
"http://google.co.uk/") @fetch

Io> "I'm in the foreground" println
I'm in the foreground

Io> page size println
16906

Pros
Tiny – ideal for embedded systems
Prototypes and duck typing are very ﬂexible

Concurrency support
Simple, consistent syntax
Raw execution speed
Small user community
Cons

Sometimes spectacularly smart, other times just as frustrating. You’ll
get astounding answers only if you know how to ask the question.

Simple facts
likes(wallace, cheese).
likes(gromit, cheese).
likes(wendolene, sheep).

?- likes(wallace, sheep).
false.
?- likes(gromit, cheese).
true.
?- likes(Who, cheese).
Who = wallace ;
Who = grommit.

Evaluating rules
friend(X, Y) :- +(X = Y),
likes(X, Z), likes(Y, Z).

?- friend(wallace, wallace).
false.
?- friend(wendolene, gromit).
false.
?- friend(wallace, gromit).
true.

Recursive rules
father(grandpa, homer).
father(homer, bart).

ancestor(X, Y) :- father(X, Y).
ancestor(X, Y) :- father(X, Z),
ancestor(Z, Y).

?- ancestor(Who, bart).
Who = homer ;
Who = grandpa ;
false.

Tuples
?- (X, Y, Z) = (1, 2, "foo").
X = 1,
Y = 2,
Z = [102, 111, 111].

?- (X, _, Z) = (1, 2, "foo").
X = 1,
Z = [102, 111, 111].

Calculation with lists
sum(0, []).
sum(Total, [Head|Tail]) :-
sum(Sum, Tail),
Total is Head + Sum.

?- sum(What, [1, 2, 3]).
What = 6.

Solving (4×4) sudoku
?- sudoku([_, _, 2, 3,
_, _, _, _,
_, _, _, _,
3, 4, _, _],
Solution).
Solution = [4,1,2,3,2,3,4,1,1,2,3,4,3,4,1,2].

Solving (4×4) sudoku
sudoku(Puzzle, Solution) :-
Solution = Puzzle,
Puzzle = [S11, S12, S13, S14,
S21, S22, S23, S24,
S31, S32, S33, S34,
S41, S42, S43, S44],
Solution ins 1..4,
Row1 = [S11, S12, S13, S14], Row2 = [S21, S22, S23, S24],
Row3 = [S31, S32, S33, S34], Row4 = [S41, S42, S43, S44],
Col1 = [S11, S21, S31, S41], Col2 = [S12, S22, S32, S42],
Col3 = [S13, S23, S33, S43], Col4 = [S14, S24, S34, S44],
Square1 = [S11, S12, S21, S22], Square2 = [S13, S14, S23, S24],
Square3 = [S31, S32, S41, S42], Square4 = [S33, S34, S43, S44],
valid([Row1, Row2, Row3, Row4,
Col1, Col2, Col3, Col4,
Square1, Square2, Square3, Square4]).

valid([]).
valid([Head|Tail]) :-
all_different(Head),
valid(Tail).

Pros

Solving logical and scheduling problems
Natural language processing
Artiﬁcial intelligence
Scaling requires deep understanding
Not a general-purpose language
Simple procedural tasks are difﬁcult
Cons

He was often awkward, was sometimes amazing, but always had a
unique expression. Sometimes, his scissors let him do incredible things.
Other times, he was awkward and humiliated.

Type inference & coercion
scala> "foo"
res0: java.lang.String = foo

scala> 123
res1: Int = 123

scala> 45.6
res2: Double = 45.6

scala> 1 + 2.3
res3: Double = 3.3

scala> "The answer is " + 42
res4: java.lang.String = The answer is 42

var & val
scala> var a = 42
a: Int = 42

scala> a = a + 1
a: Int = 43

scala> val earth = "Harmless"
earth: java.lang.String = Harmless

scala> earth = "Mostly Harmless"
<console>:8: error: reassignment to val
earth = "Mostly Harmless"
^

Ranges & tuples
scala> val range = 0 to 3
range: scala.collection.immutable.Range.Inclusive
= Range(0, 1, 2, 3)

scala> val range = 0 to 5 by 2
range: scala.collection.immutable.Range
= Range(0, 2, 4)

scala> val tuple = ("Kerry", 42)
tuple: (java.lang.String, Int) = (Kerry,42)

scala> val (name, age) = tuple
name: java.lang.String = Kerry
age: Int = 42

Lists, sets & maps
scala> val list = List(1, 2, 3)
list: List[Int] = List(1, 2, 3)

scala> list(1)
res0: Int = 2

scala> "zero"::list
res1: List[Any] = List(zero, 1, 2, 3)

scala> Set(1, 2, 3) ++ Set(2, 3, 4)
res2: scala.collection.immutable.Set[Int] = Set(1, 2, 3, 4)

scala> val map = Map(1 -> "One", 2 -> "Two")
map: scala.collection.immutable.Map[Int,java.lang.String] =
Map(1 -> One, 2 -> Two)

scala> map(1)
res3: java.lang.String = One

Defining classes
class Person(firstName: String,
lastName: String) {
def greet(name: String) {
println("Hello " + name +
", I'm " + firstName + ".")
}
}

scala> val kerry = new Person("Kerry",
"Buckley")
kerry: Person = Person@276bab54

scala> kerry.greet("DevCon")
Hello DevCon, I'm Kerry.

Companion objects
class Person(firstName: String,
lastName: String) {
...
}

object Person {
def find(id: Int) {
val (first, last) = DB.people.find(id)
new Person(first, last
}
}

Extending & traits
class Person(val name:String)

trait Nice {
def greet() = println("Howdily doodily.")
}

class Character(override val name:String)
extends Person(name) with Nice

scala> val flanders = new Character("Ned")
scala> flanders.greet
Howdily doodily.

List processing
scala> val jedi = List("Yoda", "Obiwan", "Luke")
jedi: List[java.lang.String] = List(Yoda, Obiwan, Luke)

scala> jedi.filter(name => name.size < 5)
res0: List[java.lang.String] = List(Yoda, Luke)

scala> jedi.map(name => name.size)
res1: List[Int] = List(4, 6, 4)

scala> val numbers = List(1, 2, 3)
numbers: List[Int] = List(1, 2, 3)

scala> numbers.foldLeft(0)((a, n) => a + n)
res2: Int = 6

Class hierarchy
Any

AnyVal AnyRef

Float Int … ScalaObject …

List Map

Nothing
Null

First-class XML
scala> val pets = <pets>
<chicken>Babs</chicken>
<chicken>Bunty</chicken>
<chicken>Lily</chicken>
<cat>Pebbles</cat>
<chicken>Pepper</chicken>
<cat>Twiglet</cat>
<cat>Willow</cat>
<cat>Zorro</cat>
</pets>

scala> pets "cat"
res0: scala.xml.NodeSeq = NodeSeq(<cat>Twiglet</cat>,
<cat>Pebbles</cat>, <cat>Willow</cat>,
<cat>Zorro</cat>)

Pattern matching
scala> (pets "_").foreach {pet =>
pet match {
case <cat>{name}</cat> =>
println(name + " says 'meow'.")
case <chicken>{name}</chicken> =
println(name + " says 'cluck'.")
}
}

Babs says 'cluck'.
Bunty says 'cluck'.
Lily says 'cluck'.
Pebbles says 'meow'.
Pepper says 'cluck'.
Twiglet says 'meow'.
Willow says 'meow'.
Zorro says 'meow'.

Concurrency with actors
case object Stroke
case object Feed

class Cat() extends Actor {
def act() {
loop {
react {
case Stroke => { println("Purr!") }
case Feed => { println("Om nom nom") }
}
}
}
}

scala> val cat = new Cat().start
scala> cat ! Stroke; cat ! Feed; println("Done.")
Done.
Purr!
Om nom nom

Pros

A modern version of Java
Mixins, pattern matching, blocks, XML
Concurrency with actors & immutability

Static typing
Compromises with mutable state
Cons

Agent Smith was an artiﬁcial intelligence program in the matrix that
had an amazing ability to take any form and bend the rules of
reality to be in many places at once. He was unavoidable.

The usual types (mostly)
1> 2 + 2.
4

2> "Hello".
"Hello"

3> atom.
atom

4> {foo, 123, "bar"}.
{foo,123,"bar"}

5> [1, "one", two].
[1,"one", two]

6> [87, 84, 70, 63].
"WTF?"

Variables don’t change
1> Foo = 42.
42

2> Foo = Foo + 1.
** exception error: no match of right hand side value 43

3> Values = [1, 2, 3].
[1,2,3]

4> lists:append(Values, [4]).
[1,2,3,4]

5> Values.
[1,2,3]

Pattern matching
1> Pet = {{name, "Zorro"}, {type, "Cat"}}.
{{name,"Zorro"},{type,"Cat"}}

2> {{name, Name}, {type, Type}} = Pet.
{{name,"Zorro"},{type,"Cat"}}

3> Name.
"Zorro"

4> Type.
"Cat"

5> [Head|Tail] = [foo, bar, baz].
[foo,bar,baz]

6> {Head, Tail}.
{foo,[bar,baz]}

Bit matching
1> [A, B, C, D] = [1, 0, 50, 200].
[1,0,50,200]

2> Packed = <<A:1, B:1, C:6, D:8>>.
<<"²È">>

3> <<P:1, Q:1, R:6, S:8>> = Packed.
<<"²È">>

4> {P, Q, R, S}.
{1,0,50,200}

Basic functions
-module(demo).
-export([echo/1]).
echo(Value) -> Value.

1> c(demo).
{ok,demo}

5> demo:echo("Hello").
"Hello"

6> demo:echo(42).
42

Patterns in functions
-module(fact).
-export([fact/1]).

fact(0) -> 1;
fact(N) -> N * fact(N-1).

1> c(fact).
{ok,fact}

3> fact:fact(6).
720

4> fact:fact("foo").
** exception error: bad argument in an arithmetic
expression
in function fact:fact/1 (fact.erl, line 5)

6> fact:fact(10000).

28462596809170545189064132121198688901480514017027992307941799942744113400
03764443772990786757784775815884062142317528830042339940153518739052421161
38271617481982419982759241828925978789812425312059465996259867065601615720
36032397926328736717055741975962099479720346153698119897092611277500484198
84541047554464244213657330307670362882580354896746111709736957860367019107
15127305872810411586405612811653853259684258259955846881464304255898366493
17059251717204276597407446133400054194052462303436869154059404066227828248
37151203832217864462718382292389963899282722187970245938769380309462733229
25705554596900278752822425443480211275590191694254290289169072190970836905
39873747452483372899521802363282741217040268086769210451555840567172555372
01585213282903427998981844931361064038148930449962159999935967089298019033
69984844046654192362584249471631789611920412331082686510713545168455409360
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37042392087644847615690012648892715907063064096616280387840444851916437908
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05617531786346939798024619737079041868329931016554150742308393176878366923

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Higher order functions
1> Double = fun(X) -> X * 2 end.
#Fun<erl_eval.6.111823515>

2> Double(2).
4

3> List = [1, 2, 3].
[1,2,3]

4> lists:map(Double, List).
[2,4,6]

5> lists:map(fun(X) -> X + 1 end, List).
[2,3,4]

6> lists:foldl(fun(X, Sum) -> X + Sum end, 0, List).
6

List comprehensions
1> Numbers = [1, 2, 3, 4].
[1,2,3,4]

2> Double = fun(X) -> X * 2 end.
#Fun<erl_eval.6.111823515>

3> lists:map(Double, Numbers).
[2,4,6,8]

4> [Double(X) || X <- Numbers].
[2,4,6,8]

5> [X || X <- Numbers, X > 1, X < 4].
[2,3]

6> Basket = [{pencil, 4, 0.25}, {pen, 1, 1.20}, {paper, 2, 1.00}].
[{pencil,4,0.25},{pen,1,1.2},{paper,2,1.0}]

7> Totals = [{Item, Qty * Price} || {Item, Qty, Price} <- Basket].
[{pencil,1.0},{pen,1.2},{paper,2.0}]

Message loops
-module(doubler).
-export([loop/0]).

loop() ->
receive
N ->
io:format("~b~n", [N * 2]),
loop()
end.

18> Doubler = spawn(fun doubler:loop/0).
<0.87.0>

19> Doubler ! 2.
4

Synchronous messages
-module(doubler).
-export([loop/0, double/2]).

loop() ->
receive
{Pid, N} ->
Pid ! (N * 2),
loop()
end.

double(To, N) ->
To ! {self(), N},
receive
Result -> Result
end.

27> Doubler = spawn(fun doubler:loop/0).
<0.118.0>

28> doubler:double(Doubler, 3).
6

Monitor & restart
-module(monitor).
-export([loop/0, start/0]).

loop() ->
process_flag(trap_exit, true),
receive
new ->
register(doubler, spawn_link(fun doubler:loop/0)),
loop();
{'EXIT', From, Reason} ->
io:format("~p exited: ~p.", [From, Reason]),
monitor ! new,
loop()
end.

start() ->
register(monitor, spawn(fun monitor:loop/0)),
monitor ! new.

Pros

Designed for concurrency & fault tolerance
Flexibility of dynamic typing
Lightweight processes with message passing & monitoring
Build scalable applications with the OTP library
Syntax sometimes a little clumsy
Integration with other languages

Cons

His communication style is often inverted and hard to understand.
He seems too small to make a difference, but it quickly becomes
apparent that there is more to Yoda than meets the eye.

Prefix notation
user=> (+ 2 2)
4

user=> (- 10 4 1)
5

user=> (/ 2.0 3)
0.6666666666666666

user=> (/ 2 3)
2/3

user=> (count "hello")
5

user=> (+ (count "hello") (count "clojure"))
12

Java hiding underneath
user=> (class 1)
java.lang.Integer

user=> (class "foo")
java.lang.String

user=> (class (= 2 2))
java.lang.Boolean

user=> (class (/ 1 2))
clojure.lang.Ratio

Data or code?
user=> (1 2 3)
java.lang.ClassCastException: java.lang.Integer cannot be
cast to clojure.lang.IFn (NO_SOURCE_FILE:0)

user=> (list 1 2 3)
(1 2 3)

user=> (class (list 1 2 3))
clojure.lang.PersistentList

user=> '(1 2 3)
(1 2 3)

user=> (class '(+ 2 2))
clojure.lang.PersistentList

user=> (eval '(+ 2 2))
4

Vectors, sets & maps
user=> (def numbers [1 2 3])
(1 2 3)

user=> (conj numbers "forty-two")
[1 2 3 "forty-two"]

user=> (def crew #{:zaphod :trillian :marvin})
#'user/crew

user=> (clojure.set/union crew #{:ford :zaphod :arthur})
#{:arthur :trillian :ford :zaphod :marvin}

user=> (def home {:arthur :earth, :ford :betelgeuse, :marvin :sirius})
#'user/home

user=> (home :arthur)
:earth

user=> (:arthur home)
:earth

Defining functions
user=> (defn answer [] 42)
#'user/answer

user=> (answer)
42

user=> (defn treble [a] (* 3 a))
#'user/treble

user=> (treble 20)
60

Destructuring params
user=> (def board [[:x :o :x]
[:o :x :o]
[:o :x :o]])
#'user/board

user=> (defn centre [[_ [_ c _] _]] c)
#'user/centre

user=> (centre board)
:x

Anonymous functions
user=> (defn treble [a] (* 3 a))
#'user/treble

user=> (def numbers [1 2 3])
#'user/numbers

user=> (map treble numbers)
(3 6 9)

user=> (map (fn [n] (* 3 n)) numbers)
(3 6 9)

user=> (map #(* 3 %) numbers)
(3 6 9)

Recursion: loop & recur
user=> (defn size [v]
(if (empty? v)
0
(inc (size (rest v)))))
#'user/size

user=> (size [1 2 3])
3

user=> (defn size [v]
(loop [l v, c 0]
(if (empty? l)
c
(recur (rest l) (inc c)))))

Working with sequences
user=> (def numbers [1 2 3 4])
#'user/numbers

user=> (every? odd? numbers)
false

user=> (filter odd? numbers)
(1 3)

user=> (for [x numbers] (* 2 x))
(2 4 6 8)

user=> (for [x numbers, y numbers] (* x y))
(1 2 3 4 2 4 6 8 3 6 9 12 4 8 12 16)

user=> (for [x numbers, y numbers, :when (odd? x)]
(* x y))
(1 2 3 4 3 6 9 12)

Lazy evaluation
user=> (take 5 (cycle ["I" "am" "what"]))
("I" "am" "what" "I" "am")

user=> (take 5 (drop 2 (cycle [1 2 3])))
(3 1 2 3 1)

user=> (->> [1 2 3] (cycle) (drop 2) (take 5))
(3 1 2 3 1)

user=> (take 5 (iterate inc 1))
(1 2 3 4 5)

user=> (defn factorial [n]
(apply * (take n (iterate inc 1))))
#'user/factorial

user=> (factorial 5)
120

Records & protocols
user=> (defprotocol Shape
(area [this]))
Shape

user=> (defrecord Square [width height]
Shape
(area [this] (* width height)))
user.Square

user=> (defrecord Circle [radius]
Shape
(area [this] (* (. Math PI) (* radius radius))))
user.Circle

user=> (area (Square. 2 3))
6

user=> (area (Circle. 4))
50.26548245743669

Macro expansion
user=> (defn unless [test body] (if (not test) body))
#'user/unless

user=> (unless true (println "It's a lie!"))
It's a lie!
nil

user=> (defmacro unless [test body]
(list 'if (list 'not test) body))
#'user/unless

user=> (macroexpand '(unless condition body))
(if (not condition) body)

user=> (unless true (println "It's a lie!"))
nil

user=> (unless false (println "It's true!"))
It's true!
nil

Transactional memory
user=> (def ford (ref {:name "Ford Prefect", :from "Guildford"}))
#'user/ford

user=> (deref ford)
{:name "Ford Prefect", :from "Guildford"}

user=> @ford
{:name "Ford Prefect", :from "Guildford"}

user=> (alter ford assoc :from "Betelgeuse")
java.lang.IllegalStateException: No transaction running
(NO_SOURCE_FILE:0)

user=> (dosync (alter ford assoc :from "Betelgeuse"))
{:name "Ford Prefect", :from "Betelgeuse"}

user=> @ford
{:name "Ford Prefect", :from "Betelgeuse"}

Encapsulation with atoms
user=> (def numbers (atom [1 2 3]))
#'user/numbers

user=> numbers
#<Atom@7d98d9cf: [1 2 3]>

user=> @numbers
[1 2 3]

user=> (reset! numbers [4 5 6])
[4 5 6]

user=> (swap! numbers conj 7)
[4 5 6 7]

Agents in the background
user=> (defn slow-twice [x]
(do (Thread/sleep 5000) (* 2 x)))
#'user/slow-twice

user=> (def number (agent 2))
#'user/number

user=> number
#<Agent@4c825cf3: 2>

user=> @number
2

user=> (send number slow-twice)
#<Agent@4c825cf3: 2>

user=> @number
2

user=> @number
4

Back to the futures
user=> (def ultimate-answer
(future (do
(Thread/sleep 2.4e17)
42)))
#'user/ultimate-answer

user=> @ultimate-answer
42

Pros

A good lisp implementation
Access to Java ecosystem
Concurrency provided by STM
Preﬁx notation can be confusing
(((((and all those parens don’t help)))))
Some limitations compared to other lisps

Cons

Haskell represents purity and freedom, but the power comes at a
price. Think Spock from Star Trek. His character has a single-minded
purity that has endeared him to generations.

Type inference
Prelude> :set +t
Prelude> 42
42
it :: Integer

Prelude> "Mostly harmless"
"Mostly harmless"
it :: [Char]

Prelude> "black" == "white"
False
it :: Bool

Prelude> 1/2
0.5
it :: Double

Defining functions
Prelude> let double x = x * 2
double :: Num a => a -> a

Prelude> double 4
8

Prelude> double 2.5
5.0

Prelude> let times x y = x * y
times :: Num a => a -> a -> a

Prelude> times 5 6
30

Specifying function types
module Main where

double :: Integer -> Integer
double x = 2 * x

Prelude> :load main
[1 of 1] Compiling Main ( main.hs, interpreted )
Ok, modules loaded: Main.

*Main> double 2
4

*Main> double 2.5

<interactive>:1:8:
No instance for (Fractional Integer)
arising from the literal `2.5'

Pattern matching & guards
module Main where

factorial :: Integer -> Integer
factorial 0 = 1
factorial x = x * factorial (x - 1)

factorial2 :: Integer -> Integer
factorial2 x
| x > 1 = x * factorial2 (x - 1)
| otherwise = 1

Tuples & lists
Prelude> let zaphod = ("Zaphod", "Beeblebrox", "Betelgeuse", 2)
Prelude> let (_, _, _, numberOfHeads) = zaphod

Prelude> let numbers = [1, 2, 3, 4]
Prelude> let (head:tail) = numbers
Prelude> head
1
Prelude> tail
[2,3,4]

Prelude> 1:[2, 3]
[1,2,3]

Prelude> [1, "two"]
<interactive>:1:2:
No instance for (Num [Char])
arising from the literal `1'
Possible fix: add an instance declaration for (Num [Char])
In the expression: 1
In the expression: [1, "two"]
In an equation for `it': it = [1, "two"]

Ranges & sequences
Prelude> [1..5]
[1,2,3,4,5]

Prelude> [2, 4 .. 10]
[2,4,6,8,10]

Prelude> [0, 0.5 .. 2]
[0.0,0.5,1.0,1.5,2.0]

Prelude> take 5 [1 ..]
[1,2,3,4,5]

List comprehensions
Prelude> let numbers = [1,2,3]

Prelude> [x * 2 | x <- [1, 2, 3]]
[2,4,6]

Prelude> [[x, y] | x <- numbers, y <- numbers]
[[1,1],[1,2],[1,3],[2,1],[2,2],[2,3],[3,1],[3,2],[3,3]]

Prelude> [[x, y] | x <- numbers, y <- numbers, x < y]
[[1,2],[1,3],[2,3]]

Prelude> let more_numbers = [4,5]

Prelude> [(x, y, x * y) | x <- numbers, y <- more_numbers, x < y]
[(1,4,4),(1,5,5),(2,4,8),(2,5,10),(3,4,12),(3,5,15)]

Function currying
Prelude> 4 * 5
20

Prelude> (*) 4 5
20

Prelude> :t (*)
(*) :: Num a => a -> a -> a

Prelude> let double = (* 2)
Prelude> :t double
double :: Integer -> Integer

Prelude> double 3
6

Higher order functions
Prelude> (x -> 2 * x) 5
10

Prelude> map (x -> 2 * x) [1, 2, 3]
[2,4,6]

Prelude> map (* 2) [1, 2, 3]
[2,4,6]

Prelude> filter odd [1, 2, 3, 4]
[1,3]

Prelude> foldl (+) 0 [1, 2, 3, 4]
10

Type classes
class Eq a where
(==), (/=) :: a -> a -> Bool
-- Minimal complete definition:
-- x/=y x==y
(==) or (/=)
= not(x==y) = not(x/=y)

*Main> :info Integer
data Integer
= integer-gmp:GHC.Integer.Type.S# GHC.Prim.Int#
| integer-gmp:GHC.Integer.Type.J# GHC.Prim.Int#
GHC.Prim.ByteArray#
-- Defined in integer-gmp:GHC.Integer.Type
instance Enum Integer -- Defined in GHC.Num
instance Eq Integer -- Defined in GHC.Classes
instance Integral Integer -- Defined in GHC.Real
instance Num Integer -- Defined in GHC.Num
instance Ord Integer -- Defined in GHC.Classes
instance Read Integer -- Defined in GHC.Read
instance Real Integer -- Defined in GHC.Real
instance Show Integer -- Defined in GHC.Num

User-defined types
module Main where
data Shape = Circle Float | Rectangle Float Float
deriving show
area :: Shape -> Float
area (Circle r) = pi * r ^ 2
area (Rectangle x y) = x * y

*Main> let circle = Circle 10

*Main> let rect = Rectangle 6 7

*Main> area circle
314.15927

*Main> area rect
42.0

*Main> [circle, rect]
[Circle 10.0,Rectangle 6.0 7.0]

Record syntax
module Main where
data Shape = Circle {radius :: Float}
| Rectangle {width :: Float, height :: Float}
deriving Show
area :: Shape -> Float
area (Circle r) = pi * r ^ 2
area (Rectangle x y) = x * y

*Main> let circle = Circle 10

*Main> let rect = Rectangle 6 7

*Main> [circle, rect]
[Circle {radius = 10.0},Rectangle {width = 6.0, height = 7.0}]

Monads
A monad is a construction that, given an underlying type
system, embeds a corresponding type system (called
the monadic type system) into it (that is, each monadic
type acts as the underlying type). This monadic type
system preserves all signiﬁcant aspects of the
underlying type system, while adding features particular
to the monad.
Monads must obey the following rules:
• (return x) >>= f ≡ f x
• m >>= return ≡ m
• (m >>= f) >>= g ≡ m >>= ( x -> (f x >>= g) )

The maybe monad
data Maybe a = Nothing | Just a

Prelude> case (html doc) of
Nothing -> Nothing
Just x -> case body x of
Nothing -> Nothing
Just y -> paragraph 2 y

instance Monad Maybe where
return = Just
Nothing >>= f = Nothing
(Just x) >>= f = f x

Prelude> Just someWebPage >>= html >>= body
>>= paragraph >>= return

Pros

Pure functional language with no side effects
Strong typing with powerful type inference
Laziness reduces need for recursion
Ideal for learning the functional paradigm
Inﬂexibility and lack of compromise
Small community outside academia
Steep learning curve (especially monads!)

Cons

What I learned from
Seven Languages
in Seven Weeks

Functional style
def total_price(widgets)
total = 0

widgets.each do |widget|
if widget.red?
total += widget.price
end
end
total
end

def total_price(widgets)
widgets.select{|w| w.red?}.map{|w| w.price}.reduce(&:+)
end

JVM or standalone?

• Access to Java libraries
• Deployment environments
• Escape route for Java developers
• Limitations (eg tail recursion)

Know what’s out there

• Languages
• Features
• First-class functions
• List comprehensions
• Pattern matching
• Lazy evaluation

Dealing with concurrency

• Immutability
• Coroutines
• Actors
• Futures
• Software Transactional Memory

Credits

http://pragprog.com/book/btlang/seven-languages-in-seven-weeks

http://www.ﬂickr.com/photos/oskay/472097903/

http://www.ﬂickr.com/photos/clairedancer/273214248

7li7w devcon5

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