Slides for a small Clojure presentation given at Montreal.rb on September 21st 2010.

1. Clojure for Rubyists
For Montreal.rb on September 21st 2010
By Jeff Heon

2. Clojure in a nutshell
●Functional
●Concurrency
●JVM
●Lisp

3. Similarities
Interactive development
Ruby Clojure
irb REPL (read eval loop print)
java -cp clojure.jar clojure.main

4. Similarities
Ruby symbols are Clojure keywords.
Ruby Clojure
:symbol :keyword

5. Similarities
In a conditional, nil and false evalute
to false and everything else to true.
Ruby Clojure
if 0 then true end (if 0 true)
=> true => true
if nil then true else false end (if nil true false)
=> false => false

6. Similarities
Last expression in a method is the
return value.
Ruby Clojure
def convert_farenheit_to_celcius(f) (defn convert-farenheit-to-celcius [f]
(f - 32) * 5.0 / 9 (* (- f 32)
end (/ 5.0 0)))

7. Basic types
Ruby Clojure
letter_digit = /^[a-zA-Z][0-9]$/ (def letter-digit #"^[a-zA-Z][0-9]$")
[0, 'Ruby', 42] [0 "Ruby" 42]
my_array[0] (my-vector 0)
{ :r => 'Ruby', :c => 'Clojure' } { :r "Ruby" :c "Clojure" }
#Accessing ;Accessing
my_hash[:r] (my-hash :c)

8. Functional
Ruby Clojure
def odd?(n) (n % 2) == 1 end (defn odd? [n] (= 1 (mod n 2)))
#Sum of odd numbers from 0 to 99 ;Sum of odd numbers from 0 to 99
(0..99). (reduce
select { |n| odd? n }. +
inject(0) { |acc, n| acc + n } (filter odd? (range 100)))
=> 2500 => 2500

9. Functional, but lazy
Ruby Clojure
def odd_trace?(n) (defn odd-trace? [n]
puts n (print n)
(n % 2) == 1 (= 1 (mod n 2)))
end
#Sum of odd numbers from 0 to 9 ;Sum of odd numbers from 0 to 9
(0..99). (reduce +
select { |n| odd_trace? n }. (take 10
first(10). (filter odd-trace? (range 100))))
inject(0) { |acc, n| acc + n }
=> prints 0 to 99, returns 100 => prints 0 to 31, returns 100

10. Immutable data structure
Data structure are always immutable.
Adding or removing an element to an array,
vector, hash or set will always return a new
collection.
But, they are efficient because they are persistant
(in the sense that they share structure.)

11. Concurrency
Software transactional memory
Or having multiple threads access the same
reference using transactions instead of locks.
STMs are not all the same (like GC)
Readers are never blocked.
Works hand in hand with persistant data structure.

12. Ref
1
@A A 2
@A 3

13. Where did my books go?
(def book1
{ :author "David Flanagan" :title "The Ruby Programming language" })
(def book2
{ :author "Gregory Brown" :title "Ruby Best Practices"})
(def home-books (ref #{book1 book2}))
(def lent-books (ref #{}))
(alter home-books disj book1) => No transaction running
(dosync
(alter home-books disj book1)
(alter lent-books conj book1))
@home-books => #{{:author "Gregory Brown", :title "Ruby Best Practices"}}
@lent-books => #{{:author "David Flanagan", :title "The Ruby Programming
language"}}

14. Further exploration
If nothing else: Are we there yet?
Main site: Clojure.org
News & getting started: Disclojure
Books (a comparison)
●Programming Clojure
●Clojure in Action
●The Joy of Clojure
●Practical Clojure
●And one upcoming from O'Reilly