6. Clojure concepts
Encourages Pure Functional approach
- use STM to change state
Functions as first class citizens
- functions as arguments as they return a value
Make JVM interoperation simple
- easy to use your existing Java applications
17. It s a ll b y t e c o d e in t h e
e nd ..
Any object in clojure is just a regular java object
A reference type inheriting from:
j ava. l ang. obj ec t
20. Name-spaces
; Define a namespace
( ns nam s pac e- nam
e- e)
; Include namespace code
( us e ' nam - s pac e- nam
es e)
Groups code together like a package statement
in Java
21. Defining a function
First principles Syntactic Sugar
( def nam e ( def n nam e
( f n [ ar gs ] [ ar gs ]
( f unc t i on- body ( f unc t i on-
…) )
) body . . . ) )
( def n w c om m s age [ nam ]
el e- es e
( pr i nt l n ( s t r “ W c om t o Cl oj ur e ”
el e
nam )e)
22. Special forms
Special evaluation rules for the Clojure compiler
- part of the language definition
catch, def, do, dot ('.'), finally, fn, if, let, loop,
monitor-enter, monitor-exit, new, quote,
recur, set!, throw, try and var
23. Simple function example
Define a simple function
( def n hel l o- w l d [ nam
or e]
( pr i nt l n( s t r " Hel l o "
nam ) )
e)
Call the function in the REPL or in your code
( hel l o- w l d " j r 0c ket " )
or
25. “Overloading” functions
( def n make
([ ] ; do t h e f o l l o wi n g f u n c t i o n i f
n o ar g s
( s t r uc t vec t or 0 0) )
( [ x y] ; d o i f x an d y ar e
ar g u me n t s
( s t r uc t vec t or x y) )
)
27. List – Ordered collection
( l i s t 1 3 5 7)
' ( 1 3 5 7)
( 1 2 3) ; 1 i s n o t a
f unct i on
28. Vectors – hashed ordered list
[ : m r i x- c har ac t er s [ : neo
at
: m pheus : t r i ni t y : s m t h] ]
or i
( f i r s t [ : n e o : mo r p h e u s : t r i n i t y
: s mi t h ] )
( nt h [ : mat r i x : b ab yl o n 5 : f i r e f l y
: s t ar g at e ] 2 )
( c onc at [ : n e o ] [ : t r i n i t y] )
29. Maps – unordered key/values
{ : a 1 : b 2} { : a { : a 1} }
{ : a 1 , : b 2} {: a {: a 1}}
{ :a 1 :b } { { : a 1} : a}
j ava. l an g . Ar r ayI n d e x Ou t Of Bo { { : a 1 } : a}
u n d s Ex c e p t i o n : 3
; i d i om - put : a on t he
left
{ : a 1 : b 2}
{ : a 1 , : b 2}
30. L is t s a r e f o r
c ode
Ve c t o r s a r e
fo r d a ta
31. Working with data
(last [1 1 2 3 5 8]) (doc last)
(defn penultimate [x]
(last (butlast x)) ) (doc butlast)
(penultimate [1 2 3 4 5])
42. Maven
Just like any other Java project
Step 1)
Add Clojure library jar to your POM
Step 2)
Download the Internet !!!
43. le in in g e n
Leiningen
.o rg
lein new Create a new clojure project
lein deps Download all dependencies
lein repl Start the interactive shell (repl)
lein swank Start repl server
48. A fe w
in t e r e s t in g
C lo ju r e
e x a m p le s
49. str
(str h e l l o) Concatenate strings
together
represent a
character using
(def data “boo”) Join arguments with
(str “message” data ) strings
50. Ratio
Unique data type ( / 3 5)
( / 3. 0
Allow lazy evaluation
5)
Avoid loss of precision
(/ 1 3)
(/ 1.0 3)
(class (/ 1
51. Calling Java... ooooo!!
( j avax . s wi n g . JOp t i o n Pan e /
s h o wMe s s ag e D i al o g n i l
" He l l o W r l d " )
o
52. Importing Java into Clojure
( ns dr aw ng- dem
i o
( : i m t [ j avax. s w ng J panel
por i
J Fr am e]
[ j ava. awt
Di m i on] ) )
ens
53. Working with Java
Java Classes
fullstop after class name
( J Fr am )
e.
(Math/cos 3) ; static method call
Java methods
fullstop before method name
( . get Cont ent Pane f r am ;;method name first
e)
( . f r am get Cont ent Pane) ;;object first
e
54. What class is that...
(class (str "Jr0cket"))
java.lang.String
(class (defn hello-world [name] (str "Hello
cruel world")))
clojure.lang.Var
55. Clojure calling Java web stuff
( l et [ c onn]
( dot o ( Ht t pUr l Connec t i on.
Ur l )
( . s et Reques t M hod
et
“ POST” )
( . s et DoOut put t r ue)
( . s et I ns t aneFol l ow Redi r ec t
s t r ue) ) ] )
56. Recursive functions
Functions that call Tail recursion
themselves Avoids blowing the
stack
Fractal coding
A trick as the JVM does
not support tail
recursion directly :-(
57. Tail recursion
( def n r ec ur s i ve- c ount er
( pr i nt ans w )er
( i f ( < ans w er 1000)
( r ec ur ( + ans wer 4) ) ) )
58. Where to find out more...
c l oj ur e. or g/ c heat s h
eet
63. M o re to
c o m e ...
c l oj ur e. or g
dev. c l oj ur e. or g
@ r 0c ket
j
c l oj ur e. j r 0c ket . c o
Notas del editor
Hickey's primary interest was concurrency — he wanted the ability to write multi-threaded applications, but increasingly found the mutable, stateful paradigm of object oriented programming to be part of the problem The idea of a functional Lisp integrated with a commercially accepted host platform just seemed like chocolate and peanut butter. Coming up with persistent data structures that were fast enough was the tipping point for my considering it viable. functions as first-class objects, meaning that functions can be placed into data structures, passed as arguments to other functions, evaluated in comparisons, even returned as the return value of another function. Moreover, functions do not have &quot;side effects&quot; — the ability to modify program state or data. This paradigm focuses on computation in the mathematical sense, rather than procedural algorithms, and is a completely different approach to programming. Clojure does provide persistent data structures For application developers, the most significant distinction is that Clojure defaults to making all data structures immutable developers must use one of four special mutable structures that are explicitly designed to be shared between threads: refs, vars, atoms, and agents. Clojure uses software transactional memory (STM) to coordinate changing these mutable structures while keeping them in a consistent state, much like a transactional database. This model makes it considerably simpler to write thread-safe code than it is in object oriented languages. No locks are required, therefore there are no deadlocks or race conditions.
Clojure has a programmatic macro system which allows the compiler to be extended by user code You can add your own language features with macros. Clojure itself is built out of macros such as defstruct: (defstruct person :first-name :last-name) If you need different semantics, write your own macro. If you want a variant of structs with strong typing and configurable null-checking for all fields, you can create your own defrecord macro, to be used like this: (defrecord person [String :first-name String :last-name] :allow-nulls false) This ability to reprogram the language from within the language is the unique advantage of Lisp. You will see facets of this idea described in various ways: Lisp is homoiconic - Lisp code is just Lisp data. This makes it easy for programs to write other programs. The whole language is there, all the time. Paul Graham’s essay “Revenge of the Nerds” explains why this is so powerful. http://www.paulgraham.com/icad.html Lisp syntax also eliminates rules for operator precedence and associativity, with fully parenthesized expressions, there is no possible ambiguity
The downside of Lisp’s simple, regular syntax, at least for beginners, is Lisp’s fixation on parentheses and on lists as the core data type. Clojure offers an interesting combination of features that makes Lisp more approachable for non-Lispers.
The downside of Lisp’s simple, regular syntax, at least for beginners, is Lisp’s fixation on parentheses and on lists as the core data type. Clojure offers an interesting combination of features that makes Lisp more approachable for non-Lispers.
The downside of Lisp’s simple, regular syntax, at least for beginners, is Lisp’s fixation on parentheses and on lists as the core data type. Clojure offers an interesting combination of features that makes Lisp more approachable for non-Lispers.
The downside of Lisp’s simple, regular syntax, at least for beginners, is Lisp’s fixation on parentheses and on lists as the core data type. Clojure offers an interesting combination of features that makes Lisp more approachable for non-Lispers.
The downside of Lisp’s simple, regular syntax, at least for beginners, is Lisp’s fixation on parentheses and on lists as the core data type. Clojure offers an interesting combination of features that makes Lisp more approachable for non-Lispers.
When you require a library named clojure.contrib.str-utils, Clojure looks for a file named clojure/contrib/str-utils.clj on the CLASSPATH To avoid having to use the namespace for your library, you have to use refer, like so - (refer 'examples/introduction) The use function does both require refer, like so – (use 'examples.introduction) o force a library to reload: (use :reload-all 'examples.introduction) The :reload-all flag is useful if you are making changes and want to see results without restarting the REPL.
(defn …) is syntactic sugar for (def name (function [] … ) Most people use the defn shortcut as its much more readable.
This is barfing because the evaluator has to keep around state for each call due to the expression (* x (factorial (- x 1))) . We need to make this function tail recursive. recur can be thought of as the Clojure operator for looping. Think of it like a function call for the nearest enclosing let or function definition supplied with new variables. Naively we can switch over to using this by doing: user> (defn factorial2 [x] (if (= x 0) 1 (* x (recur (- x 1))))) But this is a compile-time error (which in itself is pretty neat!). java.lang.UnsupportedOperationException: Can only recur from tail position (NO_SOURCE_FILE:4) An accumulator parameter is an extra parameter to a function that's used to gather intermediate parts of the calculation. If we do this, we can make sure that the recur call is in the tail position. Using an anonymous function we get: (defn factorial3 [x] ((fn [x y] (if (= x 0) y (recur (- x 1) (* x y)))) x 1)) Now when recur is used, it doesn't need to keep any of the previous stack frame around. This means we can finally calculate factorial 1000000, which begins with 282 and ends with lots of zeros!
Hiccup library for representing HTML in Clojure. It uses vectors to represent tags, and maps to represent a tag's attributes.