Concepts of Functional Programming for Java Brains (2010)

Concepts of
Functional Programming
“JavaAbend”, October 2010

Peter Kofler, ‘Code Cop’
@codecopkofler
www.code-cop.org

Copyright Peter Kofler, licensed under CC-BY.

PETER KOFLER, CODE-COP.ORG FANATIC ABOUT CODE QUALITY

... for Java Brain(dead)s

UML
OOA
OOP
SOAP


Peter Kofler
• dev since 11 years

• “fanatic about
code quality”

• “Code Cop”


History
• 193x Lambda
• 1958 LISP
• 197x Scheme
• 198x Haskell
• 199x Common
Lisp
• 200x Clojure


evaluation of
mathematical
functions

<code>

def f(x) x+2 end
def f(x:Int) = x+2
public int f(int x) {
return x+2;
}
</code>


Definition f(x)
x. x + 2
application f(3)
(x. x + 2) 3


no side effects
pure


final
is the new
private
(Joshua Bloch, Effective Java)


avoids
state and
mutable data

<ruby>

array = [3, 5, 7, 9]
array.find do |v|
v*v > 30
end
</ruby>

<javascript>

function derivative(f, dx) {
return function(x) {
return (f(x+dx)-f(x))/dx;
};
}
</javascript>


first-class &
anonymous
functions

<scala>

f(x,y) := x+y 
def f(x:Int,y:Int) = x+y

f1(x) returns f2(y) 
def f1(x:Int) = (i:Int) => x+i
def f2(y) = xnow constant+y
</scala>


“normal”
x y. x + y
curried
x. y. x + y


Evaluation

strict
lazy

<code>

length([2+1,1/0])

 Error
 “2”
</code>


everything is
a function

<scala>

def mywhile(cond: =>Boolean)
(body: =>Unit) {
if (cond) {
body; mywhile(cond)(body)
}
}
... mywhile( i > 0 ) { i -= 1 }
</scala>

The almighty

List
by Jonas Bonér


Iterating
(foreach/each)


Folding (foldLeft/inject)
Reducing (reduceLeft)

<scala>

def factorial(n:Int) =
((1 to n) : 1) ( _*_ )

</scala>


Mapping (map/collect)
Binding (flatMap)

<ruby>

a = [ "a", "b", "c"]
a.collect { |x| x+"!" }

=> # ["a!", "b!", "c!"]
</ruby>

<scala>

def list(folder:File) = {
folder.listFiles.
filter(_.isDirectory).
flatMap(list (_)) ++
folder.listFiles.
filter(_.isFile)
}
</scala>

<scheme>

(* (+ 1 2) 3)
=
(+ 1 2)
with continuation
(* [] 3)
</scheme>

<ruby>

def loop(interrupt)
for i in 1..10
puts "Value of i: #{i}”
if i == interrupt
callcc {|c| return c}
end
end
end
</ruby by Bruce Tate>

<ruby>

irb(main):007> cont = loop 5
Value of i: 1
...
Value of i: 5
=> #<Continuation:0x2b5a358>
irb(main):008> cont.call
Value of i: 6
...
Value of i: 10
</ruby by Bruce Tate>

Skipped...
• Monads
• Category Theory
• Pattern Matching
• Algebraic Data Types
• Type Inference
• Hindley Miller
• ...


Thank
You


Acknowledgements
• This presentation was supported by
System One (as Research Day)
(http://www.systemone.net/en/) ... 20%

• This presentation was supported by sIT
Solutions (for Developer Round Table)
(http://www.s-itsolutions.at/) ... 50%


Peter Kofler

@codecopkofler

www.code-cop.org


Links
• http://alan.dipert.org/post/307586762/polyglot-folding-ruby-clojure-scala
• http://blog.tmorris.net/what-does-functional-programming-mean/
• http://blogs.tedneward.com/2010/03/23/How+To+And+Not+To+Give+A+Talk+On+F.aspx
• http://deadprogrammersociety.blogspot.com/2007/02/ruby-blocks-closures-and-continuations.html

• http://en.wikipedia.org/wiki/Functional_programming
• http://stackoverflow.com/questions/1112773/what-are-the-core-concepts-in-functional-programming

• http://www.defmacro.org/ramblings/fp.html
• http://www.ibm.com/developerworks/java/library/j-cb03216/ (Continuations)
• http://www.infoq.com/presentations/Functional-Languages-101


CC Images #1
• zombie: http://www.flickr.com/photos/vincetemplement/3333125657/
• spray face: http://www.flickr.com/photos/iangallagher/4115047191/
• other worlds: http://www.flickr.com/photos/tohoscope/43818252/
• new ideas: http://www.flickr.com/photos/jpovey/2051196149/
• history: http://www.flickr.com/photos/shadowgate/2679760160/
• functions: http://www.flickr.com/photos/stefz/2159280574/
• lambda: http://www.flickr.com/photos/itsgreg/419031515/
• effects: http://www.flickr.com/photos/delgrossodotcom/3094902951/
• pure: http://www.flickr.com/photos/10451396@N00/429388973/
• dices: http://www.flickr.com/photos/dicemanic/19743895/
• lock: http://www.flickr.com/photos/bpc009/3328427457/
• states: http://www.flickr.com/photos/krazydad/2986774792/
• recursion: http://www.flickr.com/photos/torley/2361164281/


CC Images #2
• higher: http://www.flickr.com/photos/brent_nashville/2204427649/
• unknown: http://www.flickr.com/photos/laughingsquid/2443128847/
• closed: http://www.flickr.com/photos/functoruser/244208110/
• currying: http://www.flickr.com/photos/dumbeast/315869395/
• strict: http://www.flickr.com/photos/williac/99551756/
• lazy: http://www.flickr.com/photos/ucumari/2813269535/
• möbius: http://www.flickr.com/photos/serafa/2590342868/
• uhh: http://www.flickr.com/photos/jswaby/1178547691/
• seaside: http://www.flickr.com/photos/rgtmum/2280206308/
• skyscraper: http://www.flickr.com/photos/sanbeiji/81110217/
• monads: http://www.flickr.com/photos/adamrice/3266888223/
• questions: http://www.flickr.com/photos/seandreilinger/2326448445/

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