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Affordances inProgramming Languages

Randy CoulmanPrincipal Software Engineer

http://randycoulman.com

 @randycoulman

 randycoulman

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Satu Kyröläinen ­ http://www.satukyrolainen.com/affordances/

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Satu Kyröläinen ­ http://www.satukyrolainen.com/affordances/

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Affordance

A quality of an object or environment that allows someone toperform an action.

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Nicolas Nova ­ http://www.flickr.com/photos/nnova/2252222949/

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William Lindeke ­ http://tcsidewalks.blogspot.com/2009/06/signs­of­times­14.html

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Yoni Alter ­ http://www.yoniishappy.com/Tube­escalators

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Affordances and Software

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Amir Rajan (@amirrajan)

Coding Out Loud ­ REST APIs series

http://vimeo.com/channels/659338

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ExamplePoints

Smalltalk ­> Ruby

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class Point def initialize(x, y) @x = x @y = y endend

Point.new(3, 4)

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http://images.cryhavok.org/d/25562­2/Polar+and+Cartesian+Bears.jpg

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θ

r sinθ

r cosθ

r

x

y

http://upload.wikimedia.org/wikipedia/commons/7/78/Polar_to_cartesian.svg

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class Point def initialize(x, y) @x = x @y = y endend

Point.new(3, 4)

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Point class>>x: anX y: aY ̂self new initializeX: anX y: aY

Point>>initializeX: anX y: aY x := anX. y := aY

Point x: 3 y: 4

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Point class>> r: radius theta: angleInRadians ̂self x: radius * angleInRadians cos y: radius * angleInRadians sin

Point r: 5 theta: 0.927295

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Affordance

Named Constructors

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class Point def self.xy(x, y) new(x, y) end

def self.polar(r, theta) xy(r * Math.cos(theta), r * Math.sin(theta)) end

private_class_method :new

# ... rest same as beforeend

Point.xy(3, 4)Point.polar(5, 0.927295)

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ExampleFind/DetectSmalltalk ­> Ruby

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#(2 4 6 8) detect: [:each | each odd]

"Unhandled exception: Element Not Found"

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[2, 4, 6, 8].find { |n| n.odd? }

# => nil

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#(2 4 6 8) detect: [:each | each odd] ifNone: [#none]

"#none"

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Affordance

Multiple Blocks

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[2, 4, 6, 8].find(-> {:none}) { |n| n.odd?} # => :none

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Linguistic Relativitya.k.a. The Sapir­Whorf Hypothesis

"[T]he structure of a language affects the ways in which itsrespective speakers conceptualize their world ... or otherwiseinfluences their cognitive processes."

­­ http://en.wikipedia.org/wiki/Linguistic_relativity

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When Code CriesCory Foy at SCNA 2012

http://vimeo.com/53986875

What does a language allow you to say?

What does a language force you to say?

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The Power and Philsophy of RubyMatz at OSCON 2003

http://www.rubyist.net/~matz/slides/oscon2003/mgp00001.html

"Languages are not only tools to communicate, but also toolsto think."

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ExampleCleaning Up After Yourself

C++ ­> Ruby

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if ((err = SSLFreeBuffer(&hashCtx)) != 0) goto fail;if ((err = ReadyHash(&SSLHashSHA1, &hashCtx)) != 0) goto fail;if ((err = SSLHashSHA1.update(&hashCtx, &clientRandom)) != goto fail;if ((err = SSLHashSHA1.update(&hashCtx, &serverRandom)) != goto fail;if ((err = SSLHashSHA1.update(&hashCtx, &signedParams)) != goto fail; goto fail;if ((err = SSLHashSHA1.final(&hashCtx, &hashOut)) != 0) goto fail;

//...

fail: SSLFreeBuffer(&signedHashes); SSLFreeBuffer(&hashCtx); return err;

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#include "support.h"#include <iostream>

void foo(){ Resource* resource = acquireResource(); bar(resource);

if (baz(resource) != 42) return;

std::cout << "Completed successfully!" << std::endl;

releaseResource(resource);}

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$ ./brokenAcquiring resourceCaught exception: oops!

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#include "support.h"#include <iostream>

void foo(){ Resource* resource = acquireResource();

try { bar(resource);

if (baz(resource) == 42) { std::cout << "Completed successfully!" << std::endl; } } catch(std::exception& e) { releaseResource(resource); throw; }

releaseResource(resource);}

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$ ./wordyAcquiring resourceReleasing resourceCaught exception: oops!

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#include "support.h"#include <iostream>

void foo(){ Resource* resource = acquireResource();

try { bar(resource);

if (baz(resource) == 42) { std::cout << "Completed successfully!" << std::endl; } } catch(std::exception& e) { releaseResource(resource); throw; }

releaseResource(resource);}

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RAIIResource Acquisition Is Initialization

Acquire resources in the constructor

Release them in the destructor

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#include "SafeResource.h"#include "support.h"

SafeResource::SafeResource() : resource(acquireResource()){}

SafeResource::~SafeResource(){ releaseResource(resource);}

Resource* SafeResource::get(){ return resource;}

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#include "SafeResource.h"#include "support.h"#include <iostream>

void foo(){ SafeResource resource; bar(resource.get());

if (baz(resource.get()) != 42) return

std::cout << "Completed successfully!" << std::endl;}

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$ ./raiiAcquiring resourceReleasing resourceCaught exception: oops!

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Affordance

Deterministic Destructors

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Ruby doesn't have deterministic destructors.So what can we do instead?

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Blocks!

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class SafeResource def self.acquire resource = self.new yield resource ensure resource.release end

def initialize puts "Acquiring resource" @resource = Object.new end

def release puts "Releasing resource" @resource = nil end

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require_relative "support"require_relative "safe_resource"

def foo SafeResource.acquire do |resource| bar(resource)

return unless baz(resource) == 42

puts "Completed successfully!" endend

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$ ruby driver.rbAcquiring resourceReleasing resourceCaught exception: oops!

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ExampleImageReaderSmalltalk ­> Ruby

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ImageReader class>>fromFile: aFilename | readerClass imageStream reader |

imageStream := aFilename readStream binary. [readerClass := self readerClassFor: imageStream. readerClass ifNil: [̂self error: 'Unknown image type: ', aFilename asString]. reader := readerClass on: imageStream] ensure: [imageStream close]. ̂reader

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ImageReader class>>readerClassFor: imageStream ̂self subclasses detect: [:eachClass | imageStream reset. [eachClass canRead: imageStream] ensure: [imageStream reset]] ifNone: [nil]

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Affordance

Subclass Iteration

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class ImageReader

def self.read(filename) File.open(filename, "rb") do |io| reader_class = find_reader_class(io) raise "Unknown image type: #{filename}" unless reader_class reader_class.new(io) end end

#...end

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class ImageReader #...

def self.find_reader_class(io) subclasses.find { |reader| begin io.rewind reader.can_read?(io) ensure io.rewind end } end

#...end

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class ImageReader #...

def self.inherited(subclass) subclasses << subclass end

def self.subclasses @subclasses ||= [] end

#...end

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class BMPImageReader < ImageReader def self.can_read?(io) io.read(2) == "BM" end

def read_image puts "Reading BMP" endend

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class JPGImageReader < ImageReader def self.can_read?(io) io.read(2) == "\xFF\xD8".b end

def read_image puts "Reading JPG" endend

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class PNGImageReader < ImageReader def self.can_read?(io) io.read(8) == "\x89PNG\r\n\x1A\n".b end

def read_image puts "Reading PNG" endend

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require_relative "image_readers"

%w{bmp jpg png}.each do |ext| ImageReader.read("example.#{ext}")end

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$ ruby subclasses.rbReading BMPReading JPGReading PNG

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Takeaways

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Languages afford certain designsand inhibit others

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Languages influence thought

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Learning new languages willincrease your "solution space"

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Don't go too far

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Want More?http://randycoulman.com/blog/categories/affordances/

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AcknowledgementsKey Technology (http://www.key.net/)

Rogue.rb (@roguerb)

Jen Myers (@antiheroine)

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ReferencesThe Design of Every Day Things ­ Donald Norman

Coding Out Loud ­ Amir Rajan

Linguistic Relativity ­ Wikipedia article

When Code Cries ­ Cory Foy at SCNA 2012

The Power and Philosophy of Ruby ­ Matz at OSCON 2003

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Questions?

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Randy Coulmanhttp://speakerrate.com/randycoulman

http://www.slideshare.net/randycoulmanhttp://randycoulman.com

 @randycoulman

 randycoulman