affordances in programming languages

3/20/2014 Affordances

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Affordances inProgramming Languages

Randy CoulmanPrincipal Software Engineer

http://randycoulman.com

@randycoulman

randycoulman



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

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



Affordance

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



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

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



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

http://tcsidewalks.blogspot.com/2009/06/signs-of-times-14.html



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

http://www.yoniishappy.com/Tube-escalators



Affordances and Software



Amir Rajan (@amirrajan)

Coding Out Loud REST APIs series

http://vimeo.com/channels/659338




ExamplePoints

Smalltalk > Ruby



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

Point.new(3, 4)



http://images.cryhavok.org/d/255622/Polar+and+Cartesian+Bears.jpg

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



θ

r sinθ

r cosθ

r

x

y

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

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



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

Point.new(3, 4)



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



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

Point r: 5 theta: 0.927295



Affordance

Named Constructors



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)



ExampleFind/DetectSmalltalk > Ruby



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

"Unhandled exception: Element Not Found"



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

# => nil



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

"#none"



Affordance

Multiple Blocks



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



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




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?




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."




ExampleCleaning Up After Yourself

C++ > Ruby



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;



#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);}



$ ./brokenAcquiring resourceCaught exception: oops!




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; }




$ ./wordyAcquiring resourceReleasing resourceCaught exception: oops!




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; }




RAIIResource Acquisition Is Initialization

Acquire resources in the constructor

Release them in the destructor



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

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

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

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



#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;}



$ ./raiiAcquiring resourceReleasing resourceCaught exception: oops!



Affordance

Deterministic Destructors



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



Blocks!



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



require_relative "support"require_relative "safe_resource"

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

return unless baz(resource) == 42

puts "Completed successfully!" endend



$ ruby driver.rbAcquiring resourceReleasing resourceCaught exception: oops!



ExampleImageReaderSmalltalk > Ruby



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



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



Affordance

Subclass Iteration



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



class ImageReader #...

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

#...end



class ImageReader #...

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

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

#...end



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

def read_image puts "Reading BMP" endend



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

def read_image puts "Reading JPG" endend



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



require_relative "image_readers"

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



$ ruby subclasses.rbReading BMPReading JPGReading PNG



Takeaways



Languages afford certain designsand inhibit others



Languages influence thought



Learning new languages willincrease your "solution space"



Don't go too far



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

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

Rogue.rb (@roguerb)

Jen Myers (@antiheroine)

http://www.key.net/



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

http://www.amazon.com/Design-Everyday-Things-Donald-Norman/dp/0465067107







Questions?



Randy Coulmanhttp://speakerrate.com/randycoulman

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

@randycoulman

randycoulman

http://speakerrate.com/randycoulman

http://www.slideshare.net/randycoulman