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Affordances in
Programming Languages
Randy Coulman
Principal 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 to
perform 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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Example
Points
Smalltalk ­> Ruby

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class Point
def initialize(x, y)
@x = x
@y = y
end
end
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
end
end
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 before
end
Point.xy(3, 4)
Point.polar(5, 0.927295)

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Example
Find/Detect
Smalltalk ­> 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 Relativity
a.k.a. The Sapir­Whorf Hypothesis
"[T]he structure of a language affects the ways in which its
respective speakers conceptualize their world ... or otherwise
influences their cognitive processes."
­­ http://en.wikipedia.org/wiki/Linguistic_relativity

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When Code Cries
Cory 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 Ruby
Matz at OSCON 2003
http://www.rubyist.net/~matz/slides/oscon2003/mgp00001.html
"Languages are not only tools to communicate, but also tools
to think."

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Example
Cleaning 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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$ ./broken
Acquiring resource
Caught 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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$ ./wordy
Acquiring resource
Releasing resource
Caught 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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RAII
Resource 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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$ ./raii
Acquiring resource
Releasing resource
Caught 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!"
end
end

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$ ruby driver.rb
Acquiring resource
Releasing resource
Caught exception: oops!

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Example
ImageReader
Smalltalk ­> 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"
end
end

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class JPGImageReader < ImageReader
def self.can_read?(io)
io.read(2) == "xFFxD8".b
end
def read_image
puts "Reading JPG"
end
end

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class PNGImageReader < ImageReader
def self.can_read?(io)
io.read(8) == "x89PNGrnx1An".b
end
def read_image
puts "Reading PNG"
end
end

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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.rb
Reading BMP
Reading JPG
Reading PNG

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Takeaways

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Languages afford certain designs
and inhibit others

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Languages influence thought

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Learning new languages will
increase 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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Acknowledgements
Key Technology (http://www.key.net/)
Rogue.rb (@roguerb)
Jen Myers (@antiheroine)

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References
The 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 Coulman
http://speakerrate.com/randycoulman
http://www.slideshare.net/randycoulman
http://randycoulman.com
@randycoulman
randycoulman