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Too hard today…
(𝑓 ◦ 𝑔)(𝑥) = 𝑓(𝑔(𝑥))
Rx is a library for composing
asynchronous and event-based programs
using observable collections.
Queries? LINQ?
MSDN DevLabs
• .NET 2.0, .NET 4.0, Silverlight
• JavaScript, more…?
interface IEnumerable out
IEnumerator
Get
C# 4.0 covariance
interface IEnumerator out
bool
Move
get
void
IDisposable
You could get
stuck
(Waiting to move next)
C# 4.0 covariance
moving on
You could get
stuck
Because the Dutch are (said to be) cheap
¬ 𝐴 ∧ 𝐵 ≡ ¬𝐴 ∨ ¬𝐵
¬ 𝐴 ∨ 𝐵 ≡ ¬𝐴 ∧ ¬𝐵
The recipe to dualization
http://en.wikipedia.org/wiki/Dual_(category_theory)
Reversing arrows…
Input becomes output and vice versa
Making a U-turn
in synchrony
interface IEnumerable
IEnumerator
void
interface IEnumerator
bool
void
IDisposable
void throws Exception
get
void
C# didn’t borrow Java
checked exceptions
interface IEnumerable<T>
GetEnumerator(
(IEnumerator<T>
&
) void);
We really got an enumerator
and a disposable
interface IEnumerator
bool
IDisposable
void throws Exception
void
interface IEnumerableDual<T>
IEnumerable<T>
IDisposable
Set
IEnumerator<T>) GetEnumerator(void
interface IEnumerator
bool
Will only dualize the
synchrony aspect
void throws Exception
void
interface IEnumerableDual
IDisposable
IEnumerator<T> x
interface IEnumerator
bool
This is an
output too
void throws Exception
void
interface IEnumerableDual
IDisposable
IEnumerator<T> x
interface IEnumerator
bool
Exception
void
void
Discrete domain
with true and false
interface IEnumerableDual
IDisposable
IEnumerator<T> x
interface IEnumerator
T
true
false
Exception
void
If you got true, you
really got a T
void
interface IEnumerableDual
IDisposable
IEnumerator<T> x
interface IEnumerator
void
false
Exception
If you got false,
you really got void
void
interface IEnumerableDual
IDisposable
IEnumeratorDual>);
IEnumerator<T>
x);
IEnumerator<T>
interface IEnumeratorDual<T>
Got
(T | void | Exception) MoveNext(void
But C# doesn’t have discriminated unions…
Let’s splat this into three methods instead!
interface IEnumerableDual
IDisposable
IEnumeratorDual<T>
interface IEnumeratorDual
void
void
void
Exception
void
interface IObservable
IDisposable
IObserver<T> observer
interface IObserver
Exception
Source: http://amazon.com
void
void
void
interface IObservable out
IDisposable
Do you really know C# 4.0?
IObserver<T> observer
Used to detach the observer…
interface IObserver in
void
void
void
Exception
interface IObservable out
IDisposable
IObserver<T> observer
interface IObserver in
void
void
void
Exception
(*) Visit http://en.wikipedia.org/wiki/Color_of_the_bikeshed
interface IObservable out
IDisposable Subscribe IObserver
C# 4.0 contravariance
interface IObserver in
void
void
void
On
On
On
Exception
You could get
flooded
MoveNext
IEnumerable<T>
IEnumerator<T>
Environment
Have next!
IObservable<T>
IObserver<T>
Reactive
Got next?
OnNext
Interactive
Application
OnCompleted
.Empty<int>()
new int[0]
OnNext
new[] { 42 }
.Return(42)
OnError
.Throw<int>(ex)
Throwing iterator
.Never<int>()
Notion of time
Iterator that got stuck
OnNext
OnError
OnNext(0)
OnCompleted
OnNext(1)
OnNext(2)
yield 1
yield 2
.Range(0, 3)
yield 0
.Range(0, 3)
A variant with time notion
exists (GenerateWithTime)
Observable
Hypothetical anonymous
iterator syntax in C#
new IEnumerable int
for int
yield return
Asynchronous
Console
Synchronous
foreach var
Console
in
IObservable int
Observable
int
// Assume we introduce concurrency (see later)…
IDisposable
Console
Console
Console
C# 4.0 named
parameter syntax
"Next: "
"Oops: "
"Done"
C# doesn’t have anonymous interface
implementation, so we provide various
extension methods that take lambdas.
IObservable int
Observable
int
// Assume we introduce concurrency (see later)…
IDisposable
Console
Console
Console
"Next: "
"Oops: "
"Done"
Thread.Sleep(30000); // Main thread is blocked…
F10
IObservable int
Observable
int
// Assume we introduce concurrency (see later)…
IDisposable
Console
Console
Console
"Next: "
"Oops: "
"Done"
Thread.Sleep(30000); // Main thread is blocked…
F10
IObservable int
Observable
int
// Assume we introduce concurrency (see later)…
IDisposable
Console
Console
Console
"Next: "
"Oops: "
"Done"
Thread.Sleep(30000); // Main thread is blocked…
F5
IObservable int
Observable
int
// Assume we introduce concurrency (see later)…
Breakpoint
got hit
IDisposable
Console
Console
Console
"Next: "
"Oops: "
"Done"
Thread.Sleep(30000); // Main thread is blocked…
.MouseMove
args
if (
==
)
// I’d like to raise another event
Lack of composition
-= /* what goes here? */
Resource maintenance?
IObservable<Point>
Observable.FromEvent
var
.Where(
"MouseMove"
Can define operators
var
.Dispose()
Resource maintenance!
)
Exceptions?
FileStream
byte
File
new byte
"data.txt"
new AsyncCallback
int
Cancel?
// Do something with bs[0..bytesRead-1]
Lack of composition
null
State?
Synchronous
completion?
FileStream
Func
File
"data.txt"
IObservable<
>
Observable.FromAsyncPattern
byte
new byte
// Do something with bs[0..bytesRead-1]
Cold
.Return
.Subscribe(
.Subscribe(
Hot
)
)
.FromEvent
"
"
IScheduler
.Return
Scheduler.ThreadPool
duality
.ToObservable();
.ToEnumerable()
reactive
.Amb(
)
.Return
Scheduler.ThreadPool
"
"
.ObserveOn(frm)
"
"
push based
notion of time
// Producing an IObservable<Point> using Select
select
.Location
// Filtering for the first bisector using Where
where
Asynchronous
request
React
TextChanged
Reaction
Reactive
Reactor
Data binding
on UI thread
// IObservable<string> from TextChanged events
var
Observable
EventArgs
var
from
in
select
TextBox
"TextChanged"
TimeSpan
// Bridge with the dictionary web service
var
new DictServiceSoapClient
var
Observable
string
DictionaryWord
// Compose both sources using SelectMany
var
from
in
input.SelectMany(term =>
from
in
lookup(term))
select
Reactive|
Re|
React|
Reac|
Rea|
|Reacti|
Reactiv|
R|
Reactive
Reaction
Reactive
Reactor
input
Service call 1
Service call 2
UI data binding
Source: http://scrapetv.com
Reactive|
Re|
React|
Reac|
Rea|
|Reacti|
Reactiv|
R|
input
Until
Reactive
Service call 1
Take
Service call 2
UI data binding
// IObservable<string> from TextChanged events
var
Observable
EventArgs
var
from
in
select
TextBox
"TextChanged"
TimeSpan
// Bridge with the dictionary web service
var
new DictServiceSoapClient
var
Observable
string
DictionaryWord
Very local fix 
Compose bothapproach
sources for
using
SelectManyusing:
// Alternative
composition
//
Switch<T>(IObservable<IObservable<T>>
sources)
var IObservable<T>
from
in
var res = (from
from term in input
.TakeUntil(input)
select lookup(term))
Hops from source to source
select
.Switch();
"#input"
"keyUp"
function
return
function
return
function
"#results"
function
"#results"
"Error: "
How?
(Expression trees)
IQueryable<T>
IQbservable<T>
E.g. LINQ to PowerShell
E.g. LINQ to SQL
ToQueryable
AsQueryable
AsObservable
AsEnumerable
Homo-iconic
LINQ to *.*
AsQbservable
Translatable
ToQbservable
(MSIL)
IEnumerable<T>
IObservable<T>
E.g. LINQ to Objects
E.g. LINQ to Events
ToEnumerable
Fixed
ToObservable
Pull
(interactive)
Threads
Worker pools
Duality
Push
What?
(reactive)
Message loops
Distributed
Way simpler with Rx
(𝑓 ◦ 𝑔)(𝑥) = 𝑓(𝑔(𝑥))
Rx is a library for composing
asynchronous and event-based programs
using observable collections.
Queries! LINQ!
MSDN DevLabs
• .NET 2.0, .NET 4.0, Silverlight
• JavaScript, more…?
http://blogs.msdn.com/rxteam
http://msdn.microsoft.com/en-us/devlabs/ee794896.aspx
http://social.msdn.microsoft.com/Forums/en-US/rx
http://channel9.msdn.com/Tags/Rx