Disclaimer:

The opinions expressed herein are my own

personal opinions and do not represent my

employer's view in any way.

What is a XXX Language?

XXX = dynamicXXX = imperativeXXX = OO

What is a Functional Language

A language where functions are first-class citizens

EWD 1073

Easy likeSunday

morning

Dan MeyerWTF went

wrong?

Doing IO is a side-effect

Console.WriteLine(“ha”);Console.WriteLine(“ha”);!=Console.WriteLine(“ha”);

var x = Console.ReadLine();var y = x+x;!=var y = Console.ReadLine() + Console.ReadLine()

Doing IO is a side-effect

let date = DateTime.Now()in (date, date)( DateTime.Now(), DateTime.Now())

DateTime.Now is not a mathematical

function

If I cannot even do IO

Fundamentalist

Functional Programmin

gis just

academic nonsense

Don’t eatTheYellowSnow!

Lazy evaluation and side-effects

static bool LessThanThirty(int x){ Console.Write("{0}? Less than 30;",x); return x < 30;}static bool MoretHanTwenty(int x){ Console.Write("{0}? More than 20;",x); return x > 20;}

var q0 = from x in new[]{ 1, 25, 40, 5, 23 } where LessThanThirty(x) select x;

var q1 = from x in q0 where MoreThanTwenty(x) select x;

foreach (var r in q1) Console.WriteLine("[{0}];",r);

Order of side effects?

C#

let lessThanThirty(x:int)= begin Console.WriteLine ("{0} less than 30?", x); x < 30; end let moreThanTwenty(x:int)= begin Console.WriteLine ("{0} more than 20?", x); x > 20; end let q0 = seq { for x in [1; 25; 40; 5; 23] do if lessThanThirty x then yield x } let q1 = seq { for x in q0 do if moreThanTwenty x then yield x } for r in q1 do Console.WriteLine("> {0},", r)

F#Lazy

comprehensions

let lessThanThirty(x:int)= begin Console.WriteLine ("{0} less than 30?", x); x < 30; end let moreThanTwenty(x:int)= begin Console.WriteLine ("{0} more than 20?", x); x > 20; end let q0 = [ for x in [1; 25; 40; 5; 23] do if lessThanThirty x then yield x ] let q1 = [ for x in q0 do if moreThanTwenty x then yield x ] for r in q1 do Console.WriteLine("> {0},", r)

F#Eager

comprehensions

var xs = new[]{ 9, 8, 7, 6, 5, 4, 3, 2, 1, 0 };

IEnumerable<int> q = null;

try{ q = from x in xs select 1/x;}catch{ q = new int[];}

foreach(var z in q) Console.WriteLine(z): // throws here

Exceptions

Func<string> GetContents = null;

using( var file = FileSystem.OpenTextFileReader(@"…")){ GetContents = () => file.ReadToEnd();}

// surprise! an exception Console.WriteLine(GetContents());

Disposing resources

var DelayedActions = new List<Func<int>>();var s = "";

for (var i = 4; i < 7; i++){ DelayedActions.Add(delegate(){ return i; });}

for (var k = 0; k < DelayedActions.Count; k++){ s += DelayedActions[k]();}

Console.WriteLine(s);

Variable Instantiation

Prints 777

var DelayedActions = new List<Func<int>>();var s = "";

for (var i = 4; i < 7; i++){ var j = i; DelayedActions.Add(delegate(){ return j; });}

for (var k = 0; k < DelayedActions.Count; k++){ s += DelayedActions[k]();}

Console.WriteLine(s);

Variable Instantiation

Prints 456

Prints 666

C#

JavaScript

var DelayedActions = new List<Func<int>>();var s = "";

var j;for (var i = 4; i < 7; i++){ j = i; DelayedActions.Add(delegate(){ return j; });}

for (var k = 0; k < DelayedActions.Count; k++){ s += DelayedActions[k]();}

Console.WriteLine(s);

Variable Instantiation

Prints 666

var o1 = new object();//58225482, for instanceConsole.WriteLine(o1.GetHashCode());

var o2 = new object();// 54267293, for instanceConsole.WriteLine(o2.GetHashCode());

Debug.Assert(o1 != 02);

Object creation

new_cell(X) -> spawn(fun() -> cell(X) end).cell(Value) -> receive {set, NewValue} -> cell(NewValue); {get, Pid} -> Pid!{return, Value}, cell(Value); {dispose} -> {} end.

set_cell(Cell, NewValue) -> Cell!{set, NewValue}.

get_cell(Cell) -> Cell!{get, self()}, receive {return, Value} -> Value end.dispose_cell(Cell) -> Cell!{dispose}.

Concurrency

Page 165:

There is a subtle error in on_exit and keep_alive … Write your code such that race conditions cannot happen.…Fortunately, the OTP libraries have code for building servers, supervisor trees, and so on.These libraries have been well tested and should not suffer from any race conditions.

GET …/person/age…GET …/clock/seconds…

few interestingfunctions areidempotent

*not* idempotent*not* pure

static T Identity<T>(this T me){ if(me.GetType() == typeof(Button)) throw new Exception();

var mini_me = me as string; if(mini_me != null) return “hello world”;

Console.WriteLine(me.ToString()); return Activator.CreateInstance<T>();}

“Dishonest” typesNot

parametric!

HOT Languages

To the rescue

lessThanThirty x = do { putStrLn $ (show x)++" less than 30?" ; return $ x < 30 } moreThanTwenty x = do { putStrLn $ (show x)++" more than 20?" ; return $ x > 20 } q0 = [ x | x <- [1, 25, 50, 5, 23 ] , lessThanThirty x ] q1 = [ x | x <- q0 , moreThanTwenty 20 ]

Couldn't match expected type `Bool' against inferred type `IO Bool' In a stmt of a list comprehension: lessThanThirty xIn the expression: [x | x <- [1, 25, 50, ....], lessThanThirty x]In the definition of `q0': q0 = [x | x <- [1, 25, ....], lessThanThirty x

Haskell

unsafePerformIO :: IO a -> a

unsafeCast :: a -> bunsafeCast x = unsafePerformIO $ do{ writeIORef castref x ; readIORef castref }

castref = unsafePerformIO $ do{ newIORef undefined }

Forget about effects? Forget it!

Cover upeffects

Remove all effects

(cannot make languagemore powerfulby removing features)

Don’t fight effects Embrace and extend

Mens sana in corpore sano

Mistake

DateTime.Now() DateTime

Type error!!!!!!!!!!!!!!!

Values vs Computation

How to turn something mutable into something immutable?

Time changes, the clock does not

…9:15 AM9:16 AM9:17 AM9:19 AM9:29 AM9:21 AM9:22 AM9:23 AM9:24 AM9:25 AM9:26 AM9:27 AM9:28 AM9:29 AM9:30 AM

…

It’s just a collection

All problems in computer science can be solved by

another level of indirection

D. Wheeler

Mathematics is always right

Multithreading STM<T>

Destructive updates IO<T>

Reflection Mirror<T>

Exceptions Throws<T>

Only one way to be pure,But many ways to be impure

The “M” word

Monads

M<A>

A computation that produces a value of type A with side-effects described by M

M IO Exception Animation Collection

IO

data IO a

putChar :: Char -> IO ()getChar :: IO Char

newIORef :: a -> IO (IORef a)readIORef :: IORef a -> IO awriteIORef :: IORef a -> a -> IO ()

forkIO :: IO a -> IO ThreadID

Side-effecting

computation that yields a value of type

a

CanNormal

ProgrammersUnderstand

FundamentalistFunctional

Programming?

LINQMonads are the secret sauce behind LINQ

IEnumerable<S> SelectMany<T,S>( IEnumerable<T> src, Func<T, IEnumerable<S>> selector)

Bind

Java

Throws clause

void Foo(string s) throws Exception

ExplicitEffects

FundamentalistFunctional Programming

Abstract from evaluation orderMake all effects explicit==>Compiler can freely rearrange codeRuntime can execute in code any order

Call-by-name, or call-by-value version?

Call To ActionBuy Graham Hutton’s book

Nudge and nag MS to produce a fundamentalist functional language

Buy BryanJohn & Don’sbook

Nudge and nag MS to produce a fundamentalist functional language

Thank You