Disclaimer The opinions expressed herein are my own

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 = dynamic XXX = imperative XXX = OO

What is a Functional Language A language where functions are first-class citizens

EWD 1073

Easy like Sunday morning

Dan Meyer WTF went wrong?

Doing IO is a side-effect Console. Write. Line(“ha”); != Console. Write. Line(“ha”); var x = Console. Read. Line(); var y = x+x; != var y = Console. Read. Line() + Console. Read. Line()

Doing IO is a side-effect let date = Date. Time. Now() in (date, date) ( Date. Time. Now() , Date. Time. Now() ) Date. Time. Now is not a mathematical function

If I cannot even do IO Fundamentalist Functional Programming is just academic nonsense

Don’t eat The Yellow Snow!

C# Lazy evaluation and side-effects static bool Less. Than. Thirty(int x) { Console. Write("{0}? Less than 30; ", x); return x < 30; } static bool Moret. Han. Twenty(int x) { Console. Write("{0}? More than 20; ", x); return x > 20; } var q 0 = from x in new[]{ 1, 25, 40, 5, 23 } where Less. Than. Thirty(x) select x; var q 1 = from x in q 0 where More. Than. Twenty(x) select x; Order of side effects? foreach (var r in q 1) Console. Write. Line("[{0}]; ", r);

let less. Than. Thirty(x: int)= begin Console. Write. Line ("{0} less than 30? ", x); x < 30; end let more. Than. Twenty(x: int)= begin Console. Write. Line ("{0} more than 20? ", x); x > 20; end let q 0 = seq { for x in [1; 25; 40; 5; 23] do if less. Than. Thirty x then yield x } let q 1 = seq { for x in q 0 do if more. Than. Twenty x then yield x } for r in q 1 do Console. Write. Line("> {0}, ", r) F# Lazy comprehensions

let less. Than. Thirty(x: int)= begin Console. Write. Line ("{0} less than 30? ", x); x < 30; end let more. Than. Twenty(x: int)= begin Console. Write. Line ("{0} more than 20? ", x); x > 20; end let q 0 = [ for x in [1; 25; 40; 5; 23] do if less. Than. Thirty x then yield x ] let q 1 = [ for x in q 0 do if more. Than. Twenty x then yield x ] F# Eager comprehensions for r in q 1 do Console. Write. Line("> {0}, ", r)

Exceptions var xs = new[]{ 9, 8, 7, 6, 5, 4, 3, 2, 1, 0 }; IEnumerable q = null; try { q = from x in xs select 1/x; } catch { q = new int[]; } foreach(var z in q) Console. Write. Line(z): // throws here

Disposing resources Func Get. Contents = null; using( var file = File. System. Open. Text. File. Reader(@"…")) { Get. Contents = () => file. Read. To. End(); } // surprise! an exception Console. Write. Line(Get. Contents());

Variable Instantiation var Delayed. Actions = new List>(); var s = ""; for (var i = 4; i < 7; i++) { Delayed. Actions. Add(delegate(){ return i; }); } for (var k = 0; k < Delayed. Actions. Count; k++) { s += Delayed. Actions[k](); } Console. Write. Line(s); Prints 777

Variable Instantiation var Delayed. Actions = new List>(); var s = ""; for (var i = 4; i < 7; i++) { var j = i; Delayed. Actions. Add(delegate(){ return j; }); } for (var k = 0; k < Delayed. Actions. Count; k++) { s += Delayed. Actions[k](); Prints 456 C# } Console. Write. Line(s); Prints 666 Java. Script

Variable Instantiation var Delayed. Actions = new List>(); var s = ""; var j; for (var i = 4; i < 7; i++) { j = i; Delayed. Actions. Add(delegate(){ return j; }); } for (var k = 0; k < Delayed. Actions. Count; k++) { s += Delayed. Actions[k](); } Prints 666 Console. Write. Line(s);

Object creation var o 1 = new object(); //58225482, for instance Console. Write. Line(o 1. Get. Hash. Code()); var o 2 = new object(); // 54267293, for instance Console. Write. Line(o 2. Get. Hash. Code()); Debug. Assert(o 1 != 02);

Concurrency new_cell(X) -> spawn(fun() -> cell(X) end). cell(Value) -> receive {set, New. Value} -> cell(New. Value); {get, Pid} -> Pid!{return, Value}, cell(Value); {dispose} -> {} end. set_cell(Cell, New. Value) -> Cell!{set, New. Value}. get_cell(Cell) -> Cell!{get, self()}, receive {return, Value} -> Value end. dispose_cell(Cell) -> Cell!{dispose}.

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.

*not* idempotent *not* pure GET …/person/age… GET …/clock/seconds… few interesting functions are idempotent

“Dishonest” types Not parametric! static T Identity(this T me) { if(me. Get. Type() == typeof(Button)) throw new Exception(); var mini_me = me as string; if(mini_me != null) return “hello world”; Console. Write. Line(me. To. String()); return Activator. Create. Instance(); }

HOT Languages To the rescue

less. Than. Thirty x = do { put. Str. Ln $ (show x)++" less than 30? " ; return $ x < 30 } Couldn't match expected type `Bool' more. Than. Twenty x = do against inferred type `IO Bool' { put. Str. Ln $ (show a stmt of a list comprehension: In x)++" more than 20? " ; return $ x > 20 less. Than. Thirty x } In the expression: [x | x <- [1, 25, 50, . . ], less. Than. Thirty x] In the definition of `q 0': q 0 = [ x q 0 = 23 | x <- [1, 25, 50, 5, [x | ]x <- [1, 25, . . ], less. Than. Thirty x ] q 1 = [ x | x <- q 0 , more. Than. Twenty 20 ] Haskell

Forget about effects? Forget it! unsafe. Perform. IO : : IO a -> a unsafe. Cast : : a -> b unsafe. Cast x = unsafe. Perform. IO do{ write. IORef castref x ; read. IORef castref } castref = unsafe. Perform. IO $ do{ new. IORef undefined } $

Cover up effects

Remove all effects (cannot make language more powerful by removing features)

Don’t fight effects Embrace and extend

Mens sana in corpore sano

Mistake Date. Time. Now() Date. Time Type error !!!!!!!!

Values vs Computation How to turn something mutable into something immutable? Time changes, the clock does not

It’s just a collection All problems in computer science can be solved by another level of indirection D. Wheeler … 9: 15 9: 16 9: 17 9: 19 9: 21 9: 22 9: 23 9: 24 9: 25 9: 26 9: 27 9: 28 9: 29 9: 30 … AM AM AM AM

Mathematics is always right

Only one way to be pure, But many ways to be impure Multithreading STM Exceptions Throws Destructive updates IO Reflection Mirror

The “M” word Monads M M IO Exception Animation Collection A computation that produces a value of type A with side-effects described by M