Andy On Closures

Andy Bulka Technical Director Austhink Software www.austhink.com

Overview ,[object Object],[object Object],[object Object],[object Object]

Definition ,[object Object],[object Object],[object Object]

Language support ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]

Example – C# class Program { delegate void Action(); static Action GetAction() { int x = 0; Action a = delegate { Console.WriteLine(x); }; x = 1; return a; } static void Main(string[] args) { Action a = GetAction(); a(); } }

Example – C# class Program { delegate void SomeDelegate(); public static void InvokeMethod () { SomeDelegate del = delegate() { Console.WriteLine("Hello2"); }; del(); } } class Program { delegate void SomeDelegate(); public static void InvokeMethod () { SomeDelegate del = new SomeDelegate( SomeMethod ); del(); } static void SomeMethod() { Console.WriteLine("Hello"); } } static void Main(string[] args) { InvokeMethod (); }

Example – Python ,[object Object],x = 1 def getf(y): return lambda n, m : n + m + x + y f = getf(1) print f(1,2) x += 100 print f(1,2) x = 1 def getf(y): def myf (n, m): return n + m + x + y return myf f = getf(1) print f(1,2) x += 100 print f(1,2)

Uses – Summary ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]

What’s a “block of code” ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]

How are parameters handled? Groovy ,[object Object],[object Object],[object Object],[object Object],[object Object]

How are parameters handled? Python ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]

How are parameters handled? C# ,[object Object],result = gNames.FindAll( delegate(string name) { return name.StartsWith(startingText); } ); Where there delegate is declared public delegate bool Predicate<T>(T obj)

Java – the final var problem e.g. ,[object Object],public void example() { final double use=Math.random()*10000; SwingUtilities.invokeLater( new Runnable() { public void run() { System.out.println(use); } } ); }

Java – the final var problem ,[object Object],[object Object]

Java – final – hack using array ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]

Java inner classes vs closures ,[object Object],[object Object],[object Object],[object Object]

Closures add “state” ,[object Object],[object Object],[object Object],[object Object]

Closure “tricks” ,[object Object],[object Object],[object Object],[object Object],[object Object]

Why not just use objects? ,[object Object],[object Object],[object Object]

Why not just use anonymous inner classes? ,[object Object]

Uses – injecting strategy ,[object Object],a = delegate { Console.WriteLine(x); }; a();

Uses – injecting strategy – Use OO ,[object Object],[object Object]

Uses - iteration - filtering ,[object Object],[object Object],[object Object],offices = [] for e in employees offices << e.office end offices = employees.collect {|e| e.office}

Uses – iteration – filtering – Requires Library support ,[object Object],[object Object],[object Object],managersOffices = employees. select {|e| e.manager?}. map {|m| m.office} managers, plebs = employees. partition {|e| e.manager?} allManagers = employees. all ? {|e| e.manager?} noManagers = ! employees. any ? {|e| e.manager?} sortedEmployees = employees. sort {|a,b| a.lastname <=> b.lastname} sortedEmployees = employees. sort_by {|e| e.lastname}

Uses - iteration – filtering – Use List Comprehensions ,[object Object],[object Object],[object Object],offices = [e.office for e in employees] managersOffices = [e.office for e in employees if e.isManager]

Uses - iteration – filtering – Use Loops or OO ,[object Object],[object Object],[object Object]

Uses - Caching ,[object Object],[object Object]

Uses - Caching ,[object Object],static SomeDelegate Cache( SomeDelegate f) { bool alreadyrun = false ; int result = -1; return delegate { if (!alreadyrun) { result = f(); // run it alreadyrun = true ; } return result; }; }

Uses - Caching ,[object Object], class Program { delegate int SomeDelegate (); static void Main( string [] args) { int rez; SomeDelegate del; del = Cache(Calculate); rez = del(); Console .WriteLine(rez); rez = del(); // this time, will return the cached value. Console .WriteLine(rez); } static int Calculate() // the function we are trying to cache { Console .Write( "... " ); return 5; } ... 5 5 Press any key to continue . . .

Uses – Caching – Use OO ,[object Object],[object Object],[object Object]

Uses - set up, then clean up ,[object Object],[object Object],[object Object],[object Object]

Uses - set up, then clean up ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]

Uses - set up, then clean up – Use OO ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]

Uses – delay evaluation ,[object Object]

Uses – delay evaluation ,[object Object],class View(object): def __init__(self, viewname): self.cmds = [] def SetLocation(self, node , x, y): f = lambda : "" + "view.SetLocation(%s, %d, %d)" % ( node .ToNodeName(), x, y) self.cmds.append(f) def SetOffset(self, node1, node2 , x, y): f = lambda : "" + "view.SetOffset(%s, %s, %d, %d)" % ( node1 .ToNodeName(), node2 .ToNodeName(), x, y) self.cmds.append(f) # store lambdas/closures since don't want to execute each node.ToNodeName() till each rootnode.RecalcIndexes has been called for c in self.cmds: resultstr += c() + CRLF // executes the lambda

Uses – delay evaluation ,[object Object],class Node: def __init__(self, val): self.val = val def get(node): return lambda n, m : str(n) + " " + str(m+1) + " " + str(node.val) node = Node(10) f = get(node) print f(1,2) # 1, 3, 10 node.val = 100 print f(1,2) # 1, 3, 100

Uses – delay evaluation – Use OO ,[object Object],def get2(node): class Late: def __init__(self, node): self.node = node def Execute(self, n, m): return str(n) + " " + str(m+1) + " " + str(self.node.val) return Late(node) node = Node(10) o = get2(node) print o.Execute(1,2) # 1, 3, 10 node.val = 100 print o.Execute(1,2) # 1, 3, 100

Uses – delay evaluation – Use eval ,[object Object],[object Object]

Uses – share private state ,[object Object],[object Object]

Uses – share private state ,[object Object],[object Object],[object Object]

Uses – share private state – Use OO ,[object Object],[object Object],[object Object],[object Object]

Uses – thread safety ,[object Object]

Uses – thread safety ,[object Object],[object Object]

Uses – thread safety ,[object Object],static List<string> GetNamesStartingWith(string startingText) { return g_Names.FindAll( delegate(string name) { return name.StartsWith(startingText); }); } static string g_StartingText; static bool NameStartsWith(string name) { return name.StartsWith(g_StartingText); } static List<string> GetNamesStartingWith(string startingText) { g_StartingText = startingText; return g_Names.FindAll(NameStartsWith); }

Uses – thread safety – use OO ,[object Object],class Searcher { string g_StartingText; // local to each instance  public Searcher(string s) { g_StartingText = s; } public bool NameStartsWith(string name) { return name.StartsWith(g_StartingText); } } static List<string> GetNamesStartingWith(string startingText) { Searcher s = new Searcher(startingText); // use object to hide shared state return g_Names.FindAll(s.NameStartsWith); }

Uses - new control structures ,[object Object],[object Object],[object Object]

Uses - new control structures – Use OO ,[object Object],[object Object]

Uses - function factory ,[object Object],function AdderFactory(y) { return function(x){return x + y;} } var MyFunc; if (whatever) MyFunc = AdderFactory(5); else MyFunc = AdderFactory(10); print(MyFunc(123)); // Either 133 or 128. The anonymous inner function remembers what the value of y was when it was returned, even though y has gone away by the time the inner function is called! We say that the inner function is closed over the containing scope, or for short, that the inner function is a closure.

Uses - function factory – Use OO ,[object Object],def AdderFactory(y): return lambda x : x + y if False: MyFunc = AdderFactory(5); else: MyFunc = AdderFactory(10); print MyFunc(123) class Adder: def __init__(self, y): self.y = y def Add(self, x): return x + self.y if False: o = Adder(5) else: o = Adder(10) print o.Add(123)

Uses – command Pattern ,[object Object],[object Object],[object Object],[object Object]

Uses - command Pattern ,[object Object],[object Object]

Uses - command Pattern – Use OO ,[object Object],[object Object],[object Object],[object Object],[object Object]

Are Closures really that cool? ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]

Summary – OO vs closures ,[object Object],[object Object],[object Object],[object Object],[object Object]

Andy On Closures

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Andy On Closures