Andy On Closures

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Andy Bulka Technical Director Austhink Software www.austhink.com

Overview <ul><li>What are they? </li></ul><ul><li>What languages have them? </li></ul><ul><li>What can you do with them? <...

Definition <ul><li>A closure is a block of code that can be passed around </li></ul><ul><li>It continues to have ongoing a...

Language support <ul><li>Closures originated in Lisp. F unctional programming languages use them a lot e.g. Haskell </li><...

Example – C# class Program { delegate void Action(); static Action GetAction() { int x = 0; Action a = delegate { Conso...

Example – C# class Program { delegate void SomeDelegate(); public static void InvokeMethod () { SomeDelegate del = delega...

Example – Python <ul><li>A closure is an anonymous function that ‘closes’ over its surrounding scope. Thus when the func...

Uses – Summary <ul><li>Elegant short code fragments – no wrapping class baggage </li></ul><ul><ul><li>Bits of code passed ...

What’s a “block of code” <ul><li>Code Blocks – pass in a block vs. return a block </li></ul><ul><ul><li>Pass code block as...

How are parameters handled? Groovy <ul><li>In Groovy, closure parameters are listed before the -> token, like so: </li></u...

How are parameters handled? Python <ul><li>In the case of returning a code block for someone else to use, it is returned ...

How are parameters handled? C# <ul><li>DotNet has a List.FindAll() which expects a “predicate/delegate” that takes one par...

Java – the final var problem e.g. <ul><li>Java's anonymous inner classes can access locals - but only if they are final. ...

Java – the final var problem <ul><li>The second restriction to notice is that an inner class can only refer to variables f...

Java – final – hack using array <ul><li>public interface Counter { </li></ul><ul><li>public int current(); </li></ul...

Java inner classes vs closures <ul><li>Java inner classes are a bit verbose though (need an interface name (thus not very...

Closures add “state” <ul><li>The essential difference between an anonymous/inline function and a closure is that the closu...

Closure “tricks” <ul><li>The ability of a closure to have ongoing access to state from its context allows various tricks. ...

Why not just use objects? <ul><li>Why not pass an object or return an object. The receiver code simply calls a method on ...

Why not just use anonymous inner classes? <ul><li>It is certainly true that for any program you can write using closures, ...

Uses – injecting strategy <ul><li>Write to console vs. to a file etc. – choice of strategy </li></ul>a = delegate { Consol...

Uses – injecting strategy – Use OO <ul><li>Setting up a method on an object etc. (strategy pattern). </li></ul><ul><li>Yes...

Uses - iteration - filtering <ul><li>Iterating over lists, and filtering / collecting </li></ul><ul><li>More elegant code ...

Uses – iteration – filtering – Requires Library support <ul><li>All these Ruby closure tricks require deep library support...

Uses - iteration – filtering – Use List Comprehensions <ul><li>Python has alternative facility called “list comprehension...

Uses - iteration – filtering – Use Loops or OO <ul><li>What’s wrong with a for loop? ;-) Easy to understand, though more...

Uses - Caching <ul><li>The solution is to build delayed objects so that the first time they are forced, they store the val...

Uses - Caching <ul><li>C# translation </li></ul>static SomeDelegate Cache( SomeDelegate f) { bool alreadyrun = ...

Uses - Caching <ul><li>C# usage of caching – proof it works </li></ul> class Program { delegate int ...

Uses – Caching – Use OO <ul><li>Use lazy instantiation code (manually write each time). </li></ul><ul><li>Use proxy patter...

Uses - set up, then clean up <ul><li>in encapsulating operations that must be set up, and then cleaned up after. </li></ul...

Uses - set up, then clean up <ul><li>E.g. in Ruby encapsulating operations that must be set up, and then cleaned up after....

Uses - set up, then clean up – Use OO <ul><li>Is the idea then: </li></ul><ul><ul><li>fp = open(“file.txt) // step 1 </l...

Uses – delay evaluation <ul><li>Closures delay evaluation—i.e., they do not "do" anything until they are called ...

Uses – delay evaluation <ul><li>My python situation builds a list of functions, then I execute them much later, when all t...

Uses – delay evaluation <ul><li>Original solution </li></ul>class Node: def __init__(self, val): self.val = val def get(...

Uses – delay evaluation – Use OO <ul><li>Another way to do it. Pure OO. </li></ul>def get2(node): class Late: def __in...

Uses – delay evaluation – Use eval <ul><li>Another way to do it would be to create the code as a string and then later eva...

Uses – share private state <ul><li>Share state in a local scope place. Two or more places that use a closure can store in...

Uses – share private state <ul><li>Closures can be used to create additional scopes that can be used to group interrelated...

Uses – share private state – Use OO <ul><li>Use OO instead ! </li></ul><ul><li>Instances of objects, shared state via pro...

Uses – thread safety <ul><li>Prototype cloning can help with thread safety cos making copies of code not sharing a central...

Uses – thread safety <ul><li>One possibility might be tempting to place a lock on "g_StartingText". This would c...

Uses – thread safety <ul><li>Even with the verbosity of anonymous methods, the code has been greatly reduced. And, assumin...

$Uses – thread safety – use OO <ul><li>Yeah but can use alternative OO technique: </li></ul>class Searcher { string g_Start...$

Uses - new control structures <ul><li>Closures allow you to add new control structures to your language. With closures,...

Uses - new control structures <ul><li>It’s the ability to return out of the closure OR out of the enclosing method (Ruby ...

Uses - new control structures – Use OO <ul><li>Iterator? </li></ul><ul><li>Define an object model which supports pluggab...

Uses - function factory <ul><li>Function factory – define a function with a parameter for use later e.g. javascript this ...

Uses - function factory – Use OO <ul><li>Function factory – define a function with a parameter for use later e.g. javascri...

Uses – command Pattern <ul><li>Closures are also great for implementing the Command pattern. Command implementations that ...

Uses - command Pattern <ul><li>Function 1 that returns a function 2 with state </li></ul><ul><li>Timer invokes function 2 ...

Uses - command Pattern – Use OO <ul><li>Technique came from javascript where there are no classes (as usually understood) ...

Are Closures really that cool? <ul><li>The inventor of Python – Guido van Rossum – resists adding any further closure supp...

Summary – OO vs closures <ul><li>Arguably….  Stateful functions should be implemented as methods of classes. Behaviou...

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