Related
I'll start with the exact nature of the problem and then give some background. I am trying to name a function -threadTimer- and give it a random unique identifier, such as 'threadTimer'+ID. A randomly generated ID would work fine. Then, I need to use setInterval on it, to make it fire repeatedly and therein lies my coding problem. I have tried every variation of new, function, function as an object and I just can't get my head around it. You'll notice that the function I have created is an object and perhaps this is where I'm going in circles.
OK, the background I mentioned. threadTimer is fired by a master timer co-ordinating several threads. That's why you'll see I have generated a 'global' object for reference elsewhere. similar HTML entities can fire threadTimer at the same time, hence my requirement to make each instance unique.
window['GlblThreadExe'+ID]=setInterval(function(){threadTimer(elid,parent,lft,top,diameter,point,bStyle,color,grp,startTime,size,ID,counter,div,divwth,divht,wthIncrement,htIncrement,lftStart,topStart,lftIncrement,topIncrement)},interval);
function threadTimer(elid,parent,lft,top,diameter,point,bStyle,color,grp,startTime,size,ID,counter,div,divwth,divht,wthIncrement,htIncrement,lftStart,topStart,lftIncrement,topIncrement){
// more code
}
In truth, I think its the volume of parameters that I'm passing that's confusing my syntax. Any help appreciated
Avoid polluting window
Generally instead of polluting the global namespace you can store your setInterval ids in some variable
let intervalIds = {}
intervalIds['GlblThreadExe'+ID] = setInterval(function()...)
If really necessary, then store intervalIds to window
window.intervalIds = intervalIds;
Wrap your anonymous function
When you create the "clock", do not call setInterval directly:
Here, createTimerWithId will return a function which calls threadTimer
Dirty id generation
Use a timestamp, and mix it with some random stuff. Or better use a UUID
setInterval(createTimerWithId(), 1000)
function createTimerWithId(){
let id = Date.now()+Math.random(); //no lib, oneliner. good enough to debug
return function(){//the same function you gave to setInterval in your example
threadTimer(id, ...)
}
}
We can do better
In 1. we generated an id on the fly and thus
your code is not testable (id will always change(well except if you mock Math and Date...)).
your id is ugly (a float...)
it will be hard to know from which setInterval you come from
instead, give it the ID.
function createTimerWithId(ID){
return function(){//the same function you gave to setInterval in your example
threadTimer(ID, ...)
}
}
window['..'+ID] = setInterval(createTimerWithId(ID));
shorter version being
window['..'+ID] = setInterval((id=>{
return function(){
threadTimer(id, ...)
}
})(ID),1000);
My application is a local server that receive about 2/3 requests per seconds.
At each request, it stores and update data, process some calculation, update view (react), ...
I would like to know what is faster, when i have to use closures :
Simply create the function where I need it:
var parentValue = 'ok';
randomAsyncFunction(function() {
console.log(parentValue);
}
Create a "global" function and then bind the callback with needed values:
function testCallback(value) {
console.log(value);
}
var parentValue = 'ok';
randomAsyncFunction(testCallback.bind(undefined, parentValue));
Note: theses pseudo-codes will be executed 2/3 times per seconds. For the second example, the testCallback function will be created once, and the bind will be called instead of re-creating the function.
So, is it better or worse to use the second example ?
Both bind and the closure function expression do create a new function object. Their difference in performance will be negligible. If you really care enough, run a benchmark with your actual code and real data to see which solution is faster.
In your case, you should only care which solution is more readable and maintainable. None is strictly better or worse than the other, you have to decide yourself which one you like better.
I don’t think I’ve grokked currying yet. I understand what it does, and how to do it. I just can’t think of a situation I would use it.
Where are you using currying in JavaScript (or where are the main libraries using it)? DOM manipulation or general application development examples welcome.
One of the answers mentions animation. Functions like slideUp, fadeIn take an element as an arguments and are normally a curried function returning the high order function with the default “animation function” built-in. Why is that better than just applying the higher-up function with some defaults?
Are there any drawbacks to using it?
As requested here are some good resources on JavaScript currying:
http://www.dustindiaz.com/javascript-curry/
Crockford, Douglas (2008) JavaScript: The Good Parts
http://www.svendtofte.com/code/curried_javascript/
(Takes a detour into ML so skip the whole section from “A crash course in ML” and start again at “How to write curried JavaScript”)
http://web.archive.org/web/20111217011630/http://blog.morrisjohns.com:80/javascript_closures_for_dummies
How do JavaScript closures work?
http://ejohn.org/blog/partial-functions-in-javascript (Mr. Resig on the money as per usual)
http://benalman.com/news/2010/09/partial-application-in-javascript/
I’ll add more as they crop up in the comments.
So, according to the answers, currying and partial application in general are convenience techniques.
If you are frequently “refining” a high-level function by calling it with same configuration, you can curry (or use Resig’s partial) the higher-level function to create simple, concise helper methods.
Here's an interesting AND practical use of currying in JavaScript that uses closures:
function converter(toUnit, factor, offset, input) {
offset = offset || 0;
return [((offset + input) * factor).toFixed(2), toUnit].join(" ");
}
var milesToKm = converter.curry('km', 1.60936, undefined);
var poundsToKg = converter.curry('kg', 0.45460, undefined);
var farenheitToCelsius = converter.curry('degrees C', 0.5556, -32);
milesToKm(10); // returns "16.09 km"
poundsToKg(2.5); // returns "1.14 kg"
farenheitToCelsius(98); // returns "36.67 degrees C"
This relies on a curry extension of Function, although as you can see it only uses apply (nothing too fancy):
Function.prototype.curry = function() {
if (arguments.length < 1) {
return this; //nothing to curry with - return function
}
var __method = this;
var args = toArray(arguments);
return function() {
return __method.apply(this, args.concat([].slice.apply(null, arguments)));
}
}
#Hank Gay
In response to EmbiggensTheMind's comment:
I can't think of an instance where currying—by itself—is useful in JavaScript; it is a technique for converting function calls with multiple arguments into chains of function calls with a single argument for each call, but JavaScript supports multiple arguments in a single function call.
In JavaScript—and I assume most other actual languages (not lambda calculus)—it is commonly associated with partial application, though. John Resig explains it better, but the gist is that have some logic that will be applied to two or more arguments, and you only know the value(s) for some of those arguments.
You can use partial application/currying to fix those known values and return a function that only accepts the unknowns, to be invoked later when you actually have the values you wish to pass. This provides a nifty way to avoid repeating yourself when you would have been calling the same JavaScript built-ins over and over with all the same values but one. To steal John's example:
String.prototype.csv = String.prototype.split.partial(/,\s*/);
var results = "John, Resig, Boston".csv();
alert( (results[1] == "Resig") + " The text values were split properly" );
Agreeing with Hank Gay - It's extremely useful in certain true functional programming languages - because it's a necessary part. For example, in Haskell you simply cannot take multiple parameters to a function - you cannot do that in pure functional programming. You take one param at a time and build up your function. In JavaScript it's simply unnecessary, despite contrived examples like "converter". Here's that same converter code, without the need for currying:
var converter = function(ratio, symbol, input) {
return (input*ratio).toFixed(2) + " " + symbol;
}
var kilosToPoundsRatio = 2.2;
var litersToUKPintsRatio = 1.75;
var litersToUSPintsRatio = 1.98;
var milesToKilometersRatio = 1.62;
converter(kilosToPoundsRatio, "lbs", 4); //8.80 lbs
converter(litersToUKPintsRatio, "imperial pints", 2.4); //4.20 imperial pints
converter(litersToUSPintsRatio, "US pints", 2.4); //4.75 US pints
converter(milesToKilometersRatio, "km", 34); //55.08 km
I badly wish Douglas Crockford, in "JavaScript: The Good Parts", had given some mention of the history and actual use of currying rather than his offhanded remarks. For the longest time after reading that, I was boggled, until I was studying Functional programming and realized that's where it came from.
After some more thinking, I posit there is one valid use case for currying in JavaScript: if you are trying to write using pure functional programming techniques using JavaScript. Seems like a rare use case though.
I found functions that resemble python's functools.partial more useful in JavaScript:
function partial(fn) {
return partialWithScope.apply(this,
Array.prototype.concat.apply([fn, this],
Array.prototype.slice.call(arguments, 1)));
}
function partialWithScope(fn, scope) {
var args = Array.prototype.slice.call(arguments, 2);
return function() {
return fn.apply(scope, Array.prototype.concat.apply(args, arguments));
};
}
Why would you want to use it? A common situation where you want to use this is when you want to bind this in a function to a value:
var callback = partialWithScope(Object.function, obj);
Now when callback is called, this points to obj. This is useful in event situations or to save some space because it usually makes code shorter.
Currying is similar to partial with the difference that the function the currying returns just accepts one argument (as far as I understand that).
Consider filter function. And you want to write a callback for it.
let x = [1,2,3,4,5,6,7,11,12,14,15];
let results = x.filter(callback);
Assume want to output only even numbers, so:
let callback = x => x % 2 === 0;
Now imagine we want to implement our callback such that
depending on scenario it outputs even numbers which are above some threshold number (such
number should be configurable).
We can't easily make such threshold number a parameter to callback function, because filter invokes callback and by default passes it array elements and index.
How would you implement this?
This is a good use case for currying:
let x = [1,2,3,4,5,6,7,11,12,14,15];
let callback = (threshold) => (x) => (x % 2==0 && x > threshold);
let results1 = x.filter(callback(5)); // Even numbers higher than 5
let results2 = x.filter(callback(10)); // Even numbers higher than 10
console.log(results1,results2);
I know its old thread but I will have to show how this is being used in javascript libraries:
I will use lodash.js library to describe these concepts concretely.
Example:
var fn = function(a,b,c){
return a+b+c+(this.greet || ‘');
}
Partial Application:
var partialFnA = _.partial(fn, 1,3);
Currying:
var curriedFn = _.curry(fn);
Binding:
var boundFn = _.bind(fn,object,1,3 );//object= {greet: ’!'}
usage:
curriedFn(1)(3)(5); // gives 9
or
curriedFn(1,3)(5); // gives 9
or
curriedFn(1)(_,3)(2); //gives 9
partialFnA(5); //gives 9
boundFn(5); //gives 9!
difference:
after currying we get a new function with no parameters pre bound.
after partial application we get a function which is bound with some parameters prebound.
in binding we can bind a context which will be used to replace ‘this’, if not bound default of any function will be window scope.
Advise: There is no need to reinvent the wheel. Partial application/binding/currying are very much related. You can see the difference above. Use this meaning anywhere and people will recognise what you are doing without issues in understanding plus you will have to use less code.
It's no magic or anything... just a pleasant shorthand for anonymous functions.
partial(alert, "FOO!") is equivalent to function(){alert("FOO!");}
partial(Math.max, 0) corresponds to function(x){return Math.max(0, x);}
The calls to partial (MochiKit terminology. I think some other libraries give functions a .curry method which does the same thing) look slightly nicer and less noisy than the anonymous functions.
As for libraries using it, there's always Functional.
When is it useful in JS? Probably the same times it is useful in other modern languages, but the only time I can see myself using it is in conjunction with partial application.
I would say that, most probably, all the animation library in JS are using currying. Rather than having to pass for each call a set of impacted elements and a function, describing how the element should behave, to a higher order function that will ensure all the timing stuff, its generally easier for the customer to release, as public API some function like "slideUp", "fadeIn" that takes only elements as arguments, and that are just some curried function returning the high order function with the default "animation function" built-in.
Here's an example.
I'm instrumenting a bunch of fields with JQuery so I can see what users are up to. The code looks like this:
$('#foo').focus(trackActivity);
$('#foo').blur(trackActivity);
$('#bar').focus(trackActivity);
$('#bar').blur(trackActivity);
(For non-JQuery users, I'm saying that any time a couple of fields get or lose focus, I want the trackActivity() function to be called. I could also use an anonymous function, but I'd have to duplicate it 4 times, so I pulled it out and named it.)
Now it turns out that one of those fields needs to be handled differently. I'd like to be able to pass a parameter in on one of those calls to be passed along to our tracking infrastructure. With currying, I can.
JavaScript functions is called lamda in other functional language. It can be used to compose a new api (more powerful or complext function) to based on another developer's simple input. Curry is just one of the techniques. You can use it to create a simplified api to call a complex api. If you are the develper who use the simplified api (for example you use jQuery to do simple manipulation), you don't need to use curry. But if you want to create the simplified api, curry is your friend. You have to write a javascript framework (like jQuery, mootools) or library, then you can appreciate its power. I wrote a enhanced curry function, at http://blog.semanticsworks.com/2011/03/enhanced-curry-method.html . You don't need to the curry method to do currying, it just help to do currying, but you can always do it manually by writing a function A(){} to return another function B(){}. To make it more interesting, use function B() to return another function C().
I agree that at times you would like to get the ball rolling by creating a pseudo-function that will always have the value of the first argument filled in. Fortunately, I came across a brand new JavaScript library called jPaq (http://jpaq.org/) which provides this functionality. The best thing about the library is the fact that you can download your own build which contains only the code that you will need.
Just wanted to add some resources for Functional.js:
Lecture/conference explaining some applications
http://www.youtube.com/watch?v=HAcN3JyQoyY
Updated Functional.js library:
https://github.com/loop-recur/FunctionalJS
Some nice helpers (sorry new here, no reputation :p):
/loop-recur/PreludeJS
I've been using this library a lot recently to reduce the repetition in an js IRC clients helper library. It's great stuff - really helps clean up and simplify code.
In addition, if performance becomes an issue (but this lib is pretty light), it's easy to just rewrite using a native function.
You can use native bind for quick, one line solution
function clampAngle(min, max, angle) {
var result, delta;
delta = max - min;
result = (angle - min) % delta;
if (result < 0) {
result += delta;
}
return min + result;
};
var clamp0To360 = clampAngle.bind(null, 0, 360);
console.log(clamp0To360(405)) // 45
Another stab at it, from working with promises.
(Disclaimer: JS noob, coming from the Python world. Even there, currying is not used all that much, but it can come in handy on occasion. So I cribbed the currying function - see links)
First, I am starting with an ajax call. I have some specific processing to do on success, but on failure, I just want to give the user the feedback that calling something resulted in some error. In my actual code, I display the error feedback in a bootstrap panel, but am just using logging here.
I've modified my live url to make this fail.
function ajax_batch(e){
var url = $(e.target).data("url");
//induce error
url = "x" + url;
var promise_details = $.ajax(
url,
{
headers: { Accept : "application/json" },
// accepts : "application/json",
beforeSend: function (request) {
if (!this.crossDomain) {
request.setRequestHeader("X-CSRFToken", csrf_token);
}
},
dataType : "json",
type : "POST"}
);
promise_details.then(notify_batch_success, fail_status_specific_to_batch);
}
Now, here in order to tell the user that a batch failed, I need to write that info in the error handler, because all it is getting is a response from the server.
I still only have the info available at coding time - in my case I have a number of possible batches, but I don't know which one has failed w.o. parsing the server response about the failed url.
function fail_status_specific_to_batch(d){
console.log("bad batch run, dude");
console.log("response.status:" + d.status);
}
Let's do it. Console output is:
console:
bad batch run, dude
utility.js (line 109)
response.status:404
Now, let's change things a bit and use a reusable generic failure handler, but also one that is curried at runtime with both the known-at-code-time calling context and the run-time info available from event.
... rest is as before...
var target = $(e.target).text();
var context = {"user_msg": "bad batch run, dude. you were calling :" + target};
var contexted_fail_notification = curry(generic_fail, context);
promise_details.then(notify_batch_success, contexted_fail_notification);
}
function generic_fail(context, d){
console.log(context);
console.log("response.status:" + d.status);
}
function curry(fn) {
var slice = Array.prototype.slice,
stored_args = slice.call(arguments, 1);
return function () {
var new_args = slice.call(arguments),
args = stored_args.concat(new_args);
return fn.apply(null, args);
};
}
console:
Object { user_msg="bad batch run, dude. you were calling :Run ACL now"}
utility.js (line 117)
response.status:404
utility.js (line 118)
More generally, given how widespread callback usage is in JS, currying seems like a quite useful tool to have.
https://javascriptweblog.wordpress.com/2010/04/05/curry-cooking-up-tastier-functions/
http://www.drdobbs.com/open-source/currying-and-partial-functions-in-javasc/231001821?pgno=2
I asked a similar question at https://softwareengineering.stackexchange.com/questions/384529/a-real-life-example-of-using-curry-function
But only after I use ramda do I finally appreciate the usefulness of curry. So I will argue that if we need to chain functions together to process some input data one step a time, e.g. the promise chain example in the article Favoring Curry, using curry by "function first，data last", the code does look clean!
Here you have a practical example of were currying is being used at the moment.
https://www.joshwcomeau.com/react/demystifying-styled-components/
Basically he is creating a poor man styled components and uses currying to "preload" the name of the tag when creating a new style for it.
I am basically quite sure this pattern must exist and possess a name... for now I will call it "gate pattern"...
Here it is:
In my webpage's javascript, I have to trigger various asynchronous processes. Let's not discuss how trully async js is, but anyway I have to trigger 2 or 3 AJAX calls, must be sure, the UI build-up has finished, and so on.
Only then, when all these processes have finished, I want to do run a certain function. And precisely once.
Example
1: cropStore loaded()
2: resizeEvent()
3: productStore loaded()
The Pattern:
At the end of every (sucessful) Ajax-load-callback, the end of the GUI construction routine, etc... I set a respective flag from false to true and call gatedAction()
onEvent( 'load',
{
.... // whatever has to happen in response to cropStored, resized, etc...
// lastly:
f1 = true; //resp f2, f3, ...
gatedAction();
}
Gate will check the flags, return if any flag is still unset, only calling the target function, if all flags (or as I call them: gates) are open. If all my async pre-conditions call gatedAction() exactly once, I hope I can be sure, the actual targetFunction is called exactly once().
gatedAction ()
{
// Gate
if ( ! f1) return;
if ( ! f2) return;
if ( ! f3) return;
// actual Action ( <=> f1==f2==f3==true )
targetFunction();
}
In practice it works reliably. On a side-note: I think java-typical (not js-typical) synchronization/volatile concerns can be ignored, because javascript is not truly multithreading. Afaik a function is never stopped in the middle of it, just to grant another javascript function in the same document run-time...
So, anyone, is there a name for this? :-)
I need this pattern actually quite often, especially with complex backend UIs.. (and yes, I think, I will turn the above butt-ugly implementation into a more reusable javascript... With a gates array and a target function.)
It sounds like Balking pattern to me.
It is similar to the Rendezvous pattern, although that pattern is generally used in the context of multithreaded real-time systems.
I have no idea, if your pattern has a special name, but it seems equivalent to just using a counting semaphore, which blocks the thread, which started all those other actions, until they all made a V-invocation. Of course, there are no threads and semaphores in JavaScript, but instead of using many boolean variables you could use just one integer for counting.
In addition to the actual answer to your question, you might be interested in the Rx framework for Javascript. It's a port of the .NET version and allows you to compose events, so you don't have to work with tons of flag variables. It's meant for this sort of thing.
http://msdn.microsoft.com/en-us/devlabs/ee794896.aspx
I'm learning lots of javascript these days, and one of the things I'm not quite understanding is passing functions as parameters to other functions. I get the concept of doing such things, but I myself can't come up with any situations where this would be ideal.
My question is:
When do you want to have your javascript functions take another function as a parameter? Why not just assign a variable to that function's return value and pass that variable to the function like so:
// Why not do this
var foo = doStuff(params);
callerFunction(foo);
//instead of this
callerFunction(doStuff);
I'm confused as to why I would ever choose to do things as in my second example.
Why would you do this? What are some use cases?
Thanks!!
There are several use cases for this:
1. "Wrapper" functions.
Lets say you have a bunch of different bits of code. Before and after every bit of code, you want to do something else (eg: log, or try/catch exceptions).
You can write a "Wrapper" function to handle this. EG:
function putYourHeadInTheSand(otherFunc) {
try{
otherFunc();
} catch(e) { } // ignore the error
}
....
putYourHeadInTheSand(function(){
// do something here
});
putYourHeadInTheSand(function(){
// do something else
});
2. Callbacks.
Lets say you load some data somehow. Rather than locking up the system waiting for it to load, you can load it in the background, and do something with the result when it arrives.
Now how would you know when it arrives? You could use something like a signal or a mutex, which is hard to write and ugly, or you could just make a callback function. You can pass this callback to the Loader function, which can call it when it's done.
Every time you do an XmlHttpRequest, this is pretty much what's happening. Here's an example.
function loadStuff(callback) {
// Go off and make an XHR or a web worker or somehow generate some data
var data = ...;
callback(data);
}
loadStuff(function(data){
alert('Now we have the data');
});
3. Generators/Iterators
This is similar to callbacks, but instead of only calling the callback once, you might call it multiple times. Imagine your load data function doesn't just load one bit of data, maybe it loads 200.
This ends up being very similar to a for/foreach loop, except it's asynchronous. (You don't wait for the data, it calls you when it's ready).
function forEachData(callback) {
// generate some data in the background with an XHR or web worker
callback(data1);
// generate some more data in the background with an XHR or web worker
callback(data2);
//... etc
}
forEachData(function(data){
alert('Now we have the data'); // this will happen 2 times with different data each time
});
4. Lazy loading
Lets say your function does something with some text. BUT it only needs the text maybe one time out of 5, and the text might be very expensive to load.
So the code looks like this
var text = "dsakjlfdsafds"; // imagine we had to calculate lots of expensive things to get this.
var result = processingFunction(text);
The processing function only actually needs the text 20% of the time! We wasted all that effort loading it those extra times.
Instead of passing the text, you can pass a function which generates the text, like this:
var textLoader = function(){ return "dsakjlfdsafds"; }// imagine we had to calculate lots of expensive things to get this.
var result = processingFunction(textLoader);
You'd have to change your processingFunction to expect another function rather than the text, but that's really minor. What happens now is that the processingFunction will only call the textLoader the 20% of the time that it needs it. The other 80% of the time, it won't call the function, and you won't waste all that effort.
4a. Caching
If you've got lazy loading happening, then the textLoader function can privately store the result text in a variable once it gets it. The second time someone calls the textLoader, it can just return that variable and avoid the expensive calculation work.
The code that calls textLoader doesn't know or care that the data is cached, it's transparently just faster.
There are plenty more advanced things you can do by passing around functions, this is just scratching the surface, but hopefully it points you in the right direction :-)
One of the most common usages is as a callback. For example, take a function that runs a function against every item in an array and re-assigns the result to the array item. This requires that the function call the user's function for every item, which is impossible unless it has the function passed to it.
Here is the code for such a function:
function map(arr, func) {
for (var i = 0; i < arr.length; ++i) {
arr[i] = func(arr[i]);
}
}
An example of usage would be to multiply every item in an array by 2:
var numbers = [1, 2, 3, 4, 5];
map(numbers, function(v) {
return v * 2;
});
// numbers now contains 2, 4, 6, 8, 10
You would do this if callerFunction wants to call doStuff later, or if it wants to call it several times.
The typical example of this usage is a callback function, where you pass a callback to a function like jQuery.ajax, which will then call your callback when something finishes (such as an AJAX request)
EDIT: To answer your comment:
function callFiveTimes(func) {
for(var i = 0; i < 5; i++) {
func(i);
}
}
callFiveTimes(alert); //Alerts numbers 0 through 4
Passing a function as a parameter to another function is useful in a number of situations. The simplest is a function like setTimeout, which takes a function and a time and after that time has passed will execute that function. This is useful if you want to do something later. Obviously, if you called the function itself and passed the result in to the setTimeout function, it would have already happened and wouldn't happen later.
Another situation this is nice is when you want to do some sort of setup and teardown before and after executing some blocks of code. Recently I had a situation where I needed to destroy a jQuery UI accordion, do some stuff, and then recreate the accordion. The stuff I needed to do took a number of different forms, so I wrote a function called doWithoutAccordion(stuffToDo). I could pass in a function that got executed in between the teardown and the setup of the accordion.
Callbacks. Say you're doing something asynchronous, like an AJAX call.
doSomeAjaxCall(callbackFunc);
And in doSomeAjaxCall(), you store the callback to a variable, like var ajaxCallback Then when the server returns its result, you can call the callback function to process the result:
ajaxCallback();
This probably won't be of much practical use to you as a web programmer, but there is another class of uses for functions as first-class objects that hasn't come up yet. In most functional languages, like Scheme and Haskell, passing functions around as arguments is, along with recursion, the meat-and-potatoes of programming, rather than something with an occasional use. Higher-order functions (functions that operate on functions) like map and fold enable extremely powerful, expressive, and readable idioms that are not as readily available in imperative languages.
Map is a function that takes a list of data and a function and returns a list created by applying that function to each element of the list in turn. So if I wanted to update the positions of all the bouncing balls in my bouncing ball simulator, instead of
for(ball : ball_list) {
ball.update();
ball.display();
}
I would instead write (in Scheme)
(display (map update ball-list))
or in Python, which offers a few higher-order functions and a more familiar syntax,
display( map(update, ball-list) )
Fold takes a two-place function, a default value, and a list, and applies the function to the default and the first element, then to the result of that and the second element, and so on, finally returning the last value returned. So if my server is sending in batches of account transactions, instead of writing
for(transaction t : batch) {
account_balance += t;
}
I would write
(fold + (current-account-balance) batch))
These are just the simplest uses of the most common HOFs.
I will illustrate is with sort scenario.
Let's assume that you have an object to represent Employee of the company. Employee has multiple attributes - id, age, salary, work-experience etc.
Now, you want to sort a list of employees - in one case by employee id, in another case by salary and in yet another case by age.
Now the only thing that you wish to change is how to compare.
So, instead of having multiple sort methods, you can have a sort a method that takes a reference to function that can do the comparison.
Example code:
function compareByID(l, r) { return l.id - r.id; }
function compareByAge(l, r) { return l.age - r.age; }
function compareByEx(l, r) { return l.ex - r.ex; }
function sort(emps, cmpFn) {
//loop over emps
// assuming i and j are indices for comparision
if(cmpFn(emps[i], emps[j]) < 0) { swap(emps, i, j); }
}