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R.apex is a functional library based on Apex, inspired by Lodash and Ramda.js.

Why R.apex?

Apex is a pure object-oriented lanuage, and it does not provide any builtin features to make functional programming easier. The biggest obstacle in Apex is that functions(methods) are not First Class Citizens, the building blocks in a functional world. R.apex aims to pave the way for functional programming, and makes every attempt to mimic the functional features in Apex as much as possible. So we can adopt a functional paradigm based on R.apex.


  • Handy Collection Utility
  • Pseudo First Class Functions
  • Function Currying and Partial Application
  • Function Composition
  • Function Chaining


Reverse a List

List<Integer> reversedList = R.of(new List<Integer>{ 1, 2, 3 })
// (3, 2, 1)

Custom Sorting

List<Account> accountList = ...;
List<Account> sortedList = R.of(accountList)
// Sort account list by the last name, in descending order

Combine Top 5 Elements from Two Lists

List<String> list1 = ...;
List<String> list2 = ...;
List<String> wordList = R.of(list1)
// Combine and get List<String>

Find Element

List<Account> accountList = ...;
Account acc = (Account)R.of(accountList)
    .find(R.whereEq.apply(new Map<String, Object>{
        'LastName' => 'Wilson',
        'IsDeleted' => false
// Find the first account that is not deleted and has last name as 'Wilson'

Get Account Id List

List<Account> accountList = [ select id from account limit 10 ];
List<Id> accIdList = R.of(accountList)

Convert Account List to Map by Id

List<Account> accountList = [ select id from account limit 10 ];
Map<String, SObject> accMap = R.of(accountList)

Get Started

Copy Func.cls, R.cls and RTest.cls(optional) to your Org, and you are ready to go!

Into the Functional World

HelloWorld Function

Apex does not support first class functions, and we have NO WAY to get around it. However, we can create an invocable object camouflaged as a function, and it is referred to as a Func. Or more precisely, it is an instance of class Func. In R.apex, we roughly refer to instances of Func when we mention functions, to make things clear.

Here is how we create a function that returns Hello World.

public class HelloWorldFunc extends Func {
    public HelloWorldFunc() {

    public override Object exec() {
        return 'Hello World';

And then we get a function!

Func helloworld = new HelloWorldFunc();

Let's invoke this function.

String message = (String);
// Hello World

That's how easy it is to create a function in R.apex.

Function with Arguments

HelloWorldFunc does not make much sense, though it does a good illustration. Normally we want functions that accept arguments to do complex business logic. Here comes the AddFunc.

public class AddFunc extends Func {
    public AddFunc() {

    public override Object exec(Object arg1, Object arg2) {
        Integer a = (Integer)arg1;
        Integer b = (Integer)arg2;
        return a + b;

Quite simple, right? In the constructor, we denote that this AddFunc takes two arguments(the length of the function), and correspondingly we override the exec(Object, Object) method from Func to have our custom implementation.

Try it.

Func add = new AddFunc();
Integer result = (Integer), 2);
// 3

To conclude, we set the length of the function in its constructor, and override exec(...) method with correct arguments.

For example, if the length of the function is 3, we have exec(Object, Object, Object). Easy, yeah?

What about a function with a length of 4? exec(Object, Object, Object, Object)?

Nay, when the length goes over 3, we have execMore(List<Object>) to override.

Function with Variadic Arguments

So far so good. What if we need to handle functions that take variadic arguments? Let's extend our AddFunc to allow adding multiple numbers.

public class AddFunc extends Func {
    public AddFunc() {

    public override Object execN(List<Object> args) {
        Integer sum = 0;
        for(Object arg : args) {
            sum += (Integer)arg;
        return sum;

Now this time, we specify the length of the function to be -1, which means that it takes any number of arguments. Also we have our execN(List<Object>) overridden to get the sum of all the numbers.

By the way, the default length is -1, so if we want a variadic function, we don't even need to call super(-1). In our case, we can simply omit the constructor.

Partial Application

Well, all is good until we find something is missing. In functional programming, we can easily partially apply a function with arguments.

Say, we have a function, f: (a, b, c) => a + b + c, and we should fairly easily find out that:

var newFa = f(a);
var newFb = newFa(b);
var newFc = newFb(c);

Every time function f is applied part of the arguments it expects, it will save them and return a new function. This is great, as it guarrantees us the following:

f(a, b, c) ===
f(a, b)(c) ===
f(a)(b, c) ===

Beautiful, yeah? We do hope that we can grant this magic to our function. Fortunately, R.apex has always been built with this idea in mind and implementing this is nothing but a piece of cake. See this:

Func f1 = add.apply(1);
Func f2 = f1.apply(2);
Integer result = (Integer);

Here, we have formally introduced apply, as a builtin method of Func. And also notice the biggest caveat in R.apex. Apply does not trigger function invocation, while run does. Even if the function has received enough arguments, it will not run until run is explicitly called. Naturally, we have partial application available here., b, c) ===
f.apply(a, b, c).run() ===
f.apply(a, b).apply(c).run() ===
f.apply(a).apply(b, c).run() ===

Another thing to notice is that in functional programming, we tend to put the data we are manipulating in the last position in the argument list. For example,, myList);
// NOT
//, R.isNotNull);

This is for the convenience of functional composition, which we will cover in the next section.

Function Composition

Hopefully you have got some understanding on how to write a custom function by now. But that's far not enough. Writing functions by extending Func is still somehow bloate with boilerplates and it is kind of tedious. Better ways ahead.

The charm of functional programming lies not only in the fact of function currying and partial application, but also in the ability that they can compose. It is composition and decomposition that helps us to build a large application out of small bits and pieces in a functional world. And we adore the power.

So it is not hard to understand that functional composition is strongly recommended in R.apex, to write clean and clear codes. Let's take a leap to check it out.

Func f = (Func)
Integer result = (Integer);
// 5

Dive into this snippet, and we will make everything clear step by step. First, R.add.apply(1) creates a function f2 that takes one number and adds 1 to it. R.multiply.apply(2) creates a function f1 that takes one number and multiplies 2 to it. Then compose is the magic. It invokes f1 with one argument(i.e, 2) and gets the intermediate result of 4. After that, it invokes f2 with the intermediate result(i.e, 4) and gets the final result of 5. We can see that data flows from f1 to f2, namely, bottom-up, or right-to-left.

And this is simply how functional composition works.

If you are not quite comfortable with that, you can also try this:

Func f = (Func)
Integer result = (Integer);
// 5

It is actually the same as the previous one, only different in the composition style, namely, top-bottom, or left-to-right.

Getting familiar with functional composition is the approach to harness the power of functional programming. It takes time to sharpen the skill, and still there are more tools in functional composition in R.apex waiting to be discovered.

Function Chaining

It is totally okay if you feel the last section a little too strange, not quite like the code we usually write. We have the alternate option for you: the easy way of function composition through chaining.

R.apex adopts the similar chaining style as that of jQuery, Lodash or Promise. If you have experiences of any of these libraries, you will easily pick up R.apex.

Here is what function chaining looks like:

Integer sum = R.with(1, 2, 3)

Pretty familiar, right? In fact, most functions in R.apex are designed to be avaiable both in function composition and function chaining. Check this example:

Func sum = (Func)
Integer result = (Integer), 2, 3));

This example is a rewrite of the previous example in functional composition style, and they are equivalent.

Extend R.apex

It is your personal preference to choose between functional composition and function chaining. And the key takeaway is to use whatever is suitable in your case.

It is sad that the discovery of R.apex almost comes to an end. Realizing that the functions provided by R.apex is limited, you have to come up with your functions to tackle all the difficult business logic. The suggestion will be to encapsulate your core business logic in small custom functions and glue them together with the power of R.apex to build your own application. And this is the way you extend R.apex.

Known Limitations

Limited Support for Set and Map

Support for List is actually quite good, as List<Object> can be easily casted. However, Set and Map cannot be dynamically casted in Apex, and that makes it impossible to provide a full support. Currently, R.apex supports only Set<String> and Map<String, Object>.

Reserved Keywords Conflict

Most of us have become familiar with some of the key functions in functional programming, but will come to a surprise that they are not available in R.apex. The fact is that they are indeed available in R.apex, but just under different names. The root cause is that Apex has reserved these keywords, and we are forced to use other names.

To list the renamed functions,

Original Function Name Actual Function Name
and doAnd
or doOr
not doNot
map doMap
clone doClone
insert doInsert
insertAll doInsertAll
join doJoin
merge doMerge
update doUpdate
when doWhen

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