Efficient Android: Purposeful and Reactive Programming


Writing clear code might be difficult: Libraries, frameworks, and APIs are short-term and develop into out of date shortly. However mathematical ideas and paradigms are lasting; they require years of educational analysis and should even outlast us.

This isn’t a tutorial to indicate you how you can do X with Library Y. As an alternative, we give attention to the enduring rules behind useful and reactive programming so you may construct future-proof and dependable Android structure, and scale and adapt to adjustments with out compromising effectivity.

This text lays the foundations, and in Half 2, we’ll dive into an implementation of useful reactive programming (FRP), which mixes each useful and reactive programming.

This text is written with Android builders in thoughts, however the ideas are related and helpful to any developer with expertise normally programming languages.

Purposeful Programming 101

Purposeful programming (FP) is a sample by which you construct your program as a composition of features, remodeling information from $A$ to $B$, to $C$, and so forth., till the specified output is achieved. In object-oriented programming (OOP), you inform the pc what to do instruction by instruction. Purposeful programming is completely different: You surrender the management circulation and outline a “recipe of features” to supply your consequence as a substitute.

A green rectangle on the left with the text
The useful programming sample

FP originates from arithmetic, particularly lambda calculus, a logic system of perform abstraction. As an alternative of OOP ideas comparable to loops, lessons, polymorphism, or inheritance, FP offers strictly in abstraction and higher-order features, mathematical features that settle for different features as enter.

In a nutshell, FP has two main “gamers”: information (the mannequin, or data required on your downside) and features (representations of the habits and transformations amongst information). In contrast, OOP lessons explicitly tie a specific domain-specific information construction—and the values or state related to every class occasion—to behaviors (strategies) which are supposed for use with it.

We’ll look at three key elements of FP extra carefully:

  • FP is declarative.
  • FP makes use of perform composition.
  • FP features are pure.

An excellent beginning place to dive into the FP world additional is Haskell, a strongly typed, purely useful language. I like to recommend the Be taught You a Haskell for Nice Good! interactive tutorial as a helpful useful resource.

FP Ingredient #1: Declarative Programming

The very first thing you’ll discover about an FP program is that it’s written in declarative, versus crucial, type. Briefly, declarative programming tells a program what must be accomplished as a substitute of how you can do it. Let’s floor this summary definition with a concrete instance of crucial versus declarative programming to resolve the next downside: Given a listing of names, return a listing containing solely the names with at the least three vowels and with the vowels proven in uppercase letters.

Crucial Resolution

First, let’s look at this downside’s crucial answer in Kotlin:

enjoyable namesImperative(enter: Listing<String>): Listing<String> {
    val consequence = mutableListOf<String>()
    val vowels = listOf('A', 'E', 'I', 'O', 'U','a', 'e', 'i', 'o', 'u')

    for (identify in enter) { // loop 1
        var vowelsCount = 0

        for (char in identify) { // loop 2
            if (isVowel(char, vowels)) {

                if (vowelsCount == 3) {
                    val uppercaseName = StringBuilder()

                    for (finalChar in identify) { // loop 3
                        var transformedChar = finalChar
                        // ignore that the primary letter is likely to be uppercase
                        if (isVowel(finalChar, vowels)) {
                            transformedChar = finalChar.uppercaseChar()


    return consequence

enjoyable isVowel(char: Char, vowels: Listing<Char>): Boolean {
    return vowels.comprises(char)

enjoyable principal() {
    println(namesImperative(listOf("Iliyan", "Annabel", "Nicole", "John", "Anthony", "Ben", "Ken")))
    // [IlIyAn, AnnAbEl, NIcOlE]

We’ll now analyze our crucial answer with a number of key improvement components in thoughts:

  • Most effective: This answer has optimum reminiscence utilization and performs effectively in Large O evaluation (based mostly on a minimal variety of comparisons). On this algorithm, it is sensible to research the variety of comparisons between characters as a result of that’s the predominant operation in our algorithm. Let $n$ be the variety of names, and let $okay$ be the typical size of the names.

    • Worst-case variety of comparisons: $n(10k)(10k) = 100nk^2$
    • Clarification: $n$ (loop 1) * $10k$ (for every character, we evaluate towards 10 potential vowels) * $10k$ (we execute the isVowel() examine once more to resolve whether or not to uppercase the character—once more, within the worst case, this compares towards 10 vowels).
    • End result: For the reason that common identify size gained’t be greater than 100 characters, we are able to say that our algorithm runs in $O(n)$ time.
  • Advanced with poor readability: In comparison with the declarative answer we’ll take into account subsequent, this answer is for much longer and more durable to comply with.
  • Error-prone: The code mutates the consequence, vowelsCount, and transformedChar; these state mutations can result in refined errors like forgetting to reset vowelsCount again to 0. The circulation of execution may develop into sophisticated, and it’s simple to overlook so as to add the break assertion within the third loop.
  • Poor maintainability: Since our code is complicated and error-prone, refactoring or altering the habits of this code could also be tough. For instance, if the issue was modified to pick out names with three vowels and 5 consonants, we must introduce new variables and alter the loops, leaving many alternatives for bugs.

Our instance answer illustrates how complicated crucial code may look, though you might enhance the code by refactoring it into smaller features.

Declarative Resolution

Now that we perceive what declarative programming isn’t, let’s unveil our declarative answer in Kotlin:

enjoyable namesDeclarative(enter: Listing<String>): Listing<String> = enter.filter { identify ->
    identify.depend(::isVowel) >= 3
}.map { identify ->
    identify.map { char ->
        if (isVowel(char)) char.uppercaseChar() else char

enjoyable isVowel(char: Char): Boolean =
    listOf('A', 'E', 'I', 'O', 'U', 'a', 'e', 'i', 'o', 'u').comprises(char)

enjoyable principal() {
    println(namesDeclarative(listOf("Iliyan", "Annabel", "Nicole", "John", "Anthony", "Ben", "Ken")))
    // [IlIyAn, AnnAbEl, NIcOlE]

Utilizing the identical standards that we used to guage our crucial answer, let’s see how the declarative code holds up:

  • Environment friendly: The crucial and declarative implementations each run in linear time, however the crucial one is a little more environment friendly as a result of I’ve used identify.depend() right here, which can proceed to depend vowels till the identify’s finish (even after discovering three vowels). We will simply repair this downside by writing a easy hasThreeVowels(String): Boolean perform. This answer makes use of the identical algorithm because the crucial answer, so the identical complexity evaluation applies right here: Our algorithm runs in $O(n)$ time.
  • Concise with good readability: The crucial answer is 44 strains with giant indentation in comparison with our declarative answer’s size of 16 strains with small indentation. Strains and tabs aren’t all the things, however it’s evident from a look on the two information that our declarative answer is far more readable.
  • Much less error-prone: On this pattern, all the things is immutable. We remodel a Listing<String> of all names to a Listing<String> of names with three or extra vowels after which remodel every String phrase to a String phrase with uppercase vowels. General, having no mutation, nested loops, or breaks and giving up the management circulation makes the code less complicated with much less room for error.
  • Good maintainability: You may simply refactor declarative code on account of its readability and robustness. In our earlier instance (let’s say the issue was modified to pick out names with three vowels and 5 consonants), a easy answer could be so as to add the next statements within the filter situation: val vowels = identify.depend(::isVowel); vowels >= 3 && identify.size - vowels >= 5.

As an added optimistic, our declarative answer is solely useful: Every perform on this instance is pure and has no negative effects. (Extra about purity later.)

Bonus Declarative Resolution

Let’s check out the declarative implementation of the identical downside in a purely useful language like Haskell to reveal the way it reads. If you happen to’re unfamiliar with Haskell, notice that the . operator in Haskell reads as “after.” For instance, answer = map uppercaseVowels . filter hasThreeVowels interprets to “map vowels to uppercase after filtering for the names which have three vowels.”

import Information.Char(toUpper)

namesSolution :: [String] -> [String]
namesSolution = map uppercaseVowels . filter hasThreeVowels

hasThreeVowels :: String -> Bool
hasThreeVowels s = depend isVowel s >= 3

uppercaseVowels :: String -> String
uppercaseVowels = map uppercaseVowel
 the place
   uppercaseVowel :: Char -> Char
   uppercaseVowel c
     | isVowel c = toUpper c
     | in any other case = c

isVowel :: Char -> Bool
isVowel c = c `elem` vowels

vowels :: [Char]
vowels = ['A', 'E', 'I', 'O', 'U', 'a', 'e', 'i', 'o', 'u']

depend :: (a -> Bool) -> [a] -> Int
depend _ [] = 0
depend pred (x:xs)
  | pred x = 1 + depend pred xs
  | in any other case = depend pred xs

principal :: IO ()
principal = print $ namesSolution ["Iliyan", "Annabel", "Nicole", "John", "Anthony", "Ben", "Ken"]

-- ["IlIyAn","AnnAbEl","NIcOlE"]

This answer performs equally to our Kotlin declarative answer, with some further advantages: It’s readable, easy if you happen to perceive Haskell’s syntax, purely useful, and lazy.

Key Takeaways

Declarative programming is beneficial for each FP and Reactive Programming (which we’ll cowl in a later part).

  • It describes “what” you wish to obtain—slightly than “how” to realize it, with the precise order of execution of statements.
  • It abstracts a program’s management circulation and as a substitute focuses on the issue when it comes to transformations (i.e., $A rightarrow B rightarrow C rightarrow D$).
  • It encourages much less complicated, extra concise, and extra readable code that’s simpler to refactor and alter. In case your Android code doesn’t learn like a sentence, you’re most likely doing one thing fallacious.

In case your Android code would not learn like a sentence, you are most likely doing one thing fallacious.

Nonetheless, declarative programming has sure downsides. It’s potential to finish up with inefficient code that consumes extra RAM and performs worse than an crucial implementation. Sorting, backpropagation (in machine studying), and different “mutating algorithms” aren’t a superb match for the immutable, declarative programming type.

FP Ingredient #2: Operate Composition

Operate composition is the mathematical idea on the coronary heart of useful programming. If perform $f$ accepts $A$ as its enter and produces $B$ as its output ($f: A rightarrow B$), and performance $g$ accepts $B$ and produces $C$ ($g: B rightarrow C$), then you may create a 3rd perform, $h$, that accepts $A$ and produces $C$ ($h: A rightarrow C$). We will outline this third perform because the composition of $g$ with $f$, additionally notated as $g circ f$ or $g(f())$:

A blue box labeled
Features f, g, and h, the composition of g with f.

Each crucial answer might be translated right into a declarative one by decomposing the issue into smaller issues, fixing them independently, and recomposing the smaller options into the ultimate answer by perform composition. Let’s take a look at our names downside from the earlier part to see this idea in motion. Our smaller issues from the crucial answer are:

  1. isVowel :: Char -> Bool: Given a Char, return whether or not it’s a vowel or not (Bool).
  2. countVowels :: String -> Int: Given a String, return the variety of vowels in it (Int).
  3. hasThreeVowels :: String -> Bool: Given a String, return whether or not it has at the least three vowels (Bool).
  4. uppercaseVowels :: String -> String: Given a String, return a brand new String with uppercase vowels.

Our declarative answer, achieved by perform composition, is map uppercaseVowels . filter hasThreeVowels.

A top diagram has three blue
An instance of perform composition utilizing our names downside.

This instance is a little more sophisticated than a easy $A rightarrow B rightarrow C$ formulation, but it surely demonstrates the precept behind perform composition.

Key Takeaways

Operate composition is an easy but highly effective idea.

  • It supplies a method for fixing complicated issues by which issues are break up into smaller, less complicated steps and mixed into one answer.
  • It supplies constructing blocks, permitting you to simply add, take away, or change components of the ultimate answer with out worrying about breaking one thing.
  • You may compose $g(f())$ if the output of $f$ matches the enter kind of $g$.

When composing features, you may move not solely information but in addition features as enter to different features—an instance of higher-order features.

FP Ingredient #3: Purity

There’s yet one more key ingredient to perform composition that we should tackle: The features you compose have to be pure, one other idea derived from arithmetic. In math, all features are computations that all the time yield the identical output when referred to as with the identical enter; that is the premise of purity.

Let’s take a look at a pseudocode instance utilizing math features. Assume we now have a perform, makeEven, that doubles an integer enter to make it even, and that our code executes the road makeEven(x) + x utilizing the enter x = 2. In math, this computation would all the time translate to a calculation of $2x + x = 3x = 3(2) = 6$ and is a pure perform. Nevertheless, this isn’t all the time true in programming—if the perform makeEven(x) mutated x by doubling it earlier than the code returned our consequence, then our line would calculate $2x + (2x) = 4x = 4(2) = 8$ and, even worse, the consequence would change with every makeEven name.

Let’s discover a number of forms of features that aren’t pure however will assist us outline purity extra particularly:

  • Partial features: These are features that aren’t outlined for all enter values, comparable to division. From a programming perspective, these are features that throw an exception: enjoyable divide(a: Int, b: Int): Float will throw an ArithmeticException for the enter b = 0 attributable to division by zero.
  • Whole features: These features are outlined for all enter values however can produce a special output or negative effects when referred to as with the identical enter. The Android world is stuffed with whole features: Log.d, LocalDateTime.now, and Locale.getDefault are just some examples.

With these definitions in thoughts, we are able to outline pure features as whole features with no negative effects. Operate compositions constructed utilizing solely pure features produce extra dependable, predictable, and testable code.

Tip: To make a complete perform pure, you may summary its negative effects by passing them as a higher-order perform parameter. This manner, you may simply take a look at whole features by passing a mocked higher-order perform. This instance makes use of the @SideEffect annotation from a library we look at later within the tutorial, Ivy FRP:

droop enjoyable deadlinePassed(
deadline: LocalDate, 
    currentDate: droop () -> LocalDate
): Boolean = deadline.isAfter(currentDate())

Key Takeaways

Purity is the ultimate ingredient required for the useful programming paradigm.

  • Watch out with partial features—they’ll crash your app.
  • Composing whole features just isn’t deterministic; it will probably produce unpredictable habits.
  • At any time when potential, write pure features. You’ll profit from elevated code stability.

With our overview of useful programming accomplished, let’s look at the subsequent element of future-proof Android code: reactive programming.

Reactive Programming 101

Reactive programming is a declarative programming sample by which this system reacts to information or occasion adjustments as a substitute of requesting details about adjustments.

Two main blue boxes,
The final reactive programming cycle.

The fundamental components in a reactive programming cycle are occasions, the declarative pipeline, states, and observables:

  • Occasions are indicators from the surface world, sometimes within the type of person enter or system occasions, that set off updates. The aim of an occasion is to remodel a sign into pipeline enter.
  • The declarative pipeline is a perform composition that accepts (Occasion, State) as enter and transforms this enter into a brand new State (the output): (Occasion, State) -> f -> g -> … -> n -> State. Pipelines should carry out asynchronously to deal with a number of occasions with out blocking different pipelines or ready for them to complete.
  • States are the information mannequin’s illustration of the software program utility at a given cut-off date. The area logic makes use of the state to compute the specified subsequent state and make corresponding updates.
  • Observables pay attention for state adjustments and replace subscribers on these adjustments. In Android, observables are sometimes applied utilizing Circulation, LiveData, or RxJava, and so they notify the UI of state updates so it will probably react accordingly.

There are various definitions and implementations of reactive programming. Right here, I’ve taken a realistic strategy targeted on making use of these ideas to actual initiatives.

Connecting the Dots: Purposeful Reactive Programming

Purposeful and reactive programming are two highly effective paradigms. These ideas attain past the short-lived lifespan of libraries and APIs, and can improve your programming expertise for years to return.

Furthermore, the ability of FP and reactive programming multiplies when mixed. Now that we now have clear definitions of useful and reactive programming, we are able to put the items collectively. In half 2 of this tutorial, we outline the useful reactive programming (FRP) paradigm, and put it into follow with a pattern app implementation and related Android libraries.

The Toptal Engineering Weblog extends its gratitude to Tarun Goyal for reviewing the code samples introduced on this article.

Additional Studying on the Toptal Engineering Weblog:


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