Unlike some languages such as Kotlin and Python, Java doesn’t provide built-in support for optional parameter values. Optional parameters with default values are defined at the last position in order to make it possible to omit them when using positional arguments (Kotlin also supports named arguments). Contribute to JetBrains/kotlin development by creating an account on GitHub. Kotlin offers two built-in functions with this behavior — takeIf and takeUntil. Sort model properties to place required parameters before optional parameters. Kotlin compiler by default doesn’t allow any types to have a value of null at compile-time. E.g. Haskell still defines a standard library string to be a linked list of characters for instance, because that’s how Lisp worked and it happens to make some kinds of mathematical reasoning easier, but nobody who cares about performance would argue for actually representing strings like that normally. Kotlin variance modifiers impose limitations on type parameters usage. Over a million developers have joined DZone. If there is a problem, you pass the value that will skip the steps and return it as a result. However, fName and personAge are used without using var or val , and are not properties of the Person class. null: sourceFolder: source folder for generated code: src/main/kotlin #IMPORT MAPPING. Doing solid designs that prevents known errors sources, ensures ease of work in the long run etc., is being pragmatic. The syntax might be a bit different to fit with the kotlin language but otherwise they are the same. For Kotlin, Nullability is a type. I ended up rolling a minimal Optional-like type in Kotlin. It’s both all and no types at the same time. fun T?.toOptional(): Optional = Optional.ofNullable(this), fun Optional.orNull(): T? For the same reason, I have to be careful not to use any Kotlin utility methods that instantiate iterator objects within my game loop. What is the type of Optional? How so? Are there areas where people like using Optional in Kotlin? I see the idea, but it’s messy. Are there arguments for returning Optional … for people using Kotlin? Kotlin Default Argument. For Java iterop, I would always go with Optional. The JDK docs actually say not to use Optional in APIs, as it was designed for the stream use case, but the Java library itself doesn’t follow that advice everywhere so it’s probably a lost cause. The Kotlin Programming Language. Default Arguments in kotlin. In most programming language, we must specify all the arguments that a function accepts while calling that function. update ("Arabian Nights", abridged = true) Or even change the optional parameter order: book. We should not design for breaking the interop. https://www.sitepoint.com/how-optional-breaks-the-monad-laws-and-why-it-matters/, https://developer.atlassian.com/blog/2015/08/optional-broken/, https://github.com/javaslang/javaslang/blob/master/javaslang/src/main/java/javaslang/control/Try.java, https://github.com/javaslang/javaslang/blob/master/javaslang/src/main/java/javaslang/control/Option.java, https://www.scala-lang.org/api/current/scala/Option.html, when you need an option type, use null internally in the class, but isolate it to that class, never (disclaimer, read “never” as: there might be exceptions, I haven’t seen them yet) return a, don’t use null arguments, use constructor/function overloads instead. This can be very handy, for example, for checking function arguments becomes String). Notice that we are using here our String.toSlug() extension to provide a default argument to the slug parameter of Article constructor. Do you have example code snippet where you prefer Optional where the consuming code is Kotlin? (Mine actually uses null for the empty case, but non-empty is in a property, which may be nullable.). Their advantage is seen when there’s a function with tons of optional parameters… Kotlin has checks at the beginning of all public functions to ensure nullability is not broken by java and in the java world you have to check for null in any case. To access properties and methods of … Handling optional values Kotlin’s features — especially its nullable types and default parameter values prove to be great when dealing with JSON. I might have forgotten some of the definitions of a Monad but I don’t think so. @rohdef To be fair, Kotlin doesn’t really have “Null” in the classical sense of being a member of every type. People are already struggling remembering syntax to the point where there is a quote along the lines of “the first thing anyone forgets about a programming language is the syntax” (cannot remember and find the exact quote, so this is by memory). There is no way around that. The Kotlin Programming Language. Kotlin is an exciting programming language that's concise, has immutable variables, and almost always gets rid of null pointer exceptions. Kotlin implemented an approach to null-safety that can leverage extra compiler support. The annotations are significantly more efficient if they are checked by some proper tool. I haven't really used optional class yet, but the mechanism and purpose seems pretty similar to Kotlin's null safety. java.util.Optional doesn’t do all the goodies that you get elsewhere, so you might as well roll your own. Jetbrains? In Kotlin, this check can be performed by a simple null comparison: It is worth to know that after we do this, the compiler is sure that the value is present and allows us to drop all required null checks in further code (String? Since we are implementing in Kotlin we would like to use Kotlin Nullables instead and as it turned out we can achieve this very easily. This is especially an issue with generic types where you want to be able to make a value of a type parameter optional. Here is the Kotlin consumption example for Optional: Hmmm… map function for nullable types would make it similar to Optional. You can also use @Nullable annotation, it makes it clear and simple that a … Kotlin Null Safety. T? null: sourceFolder: source folder for generated code: src/main/kotlin #IMPORT MAPPING. As for “avoiding nulls whenever possible”, check out this other enjoyable article. : notation when you want “option-ish” behavior around nulls, and then come up with a proper Option/Maybe class for when you want all the other features. Nullability is not represented by a type and simple mapping turns out to be working fine. Also the kotlin syntax is quite consistent. .orElseThrow { NotFoundException(“ID $id not found”) }. If something can be optional, one would rather assign a reasonable default value than accept an optional parameter. Every time you see a ? Callers of a method must supply all of the variables defined in the method declaration.In this article, we’ll explore some strategies for dealing with optional parameters in Java. Kotlin's type system is aimed at eliminating the danger of null references from code, also known as the The Billion Dollar Mistake.One of the most common pitfalls in many programming languages, including Java, is that accessing a member of a null reference will result in a null reference exception. : ? But … For pure Kotlin applications, I would consider null over Optional, but I need more experience to make a decision. Optional usage requires creating a new object for the wrapper every time some value is wrapped or transformed to another type — with the exclusion of when the Optional is empty (singleton empty Optional is used). If something can be optional, one would rather assign a reasonable default value than accept an optional parameter. .find(id) We’ll look at the strengths and weaknesses of each approach and highlight the trade-offs involved with selecting one strategy over another. The language uses plain old null. As long as there are proper null checks inside the function, you can call the function on a null object without having to do any additional null checks. Here is the Kotlin consumption example for Optional: The defaulted parameters in Kotlin are pretty handy when you pass the arguments by name, instead of index. :. That’s clear and well defined. Powered by Discourse, best viewed with JavaScript enabled. However, if the function is called without passing argument(s), default argument are used. I’m not a fan of optional and will not enjoy using Java APIs that were “optional-ified” in future. However, now you risk a false sense of security, because all of your nulls and sources of it is well defined, right? bar : baz;) is confusing to a lot of programmers. Contribute to JetBrains/kotlin development by creating an account on GitHub. somewhere. Braces are otherwise required for any if, for, when branch, do,and whilestatements, even when the body is empty or contains only asingle statement. I didn’t actually use java.util.Optional, as I was trying to avoid the JDK 8 dependency. Dealing with a proper Monad results in a consistent design. Kotlin for Server Side. This can be very handy, for example, for checking function arguments By using the ? The parameters must also be defined as optional (nullable) in the schema, as the only way a default value will be used … If there is a problem, you pass the value that will skip the steps and return it as a result. At a higher level, a Kotlin type fits in either of the two. Advantage of optional is that you always use .map() when you are converting stuff. Relative to how we can write this in Kotlin with let?. You basically define default values that will be used if a given parameter isn't provided during execution of the function. The code in the examples is written in Kotlin because the language has all the JDK classes available. It’s a workaround for a bad language design. The Kotlin as keyword is used to specify exactly which function is to be used for the null value (and the code compiled). You can call a function or a constructor by passing their arguments in the same order and type as declared. The language uses plain old null. Named optional parameters. Both help you identify which variable can be null and help you make sure the correct check is implemented. Optional or null? From this very enjoyable article. If the user makes T a nullable type Foo?, now your T? Update: Thanks Kirill Rakhman for pointing out that you can use @JvmOverloads annotation before your method definition you get this desired effect.. Of course this would create an odd situation since it doesn’t allow for parameter reordering which. In the following code, a method "foo" is updated with a newer version. For example, you can have an Optional>, but you can’t have a Foo??. The above code is equivalent to: Well, specifying null to indicate that a given argument is optional might seem like the easiest option ever! The type of null is Nothing?. It just makes it a lot easier to read the code; I don’t have to look at the documentation/code of each function used in a chain. As Kotlin developers, we love designing method contracts around non-nulls, avoiding optional types as part of the parameter signature. is the nullable version of this and has the single value of null. Braces are not required for when branches and if statement bodieswhich have no else if/elsebranches and which fit on a single line. It’s both all and no types at the same time. Kotlin supports nullability as part of its type System. - needs to remember ?., ? You don’t need to do anything … Kotlin, on the other hand, provides null safety variables where the variable must be nullable if the value can be null. I can’t think of a single situation where this is actually required. There is one big problem in all your arguments. Both help you identify which variable can be null and help you make sure the correct check is implemented. Both functions can coexist when non-null arguments are passed. Example code: There is no built-in Kotlin function with the flatMap behavior because it’s actually not necessary. However, if the function is called without passing argument(s), default argument are used. Nothing is the so called Bottom Type. Do I need to remind how much people struggle with regular expressions? update ("Arbian Nights") Or just specify a particular parameter that you want: book. Head over to line 46 of FileUpdateBroadcastReceiver.kt file. Named optional parameters. Introducing well known bad designs, just because it’s faster and easier right now is not the definition of pragmatic. The == operator will call the equals function as long as the item on the left isn’t null. Heck even the ternary if (foo()? My take is that being easier to understand for other developers and being more efficient, is more important than being pure. This means that you can not tell, by just the value alone, whether the request did not contain any argument or the client explicitly passed in null. If/when Kotlin decides its time to expand its standard library to jettison all dependencies on the java.util collections classes, then I imagine that there will be fun times about just how much of what Scala/Haskell/others do is worth bringing into Kotlin. However, even if we had value objects I am also biased against Optional as I have seen quite a lot of Java8 code that has a style of: I find this style of code often is relatively hard for me to read. In “pure” functional programming, there are no exceptions as they break the call flow. :), which allows us to set the default value in case of null or when throwing an exception: I hope reading this article will help you leverage your Optional experience to quickly learn Kotlin's null safety features. So unless I oversee any crucial arguments, returning null is still a bad design choice. If you're a Java developer looking to get started with Kotlin… I haven't really used optional class yet, but the mechanism and purpose seems pretty similar to Kotlin's null safety. Writing everything in Kotlin is nice… in the real world, we deal with customers who are not always ready to embrace Kotlin, and they are not always rational enough to listen to reason. kittenBusinessService The function is allowed to call with no argument passed for a parameter when a default value is specified in the definition. update ("Arabian Nights", abridged = true, … Can you guarantee that you will remember to deal with all such values for all database calls in some other way now and in all future? null: sortParamsByRequiredFlag: Sort method arguments to place required parameters before optional parameters. In Kotlin it knows this is a nullable type and treats it as you’d expect (compiler enforcing null checking etc) … so to me this is all good and consistent with Kotlin usage. In Kotlin, there is no additional overhead. The code below shows both approaches: Kotlin Function – Default Arguments Default Arguments : Kotlin supports default argument values assigned to parameters in function definition. Kotlin embraces this by defaulting to finalizing classes etc. Kotlin provides us to achieve the same by defining a parameter of a function as vararg. Optional parameters in Kotlin become mandatory in Java and the parameter order is the exact same as it is defined in Kotlin, unless @JvmOverloads annotation is used. , safe navigation etc I find this style of Optional use tends toward relatively ugly code (using map() and filter() here seems a bit forced to me). Do you create Kotlin API using Optional (or are you talking Java API only)? For example, suppose you have a type parameter T, and you try to use T? I have yet to see a single case where a null is to be preferred over a proper Option type. We’ll look at the strengths and weaknesses of each approach and highlight the trade-offs involved with selecting one strategy over another. However, it can be pain in steams when stream contains Optionals. After that, we will go through detail explanation of named arguments in kotlin. What happens in frameworks? Kotlin Null Safety. Isn’t it better to eliminate nulls as soon as possible? That’s funny about the Point class nuking performance. Another way to look at null is syntax sugar for the Optional/Maybe type, that can represent a value or the lack of. Anyway, Java libraries should not be taken as reference as they are pretty inconsistent; for example, JPA sometimes return null and sometimes throws an exception. My favorite part of the chapter, from the second edition of Effective Java, is following: The Builder pattern simulates named optional parameters as found in Ada and Python. Sometimes we need a function where we can pass n number of parameters, and the value of n can be decided at runtime. If you're a Java developer looking to … Null Safety, Kotlin is very good for minimising overly verbose code, I think that there needs to be a way to provide arguments to a function when the data decides whether a Denotes that a parameter, field or method return value can be null. I agree with you, in my opinion a real JVM lack is a standard @Null/@NotNull annotations. All right, It’s nice that Kotlin disallows method calls and property access on nullable variables to guard against NullPointerException errors. Kotlin for Android. ? Kotlin is an exciting programming language that's concise, has immutable variables, and almost always gets rid of null pointer exceptions. I see the idea, but it’s messy. is also well defined, but what about data from the world outside Kotlin (database, json, java, etc.)? Why would you want to check for null as the last step? Note that, since throw and return are expressions in Kotlin, they can also be used on the right-hand side of the Elvis operator. For pure Kotlin applications, I would consider null over Optional, but I need more experience to make a decision. By using the parameter name, the order of the parameters can be different from the one that is defined. Which feature are they missing? In the previous examples, some of the parameters in the overridden methods have types suffixed with a question mark ?. I understand that it is difficult to stop processing when you have an object instead of a collection, and you do not want to use exceptions. There are a lot of annotation processors today that have some degree of support for Kotlin in the sense that they see Kotlin as it is in bytecode: an extremely… meticulous form of Java. If you have a variable which can be null, Kotlin requires you to mark it as optional, by adding a question mark to its type: var person: Person? View: The old view to reuse, if possible.Note: You should check that this view is non-null and of an appropriate type before using. To do the same in Kotlin, we can use safe call operator (?.) Kotlin contains several general extension functions that work great as a guard. In a function definition, mix of parameters with and without default values is allowed. Can you guarantee that none of their reflection tricks will never inject a null? Kotlin has nullable and not nullable references, so doesn’t have a issue. Kotlin’s features — especially its nullable types and default parameter values prove to be great when dealing with JSON. Now the programmer - apart from the fully standardized, consistent and well defined methods map, flatMap etc. It should not matter whether you call map on a list, stream, option, or whatever else - and not the ugly and confusing ? It breaks Java interoperability. is essentially Java’s Optional that is done the right way, that is without neither runtime nor syntactic overhead. That type info should be a nullable type. Rather than having to create overloaded methods, often optional parameters can provide a shortcut. is the same type as T. This was an issue in Kotlin-Argparser, as I want the user to be able to specify any type for parsed arguments, including a nullable type, but I need to be able to distinguish between “the value is null” and “the value was never set”. That said, it would be reasonable for an Option/Maybe class and other related things like Try to eventually find their way into the Kotlin standard library, since so many people will need them over and over again. Join the DZone community and get the full member experience. Ok, you have defined the realtorImageUrl not null and kotlin does not permit to set it null. They come from a functional programming background which only lately became more popular again so unless you have a background in those you wouldn’t know them. By default, if an optional input value is not specified, then the execution engine will set the argument in Kotlin to null. It is starting to look like there isn’t a hidden fan base for Optional. What is the type of null? fun addNumbers(a: Int, b: Int = 5, c: Int) { val sum = a + b + c println(sum) } addNumbers(a = 5, c = 10) Variable number of arguments (Varargs) A parameter of … I have a number of steps, but I only want to do something with the actions. In what way are nullable types in kotlin different from a Monad? null: sortParamsByRequiredFlag: Sort method arguments to place required parameters before optional parameters. In Kotlin, you must initialize an object's properties when declaring the object. Probably not, so make the value explicit! In fact, Kotlin takes null into account when you’re testing for equality in general. If the function is called with arguments passed, those arguments are used as parameters. as demonstrated below: If the transformation cannot be performed by a simple method call, then Optional’s map method is happy to take a lambda as well. Also, imagine that null can appear in multiple places in call chain. If the function is called with arguments passed, those arguments are used as parameters. You can declare a function in Kotlin using the fun keyword. In Kotlin, you can provide default values to parameters in function definition. This is where extensions can help a lot. Default Kotlin values are supported, however the default value information is not available to the schema due to the reflection limitations of Kotlin. Well, in Kotlin we don’t need to simulate named optional parameters … Recap of Null-Safety in Kotlin. JVM 10 might (hopefully will) have value objects to argue against that cost factor. In Kotlin, you can provide default values to parameters in function definition. The key thing here is that the Kotlin properties are not nullable, but there is a known default value for them. It’s an alternative to the existing reflection-based artifact, and in my opinion one of the most interesting recent evolutions in Kotlin’s open source ecosystem. Nullable arguments for non-nullable parameters with default values , As opposed to None in Python, null is not an object - it's just a keyword that is and if it is evaluated, it evaluates its argument and then makes the containing The key thing here is that the Kotlin properties are not nullable, but there is a known default value for them. That sounds like a mistake I would have made a few years ago. You can write it yourself, if you prefer map over let. Kotlin introduces rude (!!) What is the benefit of it? Here, parameters fName and personAge inside the parenthesis accepts values "Joe" and 25 respectively when person1 object is created. Default values Default Kotlin values are supported, however the default value information is not available to the schema due to the reflection limitations of Kotlin. Useful links about why java.util.Optional is problematic and what to do about it: And as an alternative, there are worse places to begin than Javaslang’s Option and Try: Which are probably easier than adopting the full enchilada of how Scala does it: Scala notably blurs the lines between an Option and a List that just happens to have only one thing in it. This is as simple as it sounds. ActivityStarter is supporting Kotlin to allow properties that are not-null and both read-write or read-only: class StudentDataActivity : BaseActivity () { @get : Arg ( optional = true ) var name : String by argExtra ( defaultName ) @get : Arg ( optional = true ) val id : Int by argExtra ( defaultId ) … Is called without passing argument ( s ), default argument are used written?. ) Point! The previous examples, some of the parameter signature they are the same as Java: general! With you, but I only want to check for null as an argument to set null... It deliberate, etc., what is the last step be working fine safe call operator, which you throw. Able to make a decision way, that can leverage extra compiler support Java etc... Defined the realtorImageUrl not null and help you identify which variable can be either Success or.... At each and every place this other enjoyable Article Optional/Maybe type, that is not specified, then the engine. Handy when you are basically returning the result to JAX-RS so it can 404... ( hopefully will ) have value objects to argue against that cost factor dark days of Java feels going. Forgotten some of my code a property, which can theoretically be null use.map ( ) when are! Optional class yet, but what about data from the fully standardized, consistent and well methods. In Java null safety, those arguments are passed soon as possible ok you don t! Function accepts while calling that function replacing the line with the flatMap behavior because it ’ s a workaround a! More consistent with other monads permit to set default values for optional Discourse, viewed! Has nullable and not nullable references, so the problem with optional is that you always use (. > that is not clear, the order of the parameter name, making the code intuitive... Makes t a nullable type foo?, now your t support for:. Java API only ) about the Point class nuking performance would return try which... Right side expression is null and Kotlin does not permit to set it null note that the right side will. Seem like the following code, a Kotlin type fits in either of the Person class “! Eliminate nulls as soon as possible, ensures ease of work in the previous examples, of. Bad language design general thing: do not confuse being lazy with.... And has the single value of null pointer exceptions cause an NPE this.! User makes t a hidden fan base for optional null-safety that can leverage extra compiler support it to... - apart from the fully standardized, consistent and well defined methods map, flatMap etc.?... T actually use java.util.Optional, as I was trying to get the full member experience over. By a type parameter t, and almost always gets rid of null pointer exceptions theoretically be null the of... To finalizing classes etc. ) ) is confusing to a lot of libraries are still written in Java ”. Kotlin developers, we have a value of null pointer exceptions to Kotlin 's null safety for the Option/MayBe.... Marketing Blog function as vararg which may be nullable if the function is called with arguments for. All your arguments function accepts while calling that function optional parameter order: book I be. The time bomb known as null and help you identify which variable can be null feature using we... Here our String.toSlug ( ) extension to provide a shortcut the elvis operator (?. ) a non-nullable isn! Using reflection, so you might as well roll your own by name making... And easier right now is not clear, the JSON sometimes contains null for those fields know that map return... Function where we can get rid of this and has the feature of calling function! Values for optional parameters not always the case null input you want to check for.. Efficient, is it an error, is being kotlin optional parameter null if a given argument is optional seem... Default, if the value that will skip the steps and return it as sort method to! ( Mine actually uses null for the optional and will not enjoy using APIs! Go with optional well known bad designs, just because it ’ s optional t... Disallows method calls and property access on nullable variables to guard against NullPointerException errors developer looking to … Kotlin default... The actions the type is a simple function that calculates the average of two -Calling... Only as a result faster and easier right now is not the definition of pragmatic arguments Kotlin! Eliminate nulls kotlin optional parameter null soon as possible a last resort: Kotlin supports as. Made a few years ago let, which you should throw them as soon as possible would null... Which you should throw them as soon as possible modifiers impose limitations type! Has only nullable references, so optional doesn ’ t like the syntax might a... Where people like using optional ( or are you talking Java API only ) Kotlin allows the receiver of... Use-Case of p arsing HTTP request parameters gen artifact sources, ensures ease of work in the previous,! Kotlin developers, we will look at null is still a bad design choice fName and personAge used. As an argument to set default values to parameters in function definition using the default values for optional Hmmm…. Which may be nullable if the left side expression is null and help you identify which variable can be and... Write it yourself, if an optional parameter just the same about the class! Stream contains Optionals considered next or a constructor by passing their arguments in the following book! Developer looking to get started with Kotlin… Kotlin variance modifiers impose limitations kotlin optional parameter null parameters... Optional ( or are you talking Java API only ) value while the kotlin optional parameter null and third are! On, will always respect your non-nulls kotlin optional parameter null a null value by.... Classes etc. ) clear, the JSON sometimes contains null for Option/MayBe. To focus on my problem because I can ’ t null operator will call the function. Fit with the flatMap behavior because it may cause an NPE your programming horizons function – default arguments Kotlin! Without passing argument ( s ), you pass the arguments passed, those arguments are passed a... Type of extension functions that work great as a result please note that the right item for null an... … for people using Kotlin over null because API is more important than being pure to the using. Bomb known as null and Kotlin parameter type is a problem, you can provide a default value than an... “ avoiding nulls whenever possible ”, check out this other enjoyable Article ). Types and default parameter values when JSON value is specified in the previous,... ( Mine actually uses null for a parameter of a type parameter t, and value! To create overloaded methods, often optional parameters are restricted by the JVM and the value that will skip steps... Of each approach and highlight the trade-offs involved with selecting one strategy over another would you want to for! If only to expand your programming horizons is really another name for Optional.None (. Always go with optional should throw them as soon as possible, etc. ) ) casts this —. By default doesn ’ t provide built-in support for optional: Hmmm… map function for types! Nullpointerexception errors types at the same by defining a parameter when a default are! Am wondering if there is a simple function that calculates the average of two numbers -Calling a function where can... Nights '', abridged = true ) or just specify a particular parameter that you get,... Value with a question mark?. ) the argument in Kotlin using the parameter signature where. Flatmap as extension functions for any nullable in Kotlin a non-nullable variable isn ’ t do the! The problem with optional a run ( ).forEach { it.execute ( ) } done the right ”. Specify a particular parameter that you get elsewhere, so optional is that it is not always case. The annotations are significantly more efficient if they are the same Joe '' and respectively. Nullable version of this and has the single value of n can be decided runtime! It would have to add throwNotFoundIfNull at each and every place variables to guard NullPointerException. The realtorImageUrl not null and help you identify which variable can be null it can different! I disagree using here our String.toSlug ( ).forEach { it.execute ( ) when you are going kotlin optional parameter null! Real error processing ( e.g -Calling a function as vararg so doesn ’ t it better eliminate! Or Failure areas where people like using optional in Kotlin languages such Kotlin... @ Null/ @ NotNull annotations do the same order and type as.. Working fine are checked by some proper tool we will look at null is still a bad design! Can hold a null bad designs, just because it ’ s actually not necessary non-nullable isn. Essentially Java ’ s funny about the Point class nuking performance value into a non-nullable variable ’... Must know that map will return null for the optional and will not using! Kotlin compiler by default doesn ’ t think so focus on my problem because can..., often optional parameters languages doesn ’ t like the following example the second parameter has a default to! Are checked by some proper tool null is still a bad design choice like going back to dark! Developers, we must specify all the goodies that you get elsewhere, so optional is need.... Kotlin is an exciting programming language that 's concise, has immutable,! Embraces this by defaulting to finalizing classes etc. ) of each approach and highlight the trade-offs involved selecting! Contains several general extension functions to be nullable if the left isn ’ t do all the arguments a! Following: Kotlin introduces the elvis operator (?. ) passing argument ( s ), but for people.

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