I have a bunch of strings like these in my already existing and quit big template file:
Hello $World I say hello to $User, too
I was thinking I could somehow let kotlin parse/search&replace this file as a kotlin string, and I'd just have to set the variables World and User to get an evaluated string... How is this possible?
This is not a kotlin source file, but a file that's beeing read by my kotlin program.
Why I want to do this? I used bash's envsubst before, and had to move away from this, since things were getting too complicated. But now I have no easy way to replace strings in a file anymore...
Thanks
What you want is called a template engine.
You don't really need an eval (which would allow running a full Kotlin application from a String) in this case.
For example, FreeMarker templates use a syntax that's similar to Kotlin template Strings, so a template may look like this:
<h1>Welcome ${user}!</h1>
Then, from Kotlin, you can evaluate the template with a Map holding the template bindings (variables the template can use) like this:
val user = "joe"
val bindings = mapOf("user" to user)
val cfg = new Configuration(Configuration.VERSION_2_3_29)
cfg.directoryForTemplateLoading = File("/where/you/store/templates")
val template = cfg.getTemplate("test.ftlh")
// write the resolved template to stdout
val out = OutputStreamWriter(System.out)
template.process(bindings, out)
See a Java example here: https://freemarker.apache.org/docs/pgui_quickstart_all.html
There are many other template engines, and the KTor site lists a few:
https://ktor.io/docs/working-with-views.html
If you're using KTor, BTW, it makes it much easier to use template engines... your framework may even have similar.
Warning: Please see my other answer first: you probably don't want to run a full Kotlin script just to parse some text file with variables... use a template engine for that.
It's possible to execute Kotlin scripts, i.e. eval Kotlin code, but it requires that you basically ship the Kotlin compiler with your application.
This GitHub project, KtsRunner, for example, does that, so you can do this:
val scriptContent = "5 + 10"
val fromScript: Int = KtsObjectLoader().load<Int>(scriptContent))
println(fromScript)
// >> 15
It requires some hacking though, is very slow and it uses a bunch of Kotlin "internal" APIs to work.
See the full list of libraries this project uses here:
https://github.com/s1monw1/KtsRunner/blob/master/lib/build.gradle.kts#L21
A "proper" KEEP proposal (discussion here) exists to add first-class support for this in Kotlin, but it's not finalized yet.
Currently, it looks something like this:
fun evalFile(scriptFile: File): ResultWithDiagnostics<EvaluationResult> {
val compilationConfiguration = createJvmCompilationConfigurationFromTemplate<MainKtsScript>()
val evaluationConfiguration = createJvmEvaluationConfigurationFromTemplate<MainKtsScript>()
return BasicJvmScriptingHost().eval(scriptFile.toScriptSource(), compilationConfiguration, evaluationConfiguration)
}
But it'll probably change before being released.
Related
I don't or can't modify the Java source code. The goal to configure just the Kotlin compiler to know what is nullable and what isn't.
You can specify the type manually if you know something will never be null. For example, if you have the following Java code:
public static Foo test() {
return null;
}
and you call it in Kotlin like this:
val result = Foo.test()
then result will have a type of Foo! by default – which means it can be either Foo or Foo?.. the compiler doesn't have enough information to determine that.
However, you can force the type manually:
val result: Foo = Foo.test()
// use "result" as a non-nullable type
Of course, if at runtime that is not true, you'll get a NullPointerException.
For reference, please check the documentation.
I don't know of a way to configure the compiler for this, but IntelliJ IDEA has a feature that allows you to add annotations to code via an XML file called external annotations.
You can add the Jetbrains #Nullable and #NotNull annotations to library code, but when I've tried it, it only results in compiler warnings rather than errors when you use incorrect nullability in your code. These same annotations generate compiler errors when used directly in the source code. I don't know why there is a difference in behavior.
You can use extension functions for this. If you have a method String foo() in the class Test, you can define the extension function
fun Test.safeFoo(): String = this.foo()!!
The advantage is that the code is pretty obious.
The disadvantage of this approach is that you need to write a lot of boiler plate code. You also have to define the extension function in a place where all your modules or projects can see it. Also, writing that much code just to avoid !! feels like overkill.
It should also be possible to write a Kotlin compiler extension which generates them for you but the extension would need to know which methods never return null.
Is there a way to enforce 1..* parameters in Kotlin that will still allow the spread operator?
I've tried:
class Permission(
// 1..n compliance
accessiblePage: Webpage,
vararg accessiblePages: Webpage
) {
And that does enforce 1..*, but it also means that Permission(*pages) won't work, so that's a pretty awkward interface.
Is there an easy way to enforce 1..* without a runtime constructor error?
There is, unfortunately, no way to check this in Kotlin at compile time aside from the way you mentioned. Since vararg parameters are really just syntactic sugar for an array, your code is essentially
class Permission (
accessiblePage: Webpage,
accessiblePages: Array<Webpage>
)
So the question then becomes "Can you ensure that an array has at least one element in it at compile time?" For most languages, that's a clear no, although the Kotlin team did at one point experiment with it:
[C]urrently, Kotlin compiler doesn't collect static information about
collections size. FYI, at some point Kotlin team tried to collect such
information and use it for warnings about possible
IndexOutOfBoundException and stuff like that, but it was found that
there were a very little demand on such diagnostics in real-life
projects, so, given complexity of such analysis, it was abandoned[.]
(https://github.com/Kotlin/KEEP/issues/139#issuecomment-405551324)
It's possible that this metadata will be added at some point, but you shouldn't expect it soon.
That said, you could always combine a runtime check in the case of an Array with an overloaded signature in the case of varargs. This would mean that your vararg example would work the same, but passing an array to the function would subject it to a runtime check (you'd also not have to use the spread operator anymore):
class Permission (
accessiblePage: Webpage
vararg accessiblePages: Webpage
) {
constructor(accessiblePages: Array<Webpage>) {
require(accessiblePages.isNotEmpty()) {
"Must have at least one accessible page."
}
}
}
called like
val permission1 = Permission(Webpage(), Webpage())
val permission2 = Permission() // Would fail at compile time
val pages = arrayOf()
val permission3 = Permission(pages) // Would fail at runtime. Note also the lack of the spread operator.
I'm new to kotlin. Ive always used the map transform with curly braces. Then -
Why does this work ->
val x = someList.map(::SomeConstructor)
and this doesn't?
val x = someList.map{ ::SomeConstructor }
I didn't find usage of map with circular brackets anywhere on the online tutorials.
Please try to explain in detail, or provide suitable reference article.
What you ask is explained in this official documentation.
If and only if the last argument of a function is a lambda, you can extract it from the call paranthesis, to put it inline on the right of the function. It allows a nicer DSL syntax.
EDIT: Let's make an example :
One of the good use-case is context programming. Imagine you've got a closeable object. You want to delimit its usage to ensure it's properly closed once not needed anymore. In Java, you've got the try-with-resources:
try (final AutoCloseable myResource = aquireStuff()) {
// use your resource here.
}
Kotlin provide the use function. Now, you can do either :
acquireStuff().use( { doStuff1(it) ; doStuff2(it) } )
or write :
acquireStuff().use {
doStuff1(it)
doStuff2(it)
}
It looks like a Java try-w-resource, but is extensible to any of your API. Allowing you to design libraries giving advanced constructs to end-users.
Can I have serializable lambda in Kotlin? I am trying to use Jinq library from Kotlin, but it requires serializable lambdas. Is there any syntax that makes it possible?
Update:
My code:
var temp=anyDao.streamAll(Task::class.java)
.where<Exception,Task> { t->t.taskStatus== TaskStatus.accepted }
.collect(Collectors.toList<Task>());
I am getting this error:
Caused by: java.lang.IllegalArgumentException:
Could not extract code from lambda.
This error sometimes occurs because your lambda references objects that aren't Serializable.
All objects referenced in lambda are serializable (code results in no errors in java).
Update 2
After debugging it seems that kotlin lambda isn't translated into java.lang.invoke.SerializedLambda which is required by Jinq to get information from. So the problem is how to convert it to SerializedLambda.
I'm the maker of Jinq. I haven't had the time to look at Kotlin-support, but based on your description, I'm assuming that Kotlin compiles its lambdas into actual classes or something else. As such, Jinq would probably need some special code for cracking open Kotlin lambdas, and it may also need special code for handling any unusual Kotlin-isms in the generated code. Jinq should be capable of handling it because it was previously retrofitted to handle Scala lambdas.
If you file an issue in the Jinq github about it, along with a small Kotlin example (in both source and .class file form), then I can take a quick peek at what might be involved. If it's small, I can make those changes. Unfortunately, if it looks like a lot of work, I don't think I can really justify putting a lot of resources into adding Kotlin support to Jinq.
I have no experience on Jinq, but according to the implementation in GitHub and my experience of using Java Library in Kotlin.
ref: https://github.com/my2iu/Jinq/blob/master/api/src/org/jinq/orm/stream/JinqStream.java
You can always fall back to use the native Java Interface in Kotlin.
var temp = anyDao.streamAll(Task::class.java)
.where( JinqStream.Where<Task,Exception> { t -> t.taskStatus == TaskStatus.accepted } )
.collect(Collectors.toList<Task>());
// Alternatively, You you can import the interface first
import org.jinq.orm.stream.JinqStream.*
...
// then you can use Where instead of JinqStream.Where
var temp = anyDao.streamAll(Task::class.java)
.where(Where<Task,Exception> { t -> t.taskStatus == TaskStatus.accepted } )
.collect(Collectors.toList<Task>());
Or make a custom extension to wrap the implementation
fun JinqStream<T>.where(f: (T) -> Boolean): JinqStream<T> {
return this.where(JinqStream.Where<T,Exception> { f(it) })
}
Disclaimer: The above codes have not been tested.
There are some nice libraries for testing in Scala (Specs, ScalaTest, ScalaCheck). However, with Scala's powerful type system, important parts of an API being developed in Scala are expressed statically, usually in the form of some undesirable or disallowed behavior being prevented by the compiler.
So, what is the best way to test whether something is prevented by the compiler when designing an library or other API? It is unsatisfying to comment out code that is supposed to be uncompilable and then uncomment it to verify.
A contrived example testing List:
val list: List[Int] = List(1, 2, 3)
// should not compile
// list.add("Chicka-Chicka-Boom-Boom")
Does one of the existing testing libraries handle cases like this? Is there an approach that people use that works?
The approach I was considering was to embed code in a triple-quote string or an xml element and call the compiler in my test. Calling code looking something like this:
should {
notCompile(<code>
val list: List[Int] = List(1, 2, 3)
list.add("Chicka-Chicka-Boom-Boom")
</code>)
}
Or, something along the lines of an expect-type script called on the interpreter.
I have created some specs executing some code snippets and checking the results of the interpreter.
You can have a look at the Snippets trait. The idea is to store in some org.specs.util.Property[Snippet] the code to execute:
val it: Property[Snippet] = Property(Snippet(""))
"import scala.collection.List" prelude it // will be prepended to any code in the it snippet
"val list: List[Int] = List(1, 2, 3)" snip it // snip some code (keeping the prelude)
"list.add("Chicka-Chicka-Boom-Boom")" add it // add some code to the previously snipped code. A new snip would remove the previous code (except the prelude)
execute(it) must include("error: value add is not a member of List[Int]") // check the interpreter output
The main drawback I found with this approach was the slowness of the interpreter. I don't know yet how this could be sped up.
Eric.