I want to call this function in a 3rd party library (xmltree-rs):
pub fn parse<R: Read>(r: R) -> Element {
The expected use case is to give it a file, like so:
let e: Element = Element::parse(File::open("tests/data/01.xml").unwrap());
However I have a String which I want to pass, vaguely like this:
xmltree::Element::parse("<example>blah</example>".to_owned());
However this, of course, gives an error:
error: the trait `std::io::Read` is not implemented for the type `collections::string::String` [E0277]
How would I do this, short of writing a temporary file? (e.g in Python I would use the StringIO module to wrap the string in a file-like object)
To find out what implements a trait, go to the bottom of the page for that trait.
In this case, the most promising looking implementers are &'a [u8] and Cursor<Vec<u8>>.
You can obtain a &[u8] from a &str or String by calling as_bytes(), taking note of automatic dereferencing and Deref on Strings. Alternatively, you could use a byte literal if you are only using the ASCII subset.
Related
I am a newbie in Kotlin, I just started to learn it,
I get the following code example about literal/high order function:
fun myHigherOrderFun(functionArg: (Int)->String) = functionArg(5)
println ( myHigherOrderFun { "The Number is $it" })
prints "The Number is 5"
Which I have difficulty to understand: the function myHigherOrderFun get a lambda function as parameter but i can't understand, where is the (Int) input parameter? I see is passed in functionArg(5)... but i can't realize how is possible that?
Thanks in advance.
To start from the beginning, in Kotlin functions are first-class types, just like numbers and Strings and stuff. So a function can take another function as a parameter, and/or return a function as its result. A function which does this is called a ‘higher-order function’.
And that's what you have in your example! The line:
fun myHigherOrderFun(functionArg: (Int)->String) = functionArg(5)
defines myHigherOrderFun() as a function which takes one parameter, which is itself a function taking a single Int parameter and returning a String. (myHigherOrderFun() doesn't specify an explicit return type, so it's inferred to be a String too.)
The next line is probably where things are less clear:
println(myHigherOrderFun{ "The Number is $it" })
The first non-obvious thing is that it's calling myHigherOrderFun() with a parameter. Because that parameter is a lambda, Kotlin lets you omit the usual (…), and use only the braces.
The other non-obvious thing is the lambda itself: { "The Number is $it" }. This is a literal function taking one parameter (of unspecified type).
Normally, you'd have to specify any parameters explicitly, e.g.: { a: Char, b: Int -> /* … */ }. But if there's exactly one parameter, and you aren't specifying its type, then you can skip that and just refer to the parameter as it. That's what's happening here.
(If the lambda didn't reference it, then it would be a function taking no parameters at all.)
And because the lambda is being passed to something expecting a function taking an Int parameter, Kotlin knows that it must be an Int, which is why we can get away without specifying that.
So, Kotlin passes that lambda to the myHigherOrderFun(), which executes the lambda, passing 5 as it. That interpolates it into a string, which it returns as the argument to println().
Many lambdas take a single parameter, so it gets used quite a lot in Kotlin; it's more concise (and usually more readable) than the alternative. See the docs for more info.
I am new in Kotlin and I am trying to understand type alias and functions.
I have the following example:
interface EmptyInterface
typealias GenericCase<T> = T.(EmptyInterface) -> T
val myFunctionVariable: GenericCase<String> = {
_ -> "Hello world!"
}
So far what I understand is that I extend what ever T is defined with a function that accepts as argument an EmptyInterface and returns a T.
So the myFunctionVariable is a function that should be called passing an EmptyInterface
But the following code does not compile
class a: EmptyInterface
println("${myFunctionVariable(a())}")
I need to pass a String as the first parameter:
class a: EmptyInterface
println("${myFunctionVariable("",a())}")
Why is the string needed as the first parameter? T.(EmptyInterface) -> T which in this case is String.(EmptyInterface) -> String has only 1 parameter.
Could someone please explain this?
The T. in the type T.(EmptyInterface) -> T means that this function is an extension function on T. So the regular way to call this function is to acquire a T instance, and call it on that instance, as if it was a member function. In the case of your example, where you chose T to be a String, you have to call the function on a String instance:
"foo".myFunctionVariable(a())
The syntax you've used is an alternative way to call this extension, passing in the receiver as if it was the first parameter of the function (which, at the bytecode level, it actually is):
myFunctionVariable("foo", a())
If you wish to use your function with this syntax, however, it's better to declare it to take two parameters, as this invocation of an extension function can be quite unexpected.
(There's some more info about how you can go back and forth between a function that's an extension on a type and one that takes it as a first parameter in this answer.
In the type T.(EmptyInterface) -> T, the first T is the receiver: an instance that the function is called upon, and which becomes this within the function definition. (Similar to an extension function.)
Effectively, the receiver becomes a hidden first parameter to the function; as you discovered, if you try to call the function directly, you'll need to give it explicitly.
The language spec for it is here; some other answers may also help.
I know how to read/interpret Java code and I can write it. However being new to kotlin I find code like below hard to read. Perhaps I am missing key concepts in the language.
But, how would you go about interpreting this code? Where do you propose one to start reading it in order to understand this piece of code quickly and efficiently? Left to right? Right to left? Break down parameters first? Look at return values?
inline fun <T : Any, R> ifNotNull(input: T?, callback: (T) -> R): R? {
return input?.let(callback)
}
So, like Java this is a generic function. It has two type parameters T which is of type 'Any' ('Any' is like 'Object' in Java) and R. The input parameter is a nullable T, as denoted by the question mark. Nullable types mean that the value can be null. The other function parameter is a function that takes in a T (non nullable type) and returns R. The return type of the function is a nullable R. The body of the function says that if input is not null, call and pass that to the callback and return that value. If input is null, then null is what gets returned.
Let's dissect the function definition piece by piece:
inline: Indicates that the code of the function will be copied directly to the call site, rather than being called like a normal function.
fun: We're defining a function.
<T : Any, R>: The function takes two generic type parameters, T and R. The T type is restricted to the Any type (which is Kotlin's Object-type). That might seem redundant, but what it actually says is that T cannot be a nullable type (Any?).
ifNotNull: The name of the function.
input: T?: The first parameter of type T?. We can put the ? on the T type here because we restricted it to non-nullable types in the type declaration.
callback: (T) -> R: The second parameter is of type (T) -> R, which is a function type. It's the type of a function that takes a T as input and returns an R.
: R?: The function returns a value of type R or null.
return input?.let(callback): The function body. The let function takes a function parameter, calls it with its receiver (input), and then returns the result of the function. The ? after input says that let will be called only if input is not null. If input is null, then the expression will return null.
The function is equivalent to this Java method (except for the inlining and nullable types):
public <T, R> R ifNotNull(final T input, final Function<T, R> callback) {
if (input == null) {
return null;
}
return callback.apply(input);
}
Matt's answer explains everything well in one go; I'll try to look at how you might go about reading such code.
Skipping over the first word for now, the most important thing is the second word: fun. So the whole thing is defining a function. That tells you what to expect from the rest.
The braces tell you that it's a block function, not a one-liner, so the basic structure you're expecting is: fun name(params): returnType { code }. The rest is filling in the blanks! (This fits the general pattern of Kotlin declarations, where the type comes second, after a colon. The Java equivalent would of course be more like returnType name(params) { code }.)
As with Java, the stuff in angle brackets is giving generic parameters, so we can skip that for now and go straight to the next most important bit, which is the name of the function being defined: ifNotNull.
Armed with those, we can read the rest. inline is a simple modifier, telling you that the function will be inlined by the compiler. (That enables a few things and restricts a few others, but I wouldn't worry about that now.)
The <T : Any, R> gives the generic parameter types that the function uses. The first is T, which must be Any or a subtype; the second is R, which is unrestricted.
(Any is like Java's Object, but can't be null; the topmost type is the related Any?, which also allows null. So except for the nullability, that's equivalent to the Java <T extends Object, R>.)
Going on, we have the function parameters in parentheses. Again, there are two: the first is called input, and it's of type T?, which means it accepts any value of type T, and also accepts null. The second parameter is called callback, and has a more complicated type, (T) -> R: it's a function which takes a T as its parameter, and returns an R. (Java doesn't have function types as such, so that probably looks strangest. Java's nearest equivalent is Function<R, T>.)
After the parentheses comes the return type of this function itself, R?, which means it can return either an R or null.
Finally, in braces is the actual code of the function. That has one line, which returns the value of an expression. (Its effect is to check whether the value of input is null: if so, it returns the null directly. Otherwise, it calls the callback function given in the parameter, passing input as its parameter, and returns its result.)
Although that's a short declaration, it's quite abstract and packs a lot in, so it's no wonder you're finding it hard going! (The format is similar to a Java method declaration — but Kotlin's quite expressive, so equivalent code tends to be quite a bit shorter than Java. And the generics make it more complex.) If you're just starting to learn Kotlin, I'd suggest something a bit easier :-)
(The good news is that, as in Java, you don't often need to read the stdlib code. Although Kotlin's doc comments are rarely up to the exemplary level of Java's, they're still usually enough.)
How can i make an arraylist of functions, and call each function easily? I have already tried making an ArrayList<Function<Unit>>, but when i tried to do this:
functionList.forEach { it }
and this:
for(i in 0 until functionList.size) functionList[i]
When i tried doing this: it() and this: functionList[i](), but it wouldn't even compile in intellij. How can i do this in kotlin? Also, does the "Unit" in ArrayList<Function<Unit>> mean return value or parameters?
Just like this:
val funs:List<() -> Unit> = listOf({}, { println("fun")})
funs.forEach { it() }
The compiler can successfully infer the type of funs here which is List<() -> Unit>. Note that () -> Unit is a function type in Kotlin which represents a function that does not take any argument and returns Unit.
I think there are two problems with the use of the Function interface here.
The first problem is that it doesn't mean what you might think. As I understand it, it's a very general interface, implemented by all functions, however many parameters they take (or none). So it doesn't have any invoke() method. That's what the compiler is complaining about.
Function has several sub-interfaces, one for each 'arity' (i.e. one for each number of parameters): Function0 for functions that take no parameters, Function1 for functions taking one parameter, and so on. These have the appropriate invoke() methods. So you could probably fix this by replacing Function by Function0.
But that leads me on to the second problem, which is that the Function interfaces aren't supposed to be used this way. I think they're mainly for Java compatibility and/or for internal use by the compiler.
It's usually much better to use the Kotlin syntax for function types: (P1, P2...) -> R. This is much easier to read, and avoids these sorts of problems.
So the real answer is probably to replace Function<Unit> by () -> Unit.
Also, in case it's not clear, Kotlin doesn't have a void type. Instead, it has a type called Unit, which has exactly one value. This might seem strange, but makes better sense in the type system, as it lets the compiler distinguish functions that return without an explicit value, from those which don't return. (The latter might always throw an exception or exit the process. They can be defined to return Nothing -- a type with no values at all.)
I'm trying to apply Observable.zip to a list of observables. It works but I would like it to conserve the type information. The problem happens with the snippet below:
val observable1 = Observable.fromArray(1, 2, 3)
val observable2 = Observable.fromArray(1, 2, 3)
// result is a Array<Any>. Could it be an Array<Int> instead ?
val result = Observable.zip(listOf(observable1, observable2), {a -> a}).blockingFirst()
It feels like zip should be able to know that my items are of type Int. In this very specific case, I could certainly cast but I also have more complex types and would feel much better if I could conserve type. Do I miss something ?
Unfortunately, the Java (and Kotlin) type system doesn't let you (or us library writers) do such type preservation; you have to manually cast back the a Object[] elements to their respective type.
This is done for you with the 2-9 argument zip() overloads and the zipWith instance operator.
The underlying problem is that one can't create a generic array as well as the type erasure itself. If we allowed Function<T[], R> in the signature, we would still have to create new Object[] for the invocation. However, your Function<Integer[], R> implementation would try to cast Object[] into Integer[] which is not allowed and fails with ClassCastException.
This is also described in the zip javadoc:
Note on method signature: since Java doesn't allow creating a generic array with new T[], the implementation of this operator has to create an Object[] instead. Unfortunately, a Function<Integer[], R> passed to the method would trigger a ClassCastException.