I'm stuck with a simple problem: I have a java.util.Optional and want to use the orElseThrow method to throw an exception if value is not present. However I cannot figure the right syntax to do this in Xtend. In Java, I would do:
Optional<String> host = ... // get from some method
host.orElseThrow(() -> new IllegalArgumentException("Host is not provided"));
But this does not compile in Xtend. The error message in Eclipse is like this (where MyClass is the name of my custom class):
Multiple markers at this line
- Type mismatch: cannot convert from Pair<MyClass, IllegalArgumentException> to Supplier<? extends Throwable>
- no viable alternative at input ')'
Please help!
Try something like:
val Optional<String> host = ... // get from some method
host.orElseThrow[new IllegalArgumentException("Host is not provided")]
See the documentation about lambda expressions.
(-> is an operator in Xtend but it is not related to lambda expressions. See the documentation about operators and search for the Pair Operator.)
Related
Can anyone help me to understand how "read" macro is implemented? I have the feeling that "do_read" function below is actually called, but could not figure out how that is done. I'm intrigued by the "SourceInfoTransform" class. Can anyone give me a hint on its usage?
The "SyncReadMem" implementation is listed below.
Thanks in advance for any help!
Best regards,
-Fei
sealed class SyncReadMem[T <: Data] private (t: T, n: BigInt, val readUnderWrite: SyncReadMem.ReadUnderWrite) extends MemBase[T](t, n) {
def read(x: UInt, en: Bool): T = macro SourceInfoTransform.xEnArg
/** #group SourceInfoTransformMacro */
def do_read(addr: UInt, enable: Bool)(implicit sourceInfo: SourceInfo, compileOptions: CompileOptions): T = {
val a = Wire(UInt())
a := DontCare
var port: Option[T] = None
when (enable) {
a := addr
port = Some(read(a))
}
port.get
}
}
The SourceInfoTransform is a scala macro that transforms the def read into the def do_read. The code for the macro is in src/main/scala/chisel3/internal/sourceinfo/SourceInfoTransform.scala of github.com/chipsalliance/chisel3. In that file there are a lot of transform classes for handling different chisel constructs with different numbers of arguments. The main use of the SourceInfoTransform is to get the line number of the Chisel/Scala source so it can be reported in Exceptions and in generated Firrtl and Verilog. Here is an article on macros, there are many more available.
Good luck.
Chick's answer is mostly correct--it's correct on the "how" and where to look, but slightly wrong on the "why":
The main use of the SourceInfoTransform is to get the line number of the Chisel/Scala source
This is not quite true, you can get source locators with merely having implicit sourceInfo: SourceInfo, and you could have that on the original def read if you wanted to. What the SourceInfoTransform macros do is resolve ambiguity when you want to do a bit extraction immediately following invoking the read method, eg.
myMem.read(addr, en)(16, 0)
If we had defined read with the implicits directly, the compiler would think you're trying to pass 16 and 0 as the implicit arguments, when in reality, you're trying to call .apply on the resulting T (if it's a subtype of Bits). The macro resolves the ambiguity so that the compiler knowns you're not trying to pass the implicits.
Here's a link to a talk where I describe this (warning while the little part about source locators and the macro are correct, much of this talk is out-of-date): https://youtu.be/2-ZiXNd9wbc?t=2756
I am writing my own javax.swing.ListModel<E>:
class Category(...)
class CategoryListModel extends javax.swing.ListModel[Category] {
// not shown: ListModel[Category] interface methods implemented here
...
}
However, when I try to set the list model with:
val myList: JList = ...
myList.setModel(new CategoryListModel)
The compiler gives me this error:
type mismatch;
found : CategoryListModel
required: javax.swing.ListModel[?0] where type ?0
myList.setModel(new CategoryListModel)
I thought CategoryListModel did implement ListModel[Category]??? I am trying to learn Scala by practicing using it lately, but I don't know how to interperet this error.
Okay, so the error was originating in the fact that my JList did not have a type parameter specified (it was generated by a GUI builder). I changed the declaration to:
JList<Category> // this particular file is a Java file
Now everything works.
I am not sure if I am missing something here. I would like to compare two classes that uses the same interface. Is this possible? I understand that the is operator compares classes, but is there any similar function when you use interfaces?
// works
var effect1 : CrazyEffect = new CrazyEffect();
var effect2 : SaneEffect = new SaneEffect();
trace(effect1 is effect2) // false
// does not work
var effect1 : ISoundEffect = new CrazyEffect();
var effect2 : ISoundEffect = new SaneEffect();
trace(effect1 is effect2)
1067: Implicit coercion of a value of type ISoundEffect to an unrelated type Class.
Note the differences between concepts of a class and of an object. The former is a data type whereas the latter is a runtime instance of it, a variable. is operator can not compare one variable to another.
According to language reference
is Operator
Evaluates whether an object is compatible with a specific data type,
class, or interface. Use the is operator instead of the instanceof
operator for type comparisons. You can also use the is operator to
check whether an object implements an interface.
In other words, compiler expects the first operand to be a variable whereas the second operand should be a type identifier.
var sample:String = "Object is an instance of a class.";
^^^ ^^^
variable type identifier
However effect2 is not a type identifier but a variable. Hence the error message.
Unfortunately there is no generic operator to test for interface commonality. The only alternative is:
trace((s is ISoundEffect) && (t is ISoundEffect));
Update
Checking whether objects are instances of a same class can be done by comparing class names:
if (getQualifiedClassName(effect1) == getQualifiedClassName(effect2)) {
// true
}
For in depth discussion see Get the class used to create an object instance in AS3
Even though it will work with getQualifiedClassName, there's a better method to check whether two objects are instances of the same class:
a['constructor'] === b['constructor']
getQualifiedClassName is very slow and CPU intensive. Since the above code just compares property values it is lightning fast. And yes, constructor IS a property of every object, however FB will complain if you try to access it using dot-notation, that's why I use dynamic property access.
In order to avoid Java exceptions I'm using Scala's exception handling class.
However, when compiling the following snippet:
import scala.util.control.Exception._
val cls = classManifest[T].erasure
// Invoke special constructor if it's available. Otherwise use default constructor.
allCatch opt cls.getConstructor(classOf[Project]) match {
case Some(con) =>
con.newInstance(project) // use constructor with one Project param
case None =>
cls.newInstance // just use default constructor
};
I receive the following error:
error: type mismatch;
[scalac] found : java.lang.reflect.Constructor[_]
[scalac] required: java.lang.reflect.Constructor[_$1(in method init)] where
type _$1(in method init)
[scalac] allCatch opt cls.getConstructor(classOf[Project]) match {
[scalac] ^
[scalac] one error found
What's going on here and how can I fix it?
I have no explanation, but a much shorter example which I hope pinpoint where the problem occurs. I think it is not related at all to exceptions, nor to reflection. Whether this behavior is an arcane but correct consequence of the specification or a bug, I have no idea.
val untypedList : List[_] = List("a", "b")
val typedList : List[String] = List("a", "b")
def uselessByName[A](a: => A) = a
def uselessByValue[A](a: A) = a
uselessByName(untypedList) fails with the same error as your code. The other combinations do not. So combination of a method with a generic call-by-name argument, called with a parameter of a generic with an existential type.
uselessByName[List[_]](untypedList) works, so I guess if you call explicitly opt[Constructor[_]] it might work too.
The type inference scheme has gotten confused by the types available--specifically, by the type of cls. If we write generic code:
def clser[A](cls: Class[A]) = allCatch opt cls.getConstructor(classOf[Project])
then it works perfectly okay. But you're probably doing something else--I can't tell what because you didn't provide the code--and this results in a mismatch between the expected and actual types.
My currently solution is to cast the constructor explicitly.
cls.getConstructor(classOf[Project]) becomes cls.getConstructor(classOf[Project]).asInstanceOf[Constructor[Project]].
I'm still wondering about the actual error and if there are better ways to resolve it -- so I'm going to leave this open.
What do i need to do to extract the value for friends_count. i noticed that screen_name are already define in the Status object and case class. Do still require to extends Js or JsObject different
object TweetDetails extends Js { val friends_count = 'friends_count ? num }
and then pattern match it against each json object in the list of JsObjects as represented below. The symbols are confusing:
scala> val friends_count = 'friends_count ! num // I wish SO understood Scala's symbols
val twtJsonList = http(Status("username").timeline)
twtJsonList foreach {
js =>
val Status.user.screen_name(screen_name) = js
val Status.text(text) = js
val friends_counts(friends_count) = js //i cannot figure out how to extract this
println(friends_count)
println(screen_name)
println(text)
}
Normally, Scala symbols can be thought of as a unique identifier which will always be the same. Every symbol that is lexi-graphically identical refers to the exact same memory space. There's nothing else that's special about them from Scala's point of view.
However, Dispatch-Json pimps out symbols making them JSON property extractors. To see the code which is responsible for the pimping, check out the SymOp class and the rest of the JsonExtractor.scala code.
Let's write some code which solves the problem you are looking at and then analyze what's going on:
trait ExtUserProps extends UserProps with Js {
val friends_count = 'friends_count ! num
}
object ExtUser extends ExtUserProps with Js
val good_stuff = for {
item <- http(Status("username").timeline)
msg = Status.text(item)
user = Status.user(item)
screen_name = ExtUser.screen_name(user)
friend_count = ExtUser.friends_count(user)
} yield (screen_name, msg, friend_count)
The first thing that we're doing is extending the UserProps trait in the Dispatch-Twitter module to give it a friends_count extractor and then defining a ExtUser object which we can use to get access to that extractor. Because the ExtUserProps extends UserProps, which also extends Js, we get the method sym_add_operators in scope which turns our symbol 'friends_count into a SymOp case class. We then call the ! method on that SymOp which we then pass the Extractor num to, which creates an Extractor that looks for a property "friends_count" on a JSON object and then parses it as a number before returning. Quite a bit going on there for such a small bit of code.
The next part of the program is just a for-comprehension that calls out to the Twitter timeline for a user and parses it into JsObjects which represent each status item, them we apply the Status.text extractor to pull out the status message. Then we do the same to pull out the user. We then pull the screen_name and friend_count out of the user JsObject and finally we yield a Tuple3 back with all of the properties we were looking for. We're then left with a List[Tuple3[String,String,BigDecimal]] which you could then iterate on to print out or do whatever with.
I hope that clears some things up. The Dispatch library is very expressive but can be a little tough to wrap your head around as it uses a lot of Scala tricks which someone just learning Scala won't get right away. But keep plugging around and playing with, as well as looking at the tests and source code, and you'll see how to create powerful DSL's using Scala.