In JS, I can access the function arguments through the arguments object.
I want to do the same thing somehow in ScalaJS, because I want to do some logging for functions and what parameters they got.
Is it possible?
Hmm -- interesting question. I honestly don't know if there is a way to access arguments per se. I would probably address the desire for logging the function arguments by using the sourcecode library, which is designed for that sort of thing, but I'll admit that I haven't tried that from Scala.js yet...
You cannot directly access the arguments object in Scala.js. However, you can export a method with varargs and it will work as expected in JavaScript:
object Logger {
#JSExportTopLevel("log")
def log(xs: js.Any*): Unit = {
xs.foreach(println)
}
}
This defines and exports log to the top level scope. In the JavaScript code, you can now call:
log(1, {}, {a: 1}, "foo");
As #justin-du-coeur suggested, you can use sourcecode for this. For example:
object Test extends js.JSApp {
def main(): Unit = {
a(1, "a")
b()
c("foo", "bar", "baz")
}
def trace()(implicit name: sourcecode.Name, args: sourcecode.Args): Unit = {
def makeArgList(as: Seq[sourcecode.Text[_]]): String =
as.map(a => f"${a.source} = ${a.value}").mkString("(", ", ", ")")
val argStr = args.value.map(makeArgList).mkString("")
println(f"${name.value}$argStr")
}
def a(arg1: Int, arg2: String): Unit = {
trace()
}
def b(): Unit = {
trace()
}
def c(x: String*): Unit = {
trace()
}
}
The output is as follows:
[info] Running Test
a(arg1 = 1, arg2 = a)
b()
c(x = WrappedArray(foo, bar, baz))
As you can see, trace can capture everything it needs from the context, so the result is even more boilerplate free than any JS solution I can think of.
Related
I'm trying to get started with writing a compiler plugin for scala 3. At this stage, it's primarily based on https://github.com/liufengyun/scala3-plugin-example/blob/master/plugin/src/main/scala/Phases.scala (and the accompanying youtube video explaining how it works).
It's been an interesting process so far, and I'm getting a bit of a feel for some aspects of the compiler.
As a first step, I'm simply trying to wrap a method body into a block, print whatever the returned object was going to be, and then return the object.
This differs from the original plugin mainly in that there was a single side-effecting method call added to each method - this is also assigning a local variable, (which I think is probably the cause of the problem), and moving the method body into a block.
I've produced as minimal of a working example as I could in a fork here: https://github.com/robmwalsh/scala3-plugin-example
The plugin compiles fine, seems to run as part of compilation as expected, and then blows up at runtime. I'm not entirely sure if this is me doing something wrong (not unlikely) or a bug in the compiler (less likely, but a distinct possibility!).
Can anybody please shed some light on why this isn't working? I don't know what flags should be set when creating a new Symbol, so that's one possibility, but there's heaps of stuff that sorta seemed to work so I rolled with it.
Here's where I'm at (the interesting bits):
...
override def prepareForUnit(tree: Tree)(using ctx: Context): Context =
//find the printLn method
val predef = requiredModule("scala.Predef")
printlnSym = predef.requiredMethod("println", List(defn.AnyType))
ctx
override def transformDefDef(tree: DefDef)(using ctx: Context): Tree =
val sym = tree.symbol
// ignore abstract and synthetic methods
if tree.rhs.isEmpty|| sym.isOneOf(Synthetic | Deferred | Private | Accessor)
then return tree
try {
println("\n\n\n\n")
println("========================== tree ==========================")
println(tree.show)
// val body = {tree.rhs}
val body = ValDef(
newSymbol(
tree.symbol, termName("body"), tree.symbol.flags, tree.rhs.tpe),
Block(Nil, tree.rhs)
)
// println(body)
val bodyRef = ref(body.symbol)
val printRes = ref(printlnSym).appliedTo(bodyRef)
// shove it all together in a block
val rhs1 = tpd.Block(body :: printRes :: Nil, bodyRef)
//replace RHS with new
val newDefDef = cpy.DefDef(tree)(rhs = rhs1)
println("====================== transformed ======================")
println(newDefDef.show)
newDefDef
} catch {
case e =>
println("====================== error ===========================")
println(e)
println(e.printStackTrace)
tree
}
...
test program for compiler plugin
object Test extends App:
def foo: String = "forty two"
def bar(x: String): Int = x.length
def baz(x: String, y: Int): String = x + y
baz(foo, bar(foo))
output during compile using plugin (exactly what I wanted! I got very excited at this point)
========================== tree ==========================
def foo: String = "forty two"
====================== transformed ======================
def foo: String =
{
val body: ("forty two" : String) =
{
"forty two"
}
println(body)
body
}
========================== tree ==========================
def bar(x: String): Int = x.length()
====================== transformed ======================
def bar(x: String): Int =
{
val body: Int =
{
x.length()
}
println(body)
body
}
========================== tree ==========================
def baz(x: String, y: Int): String = x.+(y)
====================== transformed ======================
def baz(x: String, y: Int): String =
{
val body: String =
{
x.+(y)
}
println(body)
body
}
output during runtime :'( (this changes depending on the code it's running on, but always the same theme)
Exception in thread "main" java.lang.VerifyError: Bad local variable type
Exception Details:
Location:
testing/Test$.body$2()I #0: aload_1
Reason:
Type top (current frame, locals[1]) is not assignable to reference type
Current Frame:
bci: #0
flags: { }
locals: { 'testing/Test$' }
stack: { }
Bytecode:
0000000: 2bb6 007d ac
at testing.Test.main(Example.scala)
Edit: I'm using scala 3.1.2
I was using the existing flags when creating my new symbol. Instead, I needed to use the Local flag, which I suppose makes sense.
I am creating in Scala and Cats a function that does some I/O and that will be called by other parts of the code. I'm also learning Cats and I want my function to:
Be generic in its effect and use a F[_]
Run on a dedicated thread pool
I want to introduce async boundaries
I assume that all my functions are generic in F[_] up to the main method because I'm trying to follow these Cat's guidelines
But I struggle to make these constraint to work by using ContextShift or ExecutionContext. I have written a full example here and this is an exctract from the example:
object ComplexOperation {
// Thread pool for ComplexOperation internal use only
val cs = IO.contextShift(
ExecutionContext.fromExecutor(Executors.newSingleThreadExecutor())
)
// Complex operation that takes resources and time
def run[F[_]: Sync](input: String): F[String] =
for {
r1 <- Sync[F].delay(cs.shift) *> op1(input)
r2 <- Sync[F].delay(cs.shift) *> op2(r1)
r3 <- Sync[F].delay(cs.shift) *> op3(r2)
} yield r3
def op1[F[_]: Sync](input: String): F[Int] = Sync[F].delay(input.length)
def op2[F[_]: Sync](input: Int): F[Boolean] = Sync[F].delay(input % 2 == 0)
def op3[F[_]: Sync](input: Boolean): F[String] = Sync[F].delay(s"Complex result: $input")
}
This clearly doesn't abstract over effects as ComplexOperation.run needs a ContextShift[IO] to be able to introduce async boundaries. What is the right (or best) way of doing this?
Creating ContextShift[IO] inside ComplexOperation.run makes the function depend on IO which I don't want.
Moving the creation of a ContextShift[IO] on the caller will simply shift the problem: the caller is also generic in F[_] so how does it obtain a ContextShift[IO] to pass to ComplexOperation.run without explicitly depending on IO?
Remember that I don't want to use one global ContextShift[IO] defined at the topmost level but I want each component to decide for itself.
Should my ComplexOperation.run create the ContextShift[IO] or is it the responsibility of the caller?
Am I doing this right at least? Or am I going against standard practices?
So I took the liberty to rewrite your code, hope it helps:
import cats.effect._
object Functions {
def sampleFunction[F[_]: Sync : ContextShift](file: String, blocker: Blocker): F[String] = {
val handler: Resource[F, Int] =
Resource.make(
blocker.blockOn(openFile(file))
) { handler =>
blocker.blockOn(closeFile(handler))
}
handler.use(handler => doWork(handler))
}
private def openFile[F[_]: Sync](file: String): F[Int] = Sync[F].delay {
println(s"Opening file $file with handler 2")
2
}
private def closeFile[F[_]: Sync](handler: Int): F[Unit] = Sync[F].delay {
println(s"Closing file handler $handler")
}
private def doWork[F[_]: Sync](handler: Int): F[String] = Sync[F].delay {
println(s"Calculating the value on file handler $handler")
"The final value"
}
}
object Main extends IOApp {
override def run(args: List[String]): IO[ExitCode] = {
val result = Blocker[IO].use { blocker =>
Functions.sampleFunction[IO](file = "filePath", blocker)
}
for {
data <- result
_ <- IO(println(data))
} yield ExitCode.Success
}
}
You can see it running here.
So, what does this code does.
First, it creates a Resource for the file, since close has to be done, even on guarantee or on failure.
It is using Blocker to run the open and close operations on a blocking thread poo (that is done using ContextShift).
Finally, on the main, it creates a default Blocker for instance, for **IO*, and uses it to call your function; and prints the result.
Fell free to ask any question.
To give you a minimal example:
object Main extends JSApp
{
val someThing: String = determineSomething("test")
def main(): Unit =
{
println(someThing)
}
}
Now, two possibilities here:
private def determineSomething(s: String): String = "succeeded"
If the project is executed like this, well, I get a log entry saying
succeeded
But when I use the more functional syntax:
private val determineSomething: (s: String) => "succeeded"
I get
TypeError: this.determineSomething$1 is null
I am curious as to the why this happens as in the (JVM) repl, both ways work perfectly fine.
I think what you want is something like this:
object Main extends JSApp {
private val determineSomething: String => String = (s: String) => "succeeded"
val someThing: String = determineSomething("test")
def main(): Unit = {
println(someThing)
}
}
The declaration of determineSomething needs to come before the declaration of something, otherwise the former will be uninitialized when the compiler attempts to initialize the latter.
I am fairly new to Scala macros, and am trying to write a very basic DSL.
I have the following Scala class:
abstract class SpecialFunction {
def apply(): Unit
}
and the following Scala macro:
def mImpl(c: Context)(bodyBlock: c.Expr[Unit]): c.Expr[X] =
c.universe.reify {
new X(new SpecialFunction {
override def apply() {
bodyBlock.splice
}
})
}
def m(bodyBlock: Unit): X = macro mImpl
So far so good. For example, it allows me to write:
def example = m {
println("Hello, world")
}
And this compiles down to:
def example = new X(new SpecialFunction {
override def apply() {
println("Hello, world")
}
})
But this formulation does not allow me to have local variables in such an "m block". For example, I cannot write:
def example = m {
val x = 7
println(x.toString)
}
At compile time I get the error:
symbol value x does not exist in example
However, what I want to achieve is this:
def example = new X(new SpecialFunction {
override def apply() {
val x = 7
println(x.toString)
}
})
(I think I understand why this is the case: sub-expressions are evaluated before being passed to the macro, and thus the reference to x is invalid).
So my question is this: How can I get the above to work? (I just want the extra code defined in the macro to be "copy-pasted" around the code in the "m block", like in a C++ macro.)
Any help would be greatly appreciated :-)
Your output expression is keeping a reference to the old x symbol, but instead it should be a reference to a new one. Therefore, you can reset all the junk references in the tree you aim to reuse by applying resetLocalAttrs from Context.
How about...?
def mImpl(c: Context)(bodyBlock: c.Expr[Unit]): c.Expr[X] =
c.universe.reify {
new X(new SpecialFunction {
override def apply() {
c.Expr[Unit](c.resetLocalAttrs(bodyBlock.tree)).splice
}
})
}
In this question, you can read a great explanation by the Scala Macros guy.
coming from my other question is there a way to get by-name-parameters for constructors working? I need a way to provide a code-block which is executed on-demand/lazy/by-name inside an object and this code-block must be able to access the class-methods as if the code-block were part of the class.
Following Testcase fails:
package test
class ByNameCons(code: => Unit) {
def exec() = {
println("pre-code")
code
println("post-code")
}
def meth() = println("method")
def exec2(code2: => Unit) = {
println("pre-code")
code2
println("post-code")
}
}
object ByNameCons {
def main(args: Array[String]): Unit = {
val tst = new ByNameCons {
println("foo")
meth() // knows meth() as code is part of ByNameCons
}
tst.exec() // ByName fails (executed right as constructor)
println("--------")
tst.exec2 { // ByName works
println("foo")
//meth() // does not know meth() as code is NOT part of ByNameCons
}
}
}
Output:
foo
method
pre-code
post-code
--------
pre-code
foo
post-code
This is because when you're making an instance like this:
val tst = new ByNameCons {
...
}
.. you're actually creating an anonymous class, like in java.
The above code is the same as:
val tst = new ByNameCons() { ... }
.. while the correct syntax for passing by-name is:
val tst = new ByNameCons( { ... } )
You cant omit parentheses the same way for constructors as with functions.
val tst = new ByNameCons( {
println("foo")
} )
Thought it is probably just easier to do this:
object ByNameCons {
def apply(code: => Unit) = new ByNameCons(code)
}
val tst = ByNameCons { // no "new" here -- can't mix traits either
println("foo")
}
I dont know why, but it appears that using {} or () when creating the class changes the behavior. Using the following class,
class Coder(code: => Unit) {
def exec = {
println("before")
code
println("after")}
}
}
scala> val brackets = new Coder {println("testing")}
testing
brackets: Coder = $anon$1#1af7a03
scala> brackets exec
before
after
Now instead if defined another way,
scala> val parens = new Coder(println("testing"))
parens: Coder = Coder#dca3ed
scala> parens exec
before
testing
after
as desired. It seems as if in the first notation, the compiler is interpreting the brackets as a block to be evaluated to Unit, instead of an anonymous function which, when called, evaluates to Unit.
FWIW, using ({ ... }) also works fine.