I have found an example code for a Scala runtime scripting in answer to Generating a class from string and instantiating it in Scala 2.10, however the code seems to be obsolete for 2.11 - I cannot find any function corresponding to build.setTypeSignature. Even if it worked, the code seems hard to read and follow to me.
How can Scala scripts be compiled and executed in Scala 2.11?
Let us assume I want following:
define several variables (names and values)
compile script
(optional improvement) change variable values
execute script
For simplicity consider following example:
I want to define following variables (programmatically, from the code, not from the script text):
val a = 1
val s = "String"
I want a following script to be compiled and on execution a String value "a is 1, s is String" returned from it:
s"a is $a, s is $s"
How should my functions look like?
def setupVariables() = ???
def compile() = ???
def changeVariables() = ???
def execute() : String = ???
Scala 2.11 adds a JSR-223 scripting engine. It should give you the functionality you are looking for. Just as a reminder, as with all of these sorts of dynamic things, including the example listed in the description above, you will lose type safety. You can see below that the return type is always Object.
Scala REPL Example:
scala> import javax.script.ScriptEngineManager
import javax.script.ScriptEngineManager
scala> val e = new ScriptEngineManager().getEngineByName("scala")
e: javax.script.ScriptEngine = scala.tools.nsc.interpreter.IMain#566776ad
scala> e.put("a", 1)
a: Object = 1
scala> e.put("s", "String")
s: Object = String
scala> e.eval("""s"a is $a, s is $s"""")
res6: Object = a is 1, s is String`
An addition example as an application running under scala 2.11.6:
import javax.script.ScriptEngineManager
object EvalTest{
def main(args: Array[String]){
val e = new ScriptEngineManager().getEngineByName("scala")
e.put("a", 1)
e.put("s", "String")
println(e.eval("""s"a is $a, s is $s""""))
}
}
For this application to work make sure to include the library dependency.
libraryDependencies += "org.scala-lang" % "scala-compiler" % scalaVersion.value
Related
My question is how do I access DownloadFiles.fileURLList property in sbt console (Scala REPL)?
I created a SBT Scala project and have this code at src/main/scala/DownloadFiles.scala
import sys.process._
import biz.neumann.url.NiceURLCodecs._
import java.net._
import java.io._
object DownloadFiles extends App {
val fileURLList = Array(
"https://www.example.com/file1.txt",
"https://www.example.com/file2.txt",
"https://www.example.com/file3.txt"
)
fileURLList.foreach(url => {
val fileName = "downloaded-files/" + new File(new URI(url).getPath).getName;
println(url.decode.encode + ": " + fileName)
new URL(url.decode.encode) #> new File(fileName) !!
})
}
I opened Scala REPL by using sbt console and when I access fileURLList property of DownloadFiles object I get null value (unevaluated) as shown below.
scala> DownloadFiles.fileURLList
res0: Array[String] = null
What I instead need is evaluated value of the property of that object.
Here's the build.sbt if it helps:
scalaVersion := "2.12.10"
name := "image-downloader"
organization := "in.bhargav.inc"
version := "1.0"
libraryDependencies += "org.scala-lang.modules" %% "scala-parser-combinators" % "1.1.2"
libraryDependencies += "biz.neumann" % "nice-url-encode-decode_2.12" % "1.5"
This is a consequence of Hello extending App which indirectly extends DalayedInit which changes the initialisation semantics:
#nowarn("""cat=deprecation&origin=scala\.DelayedInit""")
trait App extends DelayedInit
Quick fix is to make fileURLList lazy
➜ scala
Welcome to Scala 2.13.5 (OpenJDK 64-Bit Server VM, Java 1.8.0_282).
Type in expressions for evaluation. Or try :help.
scala> object DownloadFiles extends App {
| val fileURLList = Array("https://www.example.com/file1.txt")
| }
object DownloadFiles
scala> DownloadFiles.fileURLList
val res1: Array[String] = null
scala> object DownloadFiles extends App {
| lazy val fileURLList = Array("https://www.example.com/file1.txt")
| }
object DownloadFiles
scala> DownloadFiles.fileURLList
val res2: Array[String] = Array(https://www.example.com/file1.txt)
This behaviour changed in Scala 3 which drops DelayedInit semantics
➜ scala3-repl
scala> object DownloadFiles extends App {
| val fileURLList = Array("https://www.example.com/file1.txt")
| }
// defined object DownloadFiles
scala> DownloadFiles.fileURLList
val res0: Array[String] = Array(https://www.example.com/file1.txt)
The previous functionality of App, which relied on the "magic"
DelayedInit trait, is no longer available. App still exists in limited
form for now, but it does not support command line arguments and will
be deprecated in the future. If programs need to cross-build between
Scala 2 and Scala 3, it is recommended to use an explicit main method
with an Array[String] argument instead.
The first part of your code uses a variable called fileURLList. The second part of your code and your SBT command use imageURLList. imageURLList is never declared as a variable, therefore it is null. Find and replace your code from fileURLList to imageURLList and I bet it will do what you were expecting.
I am a little surprised you didn't get other errors though.
I want to parse the following JSON object using Scala:
val result = """{"24h_volume_usd": "9097260000.0"}"""
normally I use:
import net.liftweb.json._
case class VolumeUSDClass(24h_volume_usd:String) //<- problem 24h_volume_usd does not work
val element = parse(result)
element.extract[CryptoDataClass]
The problem is that I cannot define a case class with an argument that starts with a number. what is the best way to circumvent this?
You can simply enclose the name of the variable into backticks:
implicit val formats = net.liftweb.json.DefaultFormats
val result = """{"24h_volume_usd": "9097260000.0"}"""
import net.liftweb.json._
case class VolumeUSDClass(`24h_volume_usd`:String)
val element = parse(result)
val vusdcl = element.extract[VolumeUSDClass]
println(vusdcl)
Recall that almost everything can be transformed into a valid Scala identifier if you enclose it in backticks. Even strange stuff like
val `]strange...O_o...stuff[` = 42
println(`]strange...O_o...stuff[`)
works.
The example is tested with "net.liftweb" %% "lift-json" % "3.2.0" and Scala 2.11.
I need to compile function and then evaluate it with different parameters of type List[Map[String, AnyRef]].
I have the following code that does not compile with such the type but compiles with simple type like List[Int].
I found that there are just certain implementations of Liftable in scala.reflect.api.StandardLiftables.StandardLiftableInstances
import scala.reflect.runtime.universe
import scala.reflect.runtime.universe._
import scala.tools.reflect.ToolBox
val tb = universe.runtimeMirror(getClass.getClassLoader).mkToolBox()
val functionWrapper =
"""
object FunctionWrapper {
def makeBody(messages: List[Map[String, AnyRef]]) = Map.empty
}""".stripMargin
val functionSymbol =
tb.define(tb.parse(functionWrapper).asInstanceOf[tb.u.ImplDef])
val list: List[Map[String, AnyRef]] = List(Map("1" -> "2"))
tb.eval(q"$functionSymbol.function($list)")
Getting compilation error for this, how can I make it work?
Error:(22, 38) Can't unquote List[Map[String,AnyRef]], consider using
... or providing an implicit instance of
Liftable[List[Map[String,AnyRef]]]
tb.eval(q"$functionSymbol.function($list)")
^
The problem comes not from complicated type but from the attempt to use AnyRef. When you unquote some literal, it means you want the infrastructure to be able to create a valid syntax tree to create an object that would exactly match the object you pass. Unfortunately this is obviously not possible for all objects. For example, assume that you've passed a reference to Thread.currentThread() as a part of the Map. How it could possible work? Compiler is just not able to recreate such a complicated object (not to mention making it the current thread). So you have two obvious alternatives:
Make you argument also a Tree i.e. something like this
def testTree() = {
val tb = universe.runtimeMirror(getClass.getClassLoader).mkToolBox()
val functionWrapper =
"""
| object FunctionWrapper {
|
| def makeBody(messages: List[Map[String, AnyRef]]) = Map.empty
|
| }
""".stripMargin
val functionSymbol =
tb.define(tb.parse(functionWrapper).asInstanceOf[tb.u.ImplDef])
//val list: List[Map[String, AnyRef]] = List(Map("1" -> "2"))
val list = q"""List(Map("1" -> "2"))"""
val res = tb.eval(q"$functionSymbol.makeBody($list)")
println(s"testTree = $res")
}
The obvious drawback of this approach is that you loose type safety at compile time and might need to provide a lot of context for the tree to work
Another approach is to not try to pass anything containing AnyRef to the compiler-infrastructure. It means you create some function-like Wrapper:
package so {
trait Wrapper {
def call(args: List[Map[String, AnyRef]]): Map[String, AnyRef]
}
}
and then make your generated code return a Wrapper instead of directly executing the logic and call the Wrapper from the usual Scala code rather than inside compiled code. Something like this:
def testWrapper() = {
val tb = universe.runtimeMirror(getClass.getClassLoader).mkToolBox()
val functionWrapper =
"""
|object FunctionWrapper {
| import scala.collection._
| import so.Wrapper /* <- here probably different package :) */
|
| def createWrapper(): Wrapper = new Wrapper {
| override def call(args: List[Map[String, AnyRef]]): Map[String, AnyRef] = Map.empty
| }
|}
| """.stripMargin
val functionSymbol = tb.define(tb.parse(functionWrapper).asInstanceOf[tb.u.ImplDef])
val list: List[Map[String, AnyRef]] = List(Map("1" -> "2"))
val tree: tb.u.Tree = q"$functionSymbol.createWrapper()"
val wrapper = tb.eval(tree).asInstanceOf[Wrapper]
val res = wrapper.call(list)
println(s"testWrapper = $res")
}
P.S. I'm not sure what are you doing but beware of performance issues. Scala is a hard language to compile and thus it might easily take more time to compile your custom code than to run it. If performance becomes an issue you might need to use some other methods such as full-blown macro-code-generation or at least caching of the compiled code.
I am working on a small GUI application written in Scala. There are a few settings that the user will set in the GUI and I want them to persist between program executions. Basically I want a scala.collections.mutable.Map that automatically persists to a file when modified.
This seems like it must be a common problem, but I have been unable to find a lightweight solution. How is this problem typically solved?
I do a lot of this, and I use .properties files (it's idiomatic in Java-land). I keep my config pretty straight-forward by design, though. If you have nested config constructs you might want a different format like YAML (if humans are the main authors) or JSON or XML (if machines are the authors).
Here's some example code for loading props, manipulating as Scala Map, then saving as .properties again:
import java.io._
import java.util._
import scala.collection.JavaConverters._
val f = new File("test.properties")
// test.properties:
// foo=bar
// baz=123
val props = new Properties
// Note: in real code make sure all these streams are
// closed carefully in try/finally
val fis = new InputStreamReader(new FileInputStream(f), "UTF-8")
props.load(fis)
fis.close()
println(props) // {baz=123, foo=bar}
val map = props.asScala // Get to Scala Map via JavaConverters
map("foo") = "42"
map("quux") = "newvalue"
println(map) // Map(baz -> 123, quux -> newvalue, foo -> 42)
println(props) // {baz=123, quux=newvalue, foo=42}
val fos = new OutputStreamWriter(new FileOutputStream(f), "UTF-8")
props.store(fos, "")
fos.close()
Here's an example of using XML and a case class for reading a config. A real class can be nicer than a map. (You could also do what sbt and at least one project do, take the config as Scala source and compile it in; saving it is less automatic. Or as a repl script. I haven't googled, but someone must have done that.)
Here's the simpler code.
This version uses a case class:
case class PluginDescription(name: String, classname: String) {
def toXML: Node = {
<plugin>
<name>{name}</name>
<classname>{classname}</classname>
</plugin>
}
}
object PluginDescription {
def fromXML(xml: Node): PluginDescription = {
// extract one field
def getField(field: String): Option[String] = {
val text = (xml \\ field).text.trim
if (text == "") None else Some(text)
}
def extracted = {
val name = "name"
val claas = "classname"
val vs = Map(name -> getField(name), claas -> getField(claas))
if (vs.values exists (_.isEmpty)) fail()
else PluginDescription(name = vs(name).get, classname = vs(claas).get)
}
def fail() = throw new RuntimeException("Bad plugin descriptor.")
// check the top-level tag
xml match {
case <plugin>{_*}</plugin> => extracted
case _ => fail()
}
}
}
This code reflectively calls the apply of a case class. The use case is that fields missing from config can be supplied by default args. No type conversions here. E.g., case class Config(foo: String = "bar").
// isn't it easier to write a quick loop to reflect the field names?
import scala.reflect.runtime.{currentMirror => cm, universe => ru}
import ru._
def fromXML(xml: Node): Option[PluginDescription] = {
def extract[A]()(implicit tt: TypeTag[A]): Option[A] = {
// extract one field
def getField(field: String): Option[String] = {
val text = (xml \\ field).text.trim
if (text == "") None else Some(text)
}
val apply = ru.newTermName("apply")
val module = ru.typeOf[A].typeSymbol.companionSymbol.asModule
val ts = module.moduleClass.typeSignature
val m = (ts member apply).asMethod
val im = cm reflect (cm reflectModule module).instance
val mm = im reflectMethod m
def getDefault(i: Int): Option[Any] = {
val n = ru.newTermName("apply$default$" + (i+1))
val m = ts member n
if (m == NoSymbol) None
else Some((im reflectMethod m.asMethod)())
}
def extractArgs(pss: List[List[Symbol]]): List[Option[Any]] =
pss.flatten.zipWithIndex map (p => getField(p._1.name.encoded) orElse getDefault(p._2))
val args = extractArgs(m.paramss)
if (args exists (!_.isDefined)) None
else Some(mm(args.flatten: _*).asInstanceOf[A])
}
// check the top-level tag
xml match {
case <plugin>{_*}</plugin> => extract[PluginDescription]()
case _ => None
}
}
XML has loadFile and save, it's too bad there seems to be no one-liner for Properties.
$ scala
Welcome to Scala version 2.10.0-RC5 (Java HotSpot(TM) 64-Bit Server VM, Java 1.7.0_06).
Type in expressions to have them evaluated.
Type :help for more information.
scala> import reflect.io._
import reflect.io._
scala> import java.util._
import java.util._
scala> import java.io.{StringReader, File=>JFile}
import java.io.{StringReader, File=>JFile}
scala> import scala.collection.JavaConverters._
import scala.collection.JavaConverters._
scala> val p = new Properties
p: java.util.Properties = {}
scala> p load new StringReader(
| (new File(new JFile("t.properties"))).slurp)
scala> p.asScala
res2: scala.collection.mutable.Map[String,String] = Map(foo -> bar)
As it all boils down to serializing a map / object to a file, your choices are:
classic serialization to Bytecode
serialization to XML
serialization to JSON (easy using Jackson, or Lift-JSON)
use of a properties file (ugly, no utf-8 support)
serialization to a proprietary format (ugly, why reinvent the wheel)
I suggest to convert Map to Properties and vice versa. "*.properties" files are standard for storing configuration in Java world, why not use it for Scala?
The common way are *. properties, *.xml, since scala supports xml natively, so it would be easier using xml config then in java.
I was trying to pull environment variables into a scala script using java Iterators and / or Enumerations and realised that Dr Frankenstein might claim parentage, so I hacked the following from the ugly tree instead:
import java.util.Map.Entry
import System._
val propSet = getProperties().entrySet().toArray()
val props = (0 until propSet.size).foldLeft(Map[String, String]()){(m, i) =>
val e = propSet(i).asInstanceOf[Entry[String, String]]
m + (e.getKey() -> e.getValue())
}
For example to print the said same environment
props.keySet.toList.sortWith(_ < _).foreach{k =>
println(k+(" " * (30 - k.length))+" = "+props(k))
}
Please, please don't set about polishing this t$#d, just show me the scala gem that I'm convinced exists for this situation (i.e java Properties --> scala.Map), thanks in advance ;#)
Scala 2.10.3
import scala.collection.JavaConverters._
//Create a variable to store the properties in
val props = new Properties
//Open a file stream to read the file
val fileStream = new FileInputStream(new File(fileName))
props.load(fileStream)
fileStream.close()
//Print the contents of the properties file as a map
println(props.asScala.toMap)
Scala 2.7:
val props = Map() ++ scala.collection.jcl.Conversions.convertMap(System.getProperties).elements
Though that needs some typecasting. Let me work on it a bit more.
val props = Map() ++ scala.collection.jcl.Conversions.convertMap(System.getProperties).elements.asInstanceOf[Iterator[(String, String)]]
Ok, that was easy. Let me work on 2.8 now...
import scala.collection.JavaConversions.asMap
val props = System.getProperties() : scala.collection.mutable.Map[AnyRef, AnyRef] // or
val props = System.getProperties().asInstanceOf[java.util.Map[String, String]] : scala.collection.mutable.Map[String, String] // way too many repetitions of types
val props = asMap(System.getProperties().asInstanceOf[java.util.Map[String, String]])
The verbosity, of course, can be decreased with a couple of imports. First of all, note that Map will be a mutable map on 2.8. On the bright side, if you convert back the map, you'll get the original object.
Now, I have no clue why Properties implements Map<Object, Object>, given that the javadocs clearly state that key and value are String, but there you go. Having to typecast this makes the implicit option much less attractive. This being the case, the alternative is the most concise of them.
EDIT
Scala 2.8 just acquired an implicit conversion from Properties to mutable.Map[String,String], which makes most of that code moot.
In Scala 2.9.1 this is solved by implicit conversions inside collection.JavaConversions._ . The other answers use deprecated functions. The details are documented here. This is a relevant snippet out of that page:
scala> import collection.JavaConversions._
import collection.JavaConversions._
scala> import collection.mutable._
import collection.mutable._
scala> val jul: java.util.List[Int] = ArrayBuffer(1, 2, 3)
jul: java.util.List[Int] = [1, 2, 3]
scala> val buf: Seq[Int] = jul
buf: scala.collection.mutable.Seq[Int] = ArrayBuffer(1, 2, 3)
scala> val m: java.util.Map[String, Int] = HashMap("abc" -> 1, "hello" -> 2)
m: java.util.Map[String,Int] = {hello=2, abc=1}
Getting from a mutable map to an immutable map is a matter of calling toMap on it.
In Scala 2.8.1 you can do it with asScalaMap(m : java.util.Map[A, B]) in a more concise way:
var props = asScalaMap(System.getProperties())
props.keySet.toList.sortWith(_ < _).foreach { k =>
println(k + (" " * (30 - k.length)) + " = " + props(k))
}
In Scala 2.13.2:
import scala.jdk.javaapi.CollectionConverters._
val props = asScala(System.getProperties)
Looks like in the most recent version of Scala (2.10.2 as of the time of this answer), the preferred way to do this is using the explicit .asScala from scala.collection.JavaConverters:
import scala.collection.JavaConverters._
val props = System.getProperties().asScala
assert(props.isInstanceOf[Map[String, String]])