In Scala, I'd like to have a subclass method conditionally elided out (and revert to the base class implementation) based on the command line priority. As an example, suppose I have the following code:
// File: mini.scala
import scala.annotation._, elidable._
trait FooBase {
def bar(msg: String) = println("FooBase: " + msg)
}
object Foo extends FooBase {
#elidable(INFO)
override def bar(msg: String) = println("Foo: " + msg)
}
object App {
def main(args: Array[String]) {
println("before")
Foo.bar("message")
println("after")
}
}
If I compile with:
scalac -Xelide-below MINIMUM mini.scala
And run it, I see (as expected):
before
Foo: message
after
Now if I compile with:
scalac -Xelide-below MAXIMUM mini.scala
Then, I hoped to see:
before
FooBase: message
after
But I actually see:
before
after
So instead of just the subclass method being elided out (as hoped) the base class seems to be gone too.
Any explanation appreciated.
Based on the response from #som-snytt, it seems that the best way to achieve the effect I'm after is something like:
trait DebugBase {
def on = false
}
object Debug extends DebugBase {
#elidable(INFO)
override def on = true
}
object Foo {
def bar(msg: String) = if (Debug.on) println("Foo (Debug): " + msg) else println("Foo: " + msg)
}
But then there is still a runtime check on the status of Debug.on instead of completely eliding out the debug version as you might with an #ifdef in C/C++.
That's a good one. I've looked at the mechanism once upon a time, but I wasn't sure about the use case.
The answer is commented:
/** Called if a tree's symbol is elidable. If it's a DefDef,
* replace only the body/rhs with 0/false/()/null; otherwise replace
* the whole tree with it.
*/
So in eliding Foo.bar, you don't remove the method definition. You merely turn it into def bar = () and any invocation into the unit value ().
You can elide a template method usefully:
scala> class X { def f() = { if (g() < 1) println("default") } ; def g() = 1 }
defined class X
scala> :se -Xelide-below 200
scala> class Y extends X { #elidable(100) override def g() = 2 }
defined class Y
scala> new Y().f()
default
scala> :se -Xelide-below 5
scala> class Y extends X { #elidable(100) override def g() = 2 }
defined class Y
scala> new Y().f()
Related
I am new to both ScalaMock and mocking in general. I am trying to test a method which calls a method in another (mocked) class and then calls a method on the returned object.
Detailed information:
So I am using ScalaTest and there are five classes involved in this test...
SubInstruction which I am testing
class SubInstruction(label: String, val result: Int, val op1: Int, val op2: Int) extends Instruction(label, "sub") {
override def execute(m: Machine) {
val value1 = m.regs(op1)
val value2 = m.regs(op2)
m.regs(result) = value1 - value2
}
}
object SubInstruction {
def apply(label: String, result: Int, op1: Int, op2: Int) =
new SubInstruction(label, result, op1, op2)
}
Machine which must be mocked for the test
case class Machine(labels: Labels, prog: Vector[Instruction]) {
private final val NUMBEROFREGISTERS = 32
val regs: Registers = new Registers(NUMBEROFREGISTERS)
override def toString(): String = {
prog.foldLeft("")(_ + _)
}
def execute(start: Int) =
start.until(prog.length).foreach(x => prog(x) execute this)
}
object Machine extends App {
if (args.length == 0) {
println("Machine: args should be sml code file to execute")
} else {
println("SML interpreter - Scala version")
val m = Translator(args(0)).readAndTranslate(new Machine(Labels(), Vector()))
println("Here is the program; it has " + m.prog.size + " instructions.")
println(m)
println("Beginning program execution.")
m.execute(0)
println("Ending program execution.")
println("Values of registers at program termination:")
println(m.regs + ".")
}
}
Registers which is required to construct a Machine object
case class Registers(size: Int) {
val registers: Array[Int] = new Array(size)
override def toString(): String =
registers.mkString(" ")
def update(k: Int, v: Int) = registers(k) = v
def apply(k: Int) = registers(k)
}
MockableMachine which I have created as the original Machine class does not have an empty constructor and therefore (as I understand) can not be mocked
class MockableMachine extends Machine(Labels(), Vector()){
}
and finally my test class SubInstructionTest which compiles but throws the exception below.
class SubInstructionTest extends FlatSpec with MockFactory with Matchers {
val label1 = "f0"
val result1 = 25
val op1_1 = 24
val op2_1 = 20
val sub1 = SubInstruction(label1, result1, op1_1, op2_1)
"A SubInstruction" should "retrieve the operands from the correct registers in the given machine " +
"when execute(m: Machine) is called, and perform the operation saving the " +
"result in the correct register." in {
val mockMachine = mock[MockableMachine]
inSequence {
(mockMachine.regs.apply _).expects(op1_1).returning(50)
(mockMachine.regs.apply _).expects(op2_1).returning(16)
(mockMachine.regs.update _).expects(result1, 34)
}
sub1.execute(mockMachine)
}
}
Throws:
java.lang.NoSuchMethodException: Registers.mock$apply$0()
-
I have been searching for a straightforward way to mock this class for hours, but have found nothing. For the time being I have settled on the workaround detailed below, but I was under the impression that mocking would offer a less convoluted solution to the problem of testing my SubInstruction class.
The workaround:
Delete the MockableMachine class and create a CustomMachine class which extends Machine and replaces the registers value with mockedRegisters provided at construction time.
class CustomMachine (mockedRegister: Registers) extends Machine(Labels(), Vector()) {
override
val regs: Registers = mockedRegister
}
a MockableRegisters class which I have created as the original does not have an empty constructor and therefore (as I understand) can not be mocked
class MockableRegisters extends Registers(32) {
}
and the SubInstructionTest class written in a slightly different way
class SubInstructionTest extends FlatSpec with MockFactory with Matchers {
val label1 = "f0"
val result1 = 25
val op1_1 = 24
val op2_1 = 20
val sub1 = SubInstruction(label1, result1, op1_1, op2_1)
"A SubInstruction" should "retrieve the operands from the correct registers in the given machine " +
"when execute(m: Machine) is called, and perform the operation saving the " +
"result in the correct register." in {
val mockRegisters = mock[MockableRegisters]
val machine = new CustomMachine(mockRegisters)
inSequence {
(mockRegisters.apply _).expects(op1_1).returning(50)
(mockRegisters.apply _).expects(op2_1).returning(16)
(mockRegisters.update _).expects(result1, 34)
}
sub1.execute(machine)
}
}
As indicated, this feels like a workaround to me, is there not a simpler way to do this (perhaps similar to my original attempt)?
I have just included the essential code to ask the question, but you can find the full code on my GitHub account.
I don't think mocking nested objects is supported by Scalamock implicitly. You'll have to mock the object returned by the first call which is what your working example does.
FWIW, Mockito supports this. Search for RETURNS_DEEP_STUBS.
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.
Having a trait
trait Persisted {
def id: Long
}
how do I implement a method that accepts an instance of any case class and returns its copy with the trait mixed in?
The signature of the method looks like:
def toPersisted[T](instance: T, id: Long): T with Persisted
This can be done with macros (that are officially a part of Scala since 2.10.0-M3). Here's a gist example of what you are looking for.
1) My macro generates a local class that inherits from the provided case class and Persisted, much like new T with Persisted would do. Then it caches its argument (to prevent multiple evaluations) and creates an instance of the created class.
2) How did I know what trees to generate? I have a simple app, parse.exe that prints the AST that results from parsing input code. So I just invoked parse class Person$Persisted1(first: String, last: String) extends Person(first, last) with Persisted, noted the output and reproduced it in my macro. parse.exe is a wrapper for scalac -Xprint:parser -Yshow-trees -Ystop-after:parser. There are different ways to explore ASTs, read more in "Metaprogramming in Scala 2.10".
3) Macro expansions can be sanity-checked if you provide -Ymacro-debug-lite as an argument to scalac. In that case all expansions will be printed out, and you'll be able to detect codegen errors faster.
edit. Updated the example for 2.10.0-M7
It is not possible to achieve what you want using vanilla scala. The problem is that the mixins like the following:
scala> class Foo
defined class Foo
scala> trait Bar
defined trait Bar
scala> val fooWithBar = new Foo with Bar
fooWithBar: Foo with Bar = $anon$1#10ef717
create a Foo with Bar mixed in, but it is not done at runtime. The compiler simply generates a new anonymous class:
scala> fooWithBar.getClass
res3: java.lang.Class[_ <: Foo] = class $anon$1
See Dynamic mixin in Scala - is it possible? for more info.
What you are trying to do is known as record concatenation, something that Scala's type system does not support. (Fwiw, there exist type systems - such as this and this - that provide this feature.)
I think type classes might fit your use case, but I cannot tell for sure as the question doesn't provide sufficient information on what problem you are trying to solve.
Update
You can find an up to date working solution, which utilizes a Toolboxes API of Scala 2.10.0-RC1 as part of SORM project.
The following solution is based on the Scala 2.10.0-M3 reflection API and Scala Interpreter. It dynamically creates and caches classes inheriting from the original case classes with the trait mixed in. Thanks to caching at maximum this solution should dynamically create only one class for each original case class and reuse it later.
Since the new reflection API isn't that much disclosed nor is it stable and there are no tutorials on it yet this solution may involve some stupid repitative actions and quirks.
The following code was tested with Scala 2.10.0-M3.
1. Persisted.scala
The trait to be mixed in. Please note that I've changed it a bit due to updates in my program
trait Persisted {
def key: String
}
2. PersistedEnabler.scala
The actual worker object
import tools.nsc.interpreter.IMain
import tools.nsc._
import reflect.mirror._
object PersistedEnabler {
def toPersisted[T <: AnyRef](instance: T, key: String)
(implicit instanceTag: TypeTag[T]): T with Persisted = {
val args = {
val valuesMap = propertyValuesMap(instance)
key ::
methodParams(constructors(instanceTag.tpe).head.typeSignature)
.map(_.name.decoded.trim)
.map(valuesMap(_))
}
persistedClass(instanceTag)
.getConstructors.head
.newInstance(args.asInstanceOf[List[Object]]: _*)
.asInstanceOf[T with Persisted]
}
private val persistedClassCache =
collection.mutable.Map[TypeTag[_], Class[_]]()
private def persistedClass[T](tag: TypeTag[T]): Class[T with Persisted] = {
if (persistedClassCache.contains(tag))
persistedClassCache(tag).asInstanceOf[Class[T with Persisted]]
else {
val name = generateName()
val code = {
val sourceParams =
methodParams(constructors(tag.tpe).head.typeSignature)
val newParamsList = {
def paramDeclaration(s: Symbol): String =
s.name.decoded + ": " + s.typeSignature.toString
"val key: String" :: sourceParams.map(paramDeclaration) mkString ", "
}
val sourceParamsList =
sourceParams.map(_.name.decoded).mkString(", ")
val copyMethodParamsList =
sourceParams.map(s => s.name.decoded + ": " + s.typeSignature.toString + " = " + s.name.decoded).mkString(", ")
val copyInstantiationParamsList =
"key" :: sourceParams.map(_.name.decoded) mkString ", "
"""
class """ + name + """(""" + newParamsList + """)
extends """ + tag.sym.fullName + """(""" + sourceParamsList + """)
with """ + typeTag[Persisted].sym.fullName + """ {
override def copy(""" + copyMethodParamsList + """) =
new """ + name + """(""" + copyInstantiationParamsList + """)
}
"""
}
interpreter.compileString(code)
val c =
interpreter.classLoader.findClass(name)
.asInstanceOf[Class[T with Persisted]]
interpreter.reset()
persistedClassCache(tag) = c
c
}
}
private lazy val interpreter = {
val settings = new Settings()
settings.usejavacp.value = true
new IMain(settings, new NewLinePrintWriter(new ConsoleWriter, true))
}
private var generateNameCounter = 0l
private def generateName() = synchronized {
generateNameCounter += 1
"PersistedAnonymous" + generateNameCounter.toString
}
// REFLECTION HELPERS
private def propertyNames(t: Type) =
t.members.filter(m => !m.isMethod && m.isTerm).map(_.name.decoded.trim)
private def propertyValuesMap[T <: AnyRef](instance: T) = {
val t = typeOfInstance(instance)
propertyNames(t)
.map(n => n -> invoke(instance, t.member(newTermName(n)))())
.toMap
}
private type MethodType = {def params: List[Symbol]; def resultType: Type}
private def methodParams(t: Type): List[Symbol] =
t.asInstanceOf[MethodType].params
private def methodResultType(t: Type): Type =
t.asInstanceOf[MethodType].resultType
private def constructors(t: Type): Iterable[Symbol] =
t.members.filter(_.kind == "constructor")
private def fullyQualifiedName(s: Symbol): String = {
def symbolsTree(s: Symbol): List[Symbol] =
if (s.enclosingTopLevelClass != s)
s :: symbolsTree(s.enclosingTopLevelClass)
else if (s.enclosingPackageClass != s)
s :: symbolsTree(s.enclosingPackageClass)
else
Nil
symbolsTree(s)
.reverseMap(_.name.decoded)
.drop(1)
.mkString(".")
}
}
3. Sandbox.scala
The test app
import PersistedEnabler._
object Sandbox extends App {
case class Artist(name: String, genres: Set[Genre])
case class Genre(name: String)
val artist = Artist("Nirvana", Set(Genre("rock"), Genre("grunge")))
val persisted = toPersisted(artist, "some-key")
assert(persisted.isInstanceOf[Persisted])
assert(persisted.isInstanceOf[Artist])
assert(persisted.key == "some-key")
assert(persisted.name == "Nirvana")
assert(persisted == artist) // an interesting and useful effect
val copy = persisted.copy(name = "Puddle of Mudd")
assert(copy.isInstanceOf[Persisted])
assert(copy.isInstanceOf[Artist])
// the only problem: compiler thinks that `copy` does not implement `Persisted`, so to access `key` we have to specify it manually:
assert(copy.asInstanceOf[Artist with Persisted].key == "some-key")
assert(copy.name == "Puddle of Mudd")
assert(copy != persisted)
}
While it's not possible to compose an object AFTER it's creation, you can have very wide tests to determine if the object is of a specific composition using type aliases and definition structs:
type Persisted = { def id: Long }
class Person {
def id: Long = 5
def name = "dude"
}
def persist(obj: Persisted) = {
obj.id
}
persist(new Person)
Any object with a def id:Long will qualify as Persisted.
Achieving what I THINK you are trying to do is possible with implicit conversions:
object Persistable {
type Compatible = { def id: Long }
implicit def obj2persistable(obj: Compatible) = new Persistable(obj)
}
class Persistable(val obj: Persistable.Compatible) {
def persist() = println("Persisting: " + obj.id)
}
import Persistable.obj2persistable
new Person().persist()
As far as I understand it, Scala creates an anonymous class if I create a class using the new keyword and follow the class name with a constructor:
class MyClass {
def doStuff() {
// ...
}
}
val mc = new MyClass {
doStuff()
}
The nice thing being that all the code in the constructor is in the scope of the new object.
Is there a way I can reproduce this syntax where the class is created by a factory method rather than the new keyword? i.e. make the following code work:
val mf = new MyFactory
val mc = mf.MyClass {
doStuff()
}
I can't find a way to do it but Scala has so much to it that this might be pretty easy!
Using an import as suggested by #Ricky below I can get:
val mf = MyFactory;
val mc = mf.MyClass
{
import mc._
doStuff()
}
(Where the blank line before the block is needed) but that code block is not a constructor.
You can do this, but you still have to keep the new keyword, and create the nested class as a path-dependent type:
class Bippy(x: Int) {
class Bop {
def getIt = x
}
}
val bip = new Bippy(7)
val bop = new bip.Bop
bop.getIt // yields 7
val bop2 = new bip.Bop{ override def getIt = 42 }
bop2.getIt // yields 42
I don't think it's possible. However, a common pattern is to add a parameter to factory methods which takes a function modifying the created object:
trait MyClass {
var name = ""
def doStuff():Unit
}
class Foo extends MyClass {
def doStuff() { println("FOO: " + name) }
}
trait MyClassFactory {
def make: MyClass
def apply( body: MyClass => Unit ) = {
val mc = make
body(mc)
mc
}
}
object FooFactory extends MyClassFactory {
def make = new Foo
}
You can then create and modify instance with a syntax close to your example:
val foo = FooFactory { f=>
f.name = "Joe"
f.doStuff
}
It sounds like you're just looking to mix in a trait. Instead of calling myFactoryMethod(classOf[Foo]] which ideally would do (if Scala permitted it):
new T {
override def toString = "My implementation here."
}
you can instead write
trait MyImplementation {
override def toString = "My implementation here."
}
new Foo with MyImplementation
However, if you are just looking to get the members of the new object accessible without qualification, remember you can import from any stable identifier:
val foo = new Bar
import foo._
println(baz) //where baz is a member of foo.
I have the following scala code:
package dummy
import javax.servlet.http.{HttpServlet,
HttpServletRequest => HSReq, HttpServletResponse => HSResp}
import scala.actors.Actor
class DummyServlet extends HttpServlet {
RNG.start
override def doGet(req: HSReq, resp: HSResp) = {
def message = <HTML><HEAD><TITLE>RandomNumber </TITLE></HEAD><BODY>
Random number = {getRandom}</BODY></HTML>
resp.getWriter().print(message)
def getRandom: String = {var d = new DummyActor;d.start;d.getRandom}
}
class DummyActor extends Actor {
var result = "0"
def act = { RNG ! GetRandom
react { case (r:Int) => result = r.toString }
}
def getRandom:String = {
Thread.sleep(300)
result
}
}
}
// below code is not modifiable. I am using it as a library
case object GetRandom
object RNG extends Actor {
def act{loop{react{case GetRandom=>sender!scala.util.Random.nextInt}}}
}
In the above code, I have to use thread.sleep to ensure that there is enough time for result to get updated, otherwise 0 is returned. What is a more elegant way of doing this without using thread.sleep? I think I have to use futures but I cannot get my head around the concept. I need to ensure that each HTTP reaquest gets a unique random number (of course, the random number is just to explain the problem). Some hints or references would be appreciated.
Either use:
!! <-- Returns a Future that you can wait for
or
!? <-- Use the one with a timeout, the totally synchronous is dangerous
Given your definition of RNG, heres some REPL code to verify:
scala> def foo = { println(RNG.!?(1000,GetRandom)) }
foo: Unit
scala> foo
Some(-1025916420)
scala> foo
Some(-1689041124)
scala> foo
Some(-1633665186)
Docs are here: http://www.scala-lang.org/api/current/scala/actors/Actor.html