I am building a DSL with Scala and I have such an object:
object block {
def apply(content: String): String = "{\n" + content + "}\n"
def apply(): String = block("empty block")
}
so that later in my DSL I can say:
block {
"good stuff goes here"
}
that's ok, but there is the second apply() method which I want to use to allow the user to write empty blocks, so to fill them later. But the compiler does not allow to call the no-arguments apply() method with curly braces... is there any way around that except using parentheses instead? If no, then why isn't this allowed?
UPDATE: For reference: the two answers should be combined for the full picture.
Instead of an apply method without any parameters, you can let it take a parameter of type Unit that is ignored:
def apply(ignored: Unit) = "{\n}\n"
Then you can write
block {}
FINALLY
object block {
def apply[T](content: => T) = "{\n" +
(content match {
case s: String => s
case u: Unit => ""
case a => a.toString
}) + "}\n"
}
EDIT
To make a block {} working:
object block { def apply(content: => Unit) = "{\n" + content + "}\n" }
I insist on using a by-name parameter, not just a Unit as in another answer. Because => Unit does actually create a function that can be executed by a block.apply() method, and content: Unit just takes as a parameter a block of code that will be executed in-place (before block.apply()).
Let's suppose that block.apply will print "Hoho". Now:
// content: Unit
block { println("Haha") }
// Haha
// Hoho
// content: => Unit
block { println("Haha") }
// Hoho
// Haha
...
Use by-name parameter:
object block { def apply(content: => String) = "{\n" + content + "}\n" }
Related
I think my question is related but not the same as this one here.
Let define my first class
case class NoteTaker() {
private var note: Seq[String] = Seq("\n")
override def toString: String = this.note.mkString("\n")
def add(newNote: String): Unit = note ++= Seq(newNote)
}
Now I have a trait
trait SilentLogger {
import scala.util.{ Failure, Success }
val notepad = NoteTaker()
def tryAndLog[X, Y](x: X, f: X => Y): Y = {
notepad.add("Run with this input: " + x.toString)
(try {
println("Before: " + notepad.toString)
val result = f(x)
println("After: " + notepad.toString)
notepad.add("Get result:-------------------------------\n" + result.toString)
println(notepad.toString)
Success(result)
} catch {
case e: Throwable => {
println(
"Exception occurs:" + "\n" +
notepad.toString + "\n" +
e.getMessage + "\n" +
e.getStackTrace.mkString("\n")
)
Failure(e)
}}).get
}
}
I intend to use this trait to mix in with any classes where I want to collect some notes and only print out the note when there is an exception. Otherwise, I maybe just save it to a log file somewhere.
I want the notepad to be created once and reused for each of the object. In fact, I don't mind if they share the same notepad. Therefore, I chose to use 'val' in my trait.
As an example, I then create a class
case class MyClass (myField : String) extends SilentLogger {
def makeAnother : MyClass = tryAndLog("makeAnother",(x: String) => {
notepad.add(x)
val result = this.copy(myField = this.myField + " addNewField " + x)
notepad.add(result.myField)
return result
})
}
And finally I try to create two objects as follow:
scala> val firstObject = MyClass("firstObject")
firstObject: MyClass = MyClass(firstObject)
scala> val secondObject = firstObject.makeAnother
Before:
Run with this input: makeAnother
Exception occurs:
Run with this input: makeAnother
makeAnother
firstObject addNewField makeAnother
null
secondObject: MyClass = MyClass(firstObject addNewField makeAnother)
I'm really confused here. Obviously an exception occurred. But the secondObject was created just fine? But the logging message get printed out on stdout with the error 'null'.
I think my question is whether my first and second objects are actually using the same notepad or separate? How are the initialisation and the scope of notepad defined here? Is there something wrong with the way I use 'Try'?
This is caused of anonymous function with explicitly return:
(x: String) => {
notepad.add(x)
val result = this.copy(myField = this.myField + " addNewField " + x)
notepad.add(result.myField)
return result
}
In Scala, when explicitly declare return in anonymous function, it will throw NonLocalReturnControl, this will skip the later code block execute, since you have catched the Throwable, so it also will go to your catch code block.
So maybe you can remove return directly to solve this issue.
I have the below requirement where I am checking whether a value is greater than 10 or not and based on that I will break, otherwise I will return a String. Below is my code:
import scala.util.control.Breaks._
class BreakInScala {
val breakException = new RuntimeException("Break happened")
def break = throw breakException
def callMyFunc(x: Int): String = breakable(myFunc(x))
def myFunc(x: Int): String = {
if (x > 10) {
"I am fine"
} else {
break
}
}
}
Now what is the happening is that I am getting the error message saying "type mismatch; found : Unit required: String" The reason is :
def breakable(op: => Unit)
But then how I will write a function which can return value as well as break if required?
The Scala compiler can evaluate that a branch throws an exception and not use it to form a minimum bound for the return type, but not if you move the throwing code out in a method: since it can be overridden, the compiler cannot be sure it will actually never return.
Your usage of the Break constructs seems confused: it already provides a break method, there is no need to provide your own, unless you want to throw your exception instead, which would make using Break unnecessary.
You are left with a couple of options then, since I believe usage of Break is unnecessary in your case.
1) Simply throw an exception on failure
def myFunc(x: Int): String = {
if (x > 10) {
"I am fine"
} else {
throw new RuntimeException("Break happened")
}
}
def usemyFunc(): Unit = {
try {
println("myFunc(11) is " + myFunc(11))
println("myFunc(5) is " + myFunc(5))
} catch {
case e: Throwable => println("myFunc failed with " + e)
}
}
2) Use the Try class (available from Scala 2.10) to return either a value or an exception. This differs from the previous suggestion because it forces the caller to inspect the result and check whether a value is available or not, but makes using the result a bit more cumbersome
import scala.util.Try
def myFunc(x: Int): Try[String] = {
Try {
if (x > 10) {
"I am fine"
} else {
throw new RuntimeException("Break happened")
}
}
}
def useMyFunc(): Unit = {
myFunc match {
case Try.Success(s) => println("myFunc succeded with " + s)
case Try.Failure(e) => println("myFunc failed with " + e)
}
}
3) If the thrown exception isn't relevant, you can use the Option class instead.
You can see how the multiple ways of working with Options relate to each other in
this great cheat sheet.
def myFunc(x: Int): Option[String] = {
if (x > 10) {
Some("I am fine") /* Some(value) creates an Option containing value */
} else {
None /* None represents an Option that has no value */
}
}
/* There are multiple ways to work with Option instances.
One of them is using pattern matching. */
def useMyFunc(): Unit = {
myFunc(10) match {
case Some(s) => println("myFunc succeded with " + s)
case None => println("myFunc failed")
}
}
/* Another one is using the map, flatMap, getOrElse, etc methods.
They usually take a function as a parameter, which is only executed
if some condition is met.
map only runs the received function if the Option contains a value,
and passes said value as a parameter to it. It then takes the result
of the function application, and creates a new Option containing it.
getOrElse checks if the Option contains a value. If it does, it is returned
directly. If it does not, then the result of the function passed to it
is returned instead.
Chaining map and getOrElse is a common idiom meaning roughly 'transform the value
contained in this Option using this code, but if there is no value, return whatever
this other piece of code returns instead'.
*/
def useMyFunc2(): Unit = {
val toPrint = myFunc(10).map{ s =>
"myFunc(10) succeded with " + s
}.getOrElse{
"myFunc(10) failed"
}
/* toPrint now contains a message to be printed, depending on whether myFunc
returned a value or not. The Scala compiler is smart enough to infer that
both code paths return String, and make toPrint a String as well. */
println(toPrint)
}
This is a slightly odd way of doing things (throwing an exception), an alternative way of doing this might be to define a "partial function" (a function which is only defined only a specific subset of it's domain a bit like this:
scala> val partial = new PartialFunction[Int, String] {
| def apply(i : Int) = "some string"
| def isDefinedAt(i : Int) = i < 10
}
partial: PartialFunction[Int, String] = <function1>
Once you've defined the function, you can then "lift" it into an Option of type Int, by doing the following:
scala> val partialLifted = partial.lift
partialOpt: Int => Option[String] = <function1>
Then, if you call the function with a value outside your range, you'll get a "None" as a return value, otherwise you'll get your string return value. This makes it much easier to apply flatMaps/ getOrElse logic to the function without having to throw exceptions all over the place...
scala> partialLifted(45)
res7: Option[String] = None
scala> partialLifted(10)
res8: Option[String] = Some(return string)
IMO, this is a slightly more functional way of doing things...hope it helps
I am trying to parse commandline arguments and execute a function that takes the parameters upon successful extraction of the parameters. I have an object called CurrencyExchangeRunner where the main method is. I have envisioned the structure of the class as follows:
object CurrencyExtractionRunner {
def main(args:Array[String]){
parseArgs(args){
(currencyType,currencyTypeArgs) =>
CurrencyExchanger(curencyType,currencyTypeArgs){
(exchanger) => exchanger.startExchange
}
}
}
}
}
What I want to accomplish above is to parse the arguments using parseArgs(args), get the (currencyType,currencyTypeArgs) as parameters and pass those into the CurrencyExchanger factory object and then that would return the appropriate exchanger on which I will execute the startExchange method. This is what I have envisioned but I am a little clueless on how would I go about creating this flow. The first thing I tried was to create a trait that parses the command-line args as follows(I am using the jcommander library for the commandline parse):
object Args {
#Parameter(
names = Array("-h", "--help"), help = true)
var help = false
#Parameter(
names = Array("-c", "--currency-type"),
description = "Type of currency exchange that needs to be performed",
required = true)
var currencyType: String = null
#Parameter(
names = Array("-d", "--denominations"),
description = "Specific denominations to be used during the exchage")
var exchangeDenomination: String = null
#Parameter(
names = Array("-s", "--someotheroptionalarg"),
description = "Additional argument for a specific currency exchange")
var someOtherOptionalArg: String = null
}
trait ParseUtils {
//How do I do this, take the args and return a function.
def parseArgs(args: Array[String]){
val jCommander = new JCommander(Args, args.toArray: _*)
if (Args.help) {
jCommander.usage()
System.exit(0)
}
//What do I do now? How do I proceed with executing the function with
//the specific arguments?
//What do I need to do to wrap the commandline arguments so that it could
//be passed to the next function
}
}
I am pretty stuck here since I am not sure how would I make the code flexible enough to take the arbitrary sequence of commandline args and execute the next step which is the factory that returns that takes these arguments and returns the correct exchanger.
It will be great if someone could point me in the right direction.
I'm not sure why you'd use such unusual syntax to pass return values to the following methods.
I would go for a simpler solution that looks like
trait ParseUtils {
//Why would you return a function here?
//Is it a strict constraint you need to fulfill?
def parseArgs(args: Array[String]): (String, String) {
val jCommander = new JCommander(Args, args.toArray: _*)
if (Args.help) {
jCommander.usage()
System.exit(0)
}
//This is the return value of the method, a pair of parameters
(Args.currencyType, Args.exchangeDenomination)
//If you need to embed additional params, you should append them to existing one
// or you could create optional values from the Args members...
// e.g. (Args.currencyType, Args.exchangeDenomination, Option(Args.someOtherOptionalArg))
// with return type (String, String, Option[String])
}
}
object CurrencyExtractionRunner with ParseUtils {
def main(args:Array[String]){
val (currencyType,currencyTypeArgs) = parseArgs(args)
CurrencyExchanger(currencyType,currencyTypeArgs).startExchange
}
}
case class CurrencyExchanger(currencyType: String, currencyTypeArgs: String) {
def startExchange = //implementation details using the costructor arguments
}
Alternative solution
since I prefer parseArgs to be more "functional" I'd change it to
trait ParseUtils {
def parseArgs(args: Array[String]): Option[(String, String)] {
val jCommander = new JCommander(Args, args.toArray: _*)
if (Args.help) {
jCommander.usage()
None
} else
Some(Args.currencyType, Args.exchangeDenomination)
}
}
object CurrencyExtractionRunner with ParseUtils {
def main(args:Array[String]){
parseArgs(args).foreach {
case (currencyType,currencyTypeArgs) =>
CurrencyExchanger(currencyType,currencyTypeArgs).startExchange
}
}
}
case class CurrencyExchanger(currencyType: String, currencyTypeArgs: String) {
def startExchange = //implementation details using the costructor arguments
}
In my DSL I want to be able to do like this:
val a = 5
Parse("TYPE") {
a match {
case 3 => info("almost")
case s: String => error("wrong type")
case 5 => info("you won!")
case _ => error("omg")
}
}
with output
[INFO] TYPE: you won!
where Parse is a function object, which has the apply(String), info(String) and error(String) methods:
object Parse {
def apply(name: String)(parseF: => Unit) { parseF }
def info(text: String) { println("[INFO] " + name + ": " + text) }
def error(text: String) { println("[ERROR] " + name + ": " + text) }
}
The trick is that the output of the info and error methods should be somehow native to the Parse object and construct the message by the example as shown above. Therefore, they
have to be accessible like in the example, without any imports.
have to have the access to the first argument passed to Parse() ("TYPE" in the example).
must not create any instances or extra objects during their working.
This is an ideal description. I think that it will require some more boilerplate. Please suggest, how can I acheive that?
EDIT: my initial guess to declare the info and error methods inside the apply method does not make them visible in the parseF method being passed. Well, unsurprisingly..
Maybe something like this:
object Parse {
val currentName = new util.DynamicVariable("<none>")
def apply(name: String)(parseF: => Unit) = currentName.withValue(name)(parseF)
def info(text: String) = println("[INFO] %s: %s" format (currentName.value, text)
}
// usage
import Parse._ // there's no other way to get ability to call 'info' without 'Parse'.
Parse("TYPE") {
// some code
info("some info") // prints: [INFO] TYPE: some info
}
info("some other info") // prints: [INFO] <none>: some other info
If needed, it's easy to make info throw exception if it is called outside Parse {} block.
There is no solution that satisfies 1-3. You can have 1 and 2 or 2 and 3 with slightly different syntax.
1 and 2)
class Parse(val s:String) {
def info....
}
new Parse("TYPE") {
}
2 and 3)
object Parse {
def apply(s:String)(f:String=>Unit) = ...
}
Parse("TYPE")(s => {
})
Let's say I want to make a little wrapper along the lines of:
def wrapper(f: (Any) => Any): Any = {
println("Executing now")
val res = f
println("Execution finished")
res
}
wrapper {
println("2")
}
Does this make sense? My wrapper method is obviously wrong, but I think the spirit of what I want to do is possible. Am I right in thinking so? If so, what's the solution? Thanks!
If you want your wrapper method to execute the wrapped method inside itself, you should change the parameter to be 'by name'. This uses the syntax => ResultType.
def wrapper(f: => Any): Any = {
println("Executing now")
val res = f
println("Execution finished")
res
}
You can now do this,
wrapper {
println("2")
}
and it will print
Executing now
2
Execution finished
If you want to be able to use the return type of the wrapped function, you can make your method generic:
def wrapper[T](f: => T): T = {
println("Executing now")
val res: T = f
println("Execution finished")
res
}
In your case you are already executing the function println and then pass the result to your wrapper while it is expecting a function with one arguments (Any) and that return Any.
Not sure if this answer to your question but you can use a generic type parameter and accept a function with no arguments that return that type:
def wrapper[T](f: () => T) = {
println("Executing now")
val res = f() // call the function
println("Execution finished")
res
}
wrapper {
()=>println("2") // create an anonymous function that will be called
}