For scala case class with number of parameters (21!!)
e.g. case class Car(type: String, brand: String, door: Int ....)
where type = jeep, brand = toyota, door = 4 ....etc
And there is a copy method which allow override with named parameter: Car.copy(brand = Kia)
where would become type = jeep, brand = Kia, door = 2...etc
My question is, is there anyway I can provide the named parameter dynamically?
def copyCar(key: String, name: String) = {
Car.copy("key" = "name") // this is something I make up and want to see if would work
}
Is scala reflection library could provide a help here?
The reason I am using copy method is that I don't want to repeat the 21 parameters assignment every time when I create a case class which only have 1 or 2 parameter changed.
Many Thanks!
FWIW, I've just implemented a Java reflection version: CaseClassCopy.scala. I tried a TypeTag version but it wasn't that useful; TypeTag was too restrictive for this purpose.
def copy(o: AnyRef, vals: (String, Any)*) = {
val copier = new Copier(o.getClass)
copier(o, vals: _*)
}
/**
* Utility class for providing copying of a designated case class with minimal overhead.
*/
class Copier(cls: Class[_]) {
private val ctor = cls.getConstructors.apply(0)
private val getters = cls.getDeclaredFields
.filter {
f =>
val m = f.getModifiers
Modifier.isPrivate(m) && Modifier.isFinal(m) && !Modifier.isStatic(m)
}
.take(ctor.getParameterTypes.size)
.map(f => cls.getMethod(f.getName))
/**
* A reflective, non-generic version of case class copying.
*/
def apply[T](o: T, vals: (String, Any)*): T = {
val byIx = vals.map {
case (name, value) =>
val ix = getters.indexWhere(_.getName == name)
if (ix < 0) throw new IllegalArgumentException("Unknown field: " + name)
(ix, value.asInstanceOf[Object])
}.toMap
val args = (0 until getters.size).map {
i =>
byIx.get(i)
.getOrElse(getters(i).invoke(o))
}
ctor.newInstance(args: _*).asInstanceOf[T]
}
}
It is not possible using case classes.
Copy method generated at compile time and named parameters handled on compile time to. There is no possibility to do it ar runtime.
Dynamic may help to solve your issue: http://hacking-scala.tumblr.com/post/49051516694/introduction-to-type-dynamic
Yes, you would need to use reflection to do that.
It is a bit involved, because copy is a synthetic method and you'll have to invoke the getters for all fields except the one you want to replace.
To give you an idea, the copy method in this class does exactly that, except using an argument index instead of name. It calls the companion object's apply method, but the effect is the same.
I'm a bit confused - how is the following not what you need?
car: Car = ... // Retrieve an instance of Car somehow.
car.copy(type = "jeep") // Copied instance, only the type has been changed.
car.copy(door = 4) // Copied instance, only the number of doors has changed.
// ...
Is it because you have a lot of parameters for the initial instance creation? In that case, can you not use default values?
case class Car(type: String = "Jeep", door: Int = 4, ...)
You seem to know about both these features and feel that they don't fit your need - could you explain why?
Related
I was receiving this error in my Hadoop job.
java.lang.NoSuchMethodException: <PackageName>.<ClassName>.<init>(<parameters>)
In most Scala code, you would have it in compile time. But since this job is called in runtime I was not catching it in compile time.
I would think default parameter would cause constructors with both signatures to be created, one taking a single argument.
class BasicDynamicBlocker(args: Args, evaluation: Boolean = false) extends Job(args) with HiveAccess {
//I NEEDED THIS TOO:
def this(args: Args) = {
this(args, false)
}
...
}
I learned the hard way that I needed to declare the overloaded constructor using this. (I wanted to write this out in case it helps someone else.)
I also have a small questions. It still seems redundant to me. Is there a reason Scala language's design restrictions require this?
It is not like when you have default parameter you will get overloads generated for each possible case, like for example:
def method(num: Int = 4, str: String = "") = ???
you expect compiler to generate
def method(num: Int) = method(num, "")
def method(str: String) = method(4, str)
def method() = method(4, "")
but that is not the case.
You will instead have generated methods (in companion object), for each default param
def method$default$1: Int = 4
def method$default$2: String = "a"
and whenever you say in your code
method(str = "a")
it will be just changed to
method(method$default$1, "a")
So in your case, constructor with signature this(args: Args) just did not exist, there was only the 2 param version.
You can read more here: http://docs.scala-lang.org/sips/completed/named-and-default-arguments.html
Let's say there is a generic method declaration that performs a set of operations based upon a designated mode that would look something like this:
def doSomethingSmart(mode: OpMode, someGoodyList: List[Any]): Boolean = { /* do foo */ }
Where OpMode is a type/enumeration consisting of:
Read
Create
Delete
Modify
Whatever
Putting the two together would obviously yield a single-mode, reusable, code block.
Now, the type/enumeration part would probably look something like this:
object OpMode extends Enumeration {
type OpMode = Value
val Read, Write, Create, Modify, Delete, Whatever = Value
}
But let's say you wanted to expand the scope of doSomethingSmart() to span what is typically done using bitwise operators, for example: Create & Modify & Whatever. Is there a "scala-way" of restricting the bit-masked argument to that limited data-set (ie, the enumeration/type). Maybe something along these lines:
def doSomethingSmarter(more: T < [BitwiseSelectionOf[OpMode]], ...
Or, is it best to simply drop back to binary-indexed value assignments - in which cases there is no "type" checking per se?
TIA.
EDIT: I guess another possibility would be to change the OpMode to be a List and then just run a series of "contains" operations.
EDIT 2: In particular, I was looking for an efficient mechanism that provides an inline construct when making a call to doSomethingSmarter()
Enumeration defines an inner type called ValueSet which gives you at least some of the functionality you are looking for. The methods on it are still Set-like (you would add a new mode using +, and check for a mode using contains), but it may serve your purposes.
EDIT: had some fun fiddling around and came up with this:
import scala.collection.BitSet
object OpMode extends Enumeration {
protected case class Val(name: String, val mask: Int) extends super.Val(nextId, name)
type OpMode = Val
val Read = Val("Read", 1)
val Write = Val("Write", 2)
val Create = Val("Create", 4)
val Modify = Val("Modify", 8)
val Delete = Val("Delete", 16)
val Whatever = Val("Whatever", 32)
case class FlagSet(bits: BitSet) {
def isSet(mode: OpMode) = bits.contains(mode.mask)
def +(mode: OpMode) = new FlagSet(bits + mode.mask)
def -(mode: OpMode) = new FlagSet(bits - mode.mask)
def &(other: FlagSet) = new FlagSet(bits & other.bits)
def &~(other: FlagSet) = new FlagSet(bits &~ other.bits)
def ^(other: FlagSet) = new FlagSet(bits ^ other.bits)
def |(other: FlagSet) = new FlagSet(bits | other.bits)
def size = bits.size
// etc.
}
object FlagSet {
def apply(flags: OpMode*): FlagSet = apply(BitSet(flags.map(_.mask):_*))
def apply(modes: ValueSet): FlagSet = apply(BitSet(modes.toSeq.map{ case m: OpMode => m.mask }:_*))
}
}
def doSomethingSmarter(modes: OpMode.FlagSet, someGoodyList: List[Any]) = modes.size
val flags = OpMode.FlagSet(OpMode.Read, OpMode.Write)
doSomethingSmarter(flags, Nil)
val modes = OpMode.ValueSet(OpMode.Read, OpMode.Write)
doSomethingSmarter(OpMode.FlagSet(modes), Nil)
Basically, I extended the Enumeration.Val type to add a suitable bit mask for each mode, and added an inner class FlagSet to interoperate between OpModes and an under-the-covers BitSet. Changing doSomethingSmarter to take in such a FlagSet enables usage that should be closer to what you are hoping for.
The above can probably be improved upon, but working with Enumeration can be tricky. As an alternative, you might find it preferable to work with a sealed trait and case classes/objects extending it - this can often give semantics closer to what is possible with Java's enum types.
With a mutable object I can write something like
var user = DAO.getUser(id)
user.name = "John"
user.email ="john#doe.com"
// logic on user
If user is immutable then I need to clone\copy it on every change operation.
I know a few ways to perform this
case class copy
method (like changeName) that creates a new object with the new property
What is the best practice?
And one more question. Is there any existing technique to get "changes" relative to the original object(for example to generate update statement)?
Both ways you've mentioned belongs to functional and OO paradigms respectively. If you prefer functional decomposition with abstract data type, which, in Scala, is represented by case classes, then choose copy method. Using mutators is not a good practice in my option, cause that will pull you back to Java/C#/C++ way of life.
On the other hand making ADT case class like
case class Person(name: String, age: String)
is more consise then:
class Person(_name: String, _age: String) {
var name = _name
var age = _a
def changeName(newName: String): Unit = { name = newName }
// ... and so on
}
(not the best imperative code, can be shorter, but clear).
Of cause there is another way with mutators, just to return a new object on each call:
class Person(val name: String,
val age: String) {
def changeName(newName: String): Unit = new Person(newName, age)
// ... and so on
}
But still case class way is more consise.
And if you go futher, to concurrent/parallel programming, you'll see that functional consept with immutable value are much better, then tring to guess in what state your object currently are.
Update
Thanks to the senia, forgot to mention two things.
Lenses
At the most basic level, lenses are sort of getters and setters for immutable data and looks like this:
case class Lens[A,B](get: A => B, set: (A,B) => A) {
def apply(a: A) = get(a)
// ...
}
That is it. A lens is a an object that contains two functions: get and set. get takes an A and returns a B. set takes an A and B and returns a new A. It’s easy to see that the type B is a value contained in A. When we pass an instance to get we return that value. When we pass an A and a B to set we update the value B in A and return a new A reflecting the change. For convenience the get is aliased to apply. There is a good intro to Scalaz Lens case class
Records
This one, ofcause, comes from the shapeless library and called Records. An implementation of extensible records modelled as HLists of associations. Keys are encoded using singleton types and fully determine the types of their corresponding values (ex from github):
object author extends Field[String]
object title extends Field[String]
object price extends Field[Double]
object inPrint extends Field[Boolean]
val book =
(author -> "Benjamin Pierce") ::
(title -> "Types and Programming Languages") ::
(price -> 44.11) ::
HNil
// Read price field
val currentPrice = book.get(price) // Inferred type is Double
currentPrice == 44.11
// Update price field, relying on static type of currentPrice
val updated = book + (price -> (currentPrice+2.0))
// Add a new field
val extended = updated + (inPrint -> true)
I know there are multiple questions addressing related problems, but I'm not sure it does attack exactly what I'm looking for. I'm still new to Scala, after several years of Java development. I'm looking for the best way to test if an object has been initialized, and if not, initialize it then. For example, in Java:
private MyObject myObj = null;
and at some point in the future:
public void initMyObj(){
if (myObj == null){
myObj = new MyObj();
}
// do something with myObj
}
After this, I might reassign myObj to a different object, but it is unlikely. In Scala, I have this:
class Test {
var myObj: MyObj = _
}
I've read that I could use Option instead, something like:
var myObj = None : Option[MyObj]
and then my check:
myObj match {
case None => ...
case Some(value) => ...
}
but it feels ackward to use this pattern when I might not make this kind of check anywhere else at any other time - though being so new to Scala, I might be wrong. Is this the best way to achieve what I want or is there any other option not involving Option?
It is not generally ideal practice in Scala to leave partially-constructed objects lying around. You would normally rethink how your objects were getting instantiated to see if you can't use a different pattern that is less fragile. For example, instead of setting uninitialized variables in methods:
class Foo { var a: String = null; var b: String = null }
def initFooA(s: String, f: Foo) { if (f.a == null) f.a = s }
def initFooB(s: String, f: Foo) { if (f.b == null) f.b = s }
f
initFooA("salmon", f)
// Do stuff
initFooB("herring", f)
you would attempt to restructure your code to generate the values you need on demand, and delay the instantiation of foo until then:
case class Bar(a: String, b: String) {}
def initBarA(s: String) = s
def initBarB(s: String) = s
val iba = initBarA("halibut")
// Do stuff
val ibb = initBarB("cod")
Bar(iba, ibb)
Because Scala has easy access to tuples (and type inference), this can be a lot less painful than in Java.
Another thing you can do is defer the late initialization to someone else.
case class Baz(a: String)(bMaker: => String) {
lazy val b = bMaker
}
Now you pass in something that will make parameter b, and arrange for it to handle any late initialization stuff that needs to be handled. This doesn't always avoid needing to set vars, but it can help push it out of your class code into your initialization logic (which is usually a better place for it).
Doing this with vars is a little less straightforward. Realistically, you're probably best off just devoting a class to it e.g. by:
class LazyVar[A](initial: => A) {
private[this] var loaded = false
private[this] var variable: A = _
def apply() = { if (!loaded) { loaded = true; variable = initial }; variable }
def update(a: A) { loaded = true; variable = a }
}
where you then (sadly) have to use () on every read and write.
scala> val lv = new LazyVar({ println("Hi!"); 5 })
lv: LazyVar[Int] = LazyVar#2626ea08
scala> lv()
Hi!
res2: Int = 5
scala> lv() = 7
scala> lv()
res4: Int = 7
Then you use an instance of this class instead of the actual var and pass through the lazy initializer. (lazy val is very much like this under the hood; the compiler just protects you from noticing.)
Finally, if you want to have a fully-functional object that is occasionally missing a value, var x: Option[X] is the construct you want to use; if you can't find a way around the standard Java creation patterns (and you don't want to try something more exotic like objects that create each other with more and more information, either because performance is critical and you can't afford it, or you dislike writing that much boilerplate to allow type-checking to verify that your object is properly created) but you otherwise want to use it, var x: X = null is what I'd choose, not _. If X is a primitive, you probably need to choose the correct value wisely anyway (for example, Double.NaN instead of 0.0, -1 rather than 0 for Int) to indicate I-am-not-initialized. If it's generic code and you want Any instead of AnyRef, asInstanceOf-ing back and forth between Any and AnyRef is probably the best way out of poorly typechecked situation (assuming you really, really can't use Option, which at that point is much clearer).
Maybe a lazy variable is what you need.
lazy val myObj: MyObj = //here you put the object creation code
In this way the object creation is postponed to the first time the code tries to access it.
In Mercury I can use:
A = B^some_field := SomeValue
to bind A to a copy of B, except that some_field is SomeValue instead of whatever it was in B. I believe the Haskell equivalent is something like:
a = b { some_field = some_value }
Does Scala have something like this for "modifying" immutable values. The alternative seems to be to have a constructor that directly sets every field in the instance, which isn't always ideal (if there are invarients the constructor should be maintaining). Plus it would be really clunky and much more fragile if I had to explicitly pass every other value in the instance I want to have a modified copy of.
I couldn't find anything about this by googling, or in a brief survey of the language reference manual or "Scala By Example" (which I have read start-to-finish, but haven't absorbed all of yet, so it may well be in there).
I can see that this feature could have some weird interactions with Java-style access protection and subclasses though...
If you define your class as a case class, a convenient copy method is generated, and calling it you can specify with named parameters new values for certain fields.
scala> case class Sample(str: String, int: Int)
defined class Sample
scala> val s = Sample("text", 42)
s: Sample = Sample(text,42)
scala> val s2 = s.copy(str = "newText")
s2: Sample = Sample(newText,42)
It even works with polymorphic case classes:
scala> case class Sample[T](t: T, int: Int)
defined class Sample
scala> val s = Sample("text", 42)
s: Sample[java.lang.String] = Sample(text,42)
scala> val s2 = s.copy(t = List(1,2,3), 42)
s2: Sample[List[Int]] = Sample(List(1, 2, 3),42)
Note that s2 has a different type than s.
You can use case classes for this, but you don't have to. Case classes are nothing magical - the modifier case just saves you a lot of typing.
The copy method is realized by the use of named and default parameters. The names are the same as the fields and the defaults are the current values of the fields. Here's an example:
class ClassWithCopy(val field1:String, val field2:Int) {
def copy(field1:String = this.field1, field2:Int = this.field2) = {
new ClassWithCopy(field1,field2);
}
}
You can use this just like the copy method on case classes. Named and default parameters are a very useful feature, and not only for copy methods.
If the object you're planning on modifying is a case class then you can use the autogenerated copy method:
scala> val user = User(2, "Sen")
user: User = User(2,Sen)
scala> val corrected = user.copy(name = "Sean")
corrected: User = User(2,Sean)