I'm trying to use discriminators in existing project and something is wrong with my classes I guess.
Consider this scodec example. If I change TurnLeft and its codec to
sealed class TurnLeft(degrees: Int) extends Command {
def getDegrees: Int = degrees
}
implicit val leftCodec: Codec[TurnLeft] = uint8or16.xmap[TurnLeft](v => new TurnLeft(v), _.getDegrees)
I get
Error:(x, x) could not find Lazy implicit value of type scodec.Codec[Command]
val codec: Codec[Either[UnrecognizedCommand, Command]] = discriminatorFallback(unrecognizedCodec, Codec[Command])
It all works if I make degrees field value field. I suspect it's something tricky with shapeless. What should I do to make it work ?
Sample project that demonstrates the issue is here.
shapeless's Generic is defined for "case-class-like" types. To a first approximation, a case-class-like type is one whose values can be deconstructed to it's constructor parameters which can then be used to reconstruct an equal value, ie.
case class Foo ...
val foo = Foo(...)
val fooGen = Generic[Foo]
assert(fooGen.from(fooGen.to(foo)) == foo)
Case classes with a single constructor parameter list meet this criterion, whereas classes which don't have public (lazy) vals for their constructor parameters, or a companion with a matching apply/unapply, do not.
The implementation of Generic is fairly permissive, and will treat (lazy) val members which correspond to constructor parameters (by type and order) as being equivalent to accessible constructor arguments, so the closest to your example that we can get would be something like this,
sealed class TurnLeft(degrees: Int) extends Command {
val getDegrees: Int = degrees
}
scala> Generic[TurnLeft]
res0: shapeless.Generic[TurnLeft]{type Repr = Int :: HNil } = ...
In this case getDegrees is treated as the accessor for the single Int constructor parameter.
Related
I am trying to understand how a case class can be passed as an argument to a function which accepts functions as arguments. Below is an example:
Consider the below function
def !: In[B] = { ... }
If I understood correctly, this is a polymorphic method which has a type parameter B and accepts a function h as a parameter. Out and In are other two classes defined previously.
This function is then being used as shown below:
case class Q(p: boolean)(val cont: Out[R])
case class R(p: Int)
def g(c: Out[Q]) = {
val rin = c !! Q(true)_
...
}
I am aware that currying is being used to avoid writing the type annotation and instead just writing _. However, I cannot grasp why and how the case class Q is transformed to a function (h) of type Out[B] => A.
EDIT 1 Updated !! above and the In and Out definitions:
abstract class In[+A] {
def future: Future[A]
def receive(implicit d: Duration): A = {
Await.result[A](future, d)
}
def ?[B](f: A => B)(implicit d: Duration): B = {
f(receive)
}
}
abstract class Out[-A]{
def promise[B <: A]: Promise[B]
def send(msg: A): Unit = promise.success(msg)
def !(msg: A) = send(msg)
def create[B](): (In[B], Out[B])
}
These code samples are taken from the following paper: http://drops.dagstuhl.de/opus/volltexte/2016/6115/
TLDR;
Using a case class with multiple parameter lists and partially applying it will yield a partially applied apply call + eta expansion will transform the method into a function value:
val res: Out[Q] => Q = Q.apply(true) _
Longer explanation
To understand the way this works in Scala, we have to understand some fundamentals behind case classes and the difference between methods and functions.
Case classes in Scala are a compact way of representing data. When you define a case class, you get a bunch of convenience methods which are created for you by the compiler, such as hashCode and equals.
In addition, the compiler also generates a method called apply, which allows you to create a case class instance without using the new keyword:
case class X(a: Int)
val x = X(1)
The compiler will expand this call to
val x = X.apply(1)
The same thing will happen with your case class, only that your case class has multiple argument lists:
case class Q(p: boolean)(val cont: Out[R])
val q: Q = Q(true)(new Out[Int] { })
Will get translated to
val q: Q = Q.apply(true)(new Out[Int] { })
On top of that, Scala has a way to transform methods, which are a non value type, into a function type which has the type of FunctionX, X being the arity of the function. In order to transform a method into a function value, we use a trick called eta expansion where we call a method with an underscore.
def foo(i: Int): Int = i
val f: Int => Int = foo _
This will transform the method foo into a function value of type Function1[Int, Int].
Now that we posses this knowledge, let's go back to your example:
val rin = c !! Q(true) _
If we just isolate Q here, this call gets translated into:
val rin = Q.apply(true) _
Since the apply method is curried with multiple argument lists, we'll get back a function that given a Out[Q], will create a Q:
val rin: Out[R] => Q = Q.apply(true) _
I cannot grasp why and how the case class Q is transformed to a function (h) of type Out[B] => A.
It isn't. In fact, the case class Q has absolutely nothing to do with this! This is all about the object Q, which is the companion module to the case class Q.
Every case class has an automatically generated companion module, which contains (among others) an apply method whose signature matches the primary constructor of the companion class, and which constructs an instance of the companion class.
I.e. when you write
case class Foo(bar: Baz)(quux: Corge)
You not only get the automatically defined case class convenience methods such as accessors for all the elements, toString, hashCode, copy, and equals, but you also get an automatically defined companion module that serves both as an extractor for pattern matching and as a factory for object construction:
object Foo {
def apply(bar: Baz)(quux: Corge) = new Foo(bar)(quux)
def unapply(that: Foo): Option[Baz] = ???
}
In Scala, apply is a method that allows you to create "function-like" objects: if foo is an object (and not a method), then foo(bar, baz) is translated to foo.apply(bar, baz).
The last piece of the puzzle is η-expansion, which lifts a method (which is not an object) into a function (which is an object and can thus be passed as an argument, stored in a variable, etc.) There are two forms of η-expansion: explicit η-expansion using the _ operator:
val printFunction = println _
And implicit η-expansion: in cases where Scala knows 100% that you mean a function but you give it the name of a method, Scala will perform η-expansion for you:
Seq(1, 2, 3) foreach println
And you already know about currying.
So, if we put it all together:
Q(true)_
First, we know that Q here cannot possibly be the class Q. How do we know that? Because Q here is used as a value, but classes are types, and like most programming languages, Scala has a strict separation between types and values. Therefore, Q must be a value. In particular, since we know class Q is a case class, object Q is the companion module for class Q.
Secondly, we know that for a value Q
Q(true)
is syntactic sugar for
Q.apply(true)
Thirdly, we know that for case classes, the companion module has an automatically generated apply method that matches the primary constructor, so we know that Q.apply has two parameter lists.
So, lastly, we have
Q.apply(true) _
which passes the first argument list to Q.apply and then lifts Q.apply into a function which accepts the second argument list.
Note that case classes with multiple parameter lists are unusual, since only the parameters in the first parameter list are considered elements of the case class, and only elements benefit from the "case class magic", i.e. only elements get accessors implemented automatically, only elements are used in the signature of the copy method, only elements are used in the automatically generated equals, hashCode, and toString() methods, and so on.
I'd like to enrich a 'graph for scala' graph. For this purpose i've created an implicit value class:
import scalax.collection.mutable
import scalax.collection.edge.DiEdge
...
type Graph = mutable.Graph[Int, DiEdge]
implicit class EnrichGraph(val G: Graph) extends AnyVal {
def roots = G.nodes.filter(!_.hasPredecessors)
...
}
...
The problem lies with the return type of its methods, e.g.:
import ....EnrichGraph
val H: Graph = mutable.Graph[Int,DiEdge]()
val roots1 = H.nodes.filter(!_.hasPredecessors) // type Iterable[H.NodeT]
val roots2 = H.roots // type Iterable[RichGraph#G.NodeT] !!
val subgraph1 = H.filter(H.having(roots1)) // works!
val subgraph2 = H.filter(H.having(roots2)) // type mismatch!
Does the cause lie with fact that 'Graph' has dependent subtypes, e.g. NodeT? Is there a way to make this enrichment work?
What usually works is propagating the singleton type as a type parameter to EnrichGraph. That means a little bit of extra boilerplate since you have to split the implicit class into a class and an implicit def.
class EnrichGraph[G <: Graph](val G: G) extends AnyVal {
def roots: Iterable[G#NodeT] = G.nodes.filter(!_.hasPredecessors)
//...
}
implicit def EnrichGraph(g: Graph): EnrichGraph[g.type] = new EnrichGraph[g.type](g)
The gist here being that G#NodeT =:= H.NodeT if G =:= H.type, or in other words (H.type)#NodeT =:= H.NodeT. (=:= is the type equality operator)
The reason you got that weird type, is that roots has a path type dependent type. And that path contains the value G. So then the type of val roots2 in your program would need to contain a path to G. But since G is bound to an instance of EnrichGraph which is not referenced by any variable, the compiler cannot construct such a path. The "best" thing the compiler can do is construct a type with that part of the path left out: Set[_1.G.NodeT] forSome { val _1: EnrichGraph }. This is the type I actually got with your code; I assume you're using Intellij which is printing this type differently.
As pointed out by #DmytroMitin a version which might work better for you is:
import scala.collection.mutable.Set
class EnrichGraph[G <: Graph](val G: G) extends AnyVal {
def roots: Set[G.NodeT] = G.nodes.filter(!_.hasPredecessors)
//...
}
implicit def EnrichGraph(g: Graph): EnrichGraph[g.type] = new EnrichGraph[g.type](g)
Since the rest of your code actually requires a Set instead of an Iterable.
The reason why this still works despite reintroducing the path dependent type is quite tricky. Actually now roots2 will receive the type Set[_1.G.NodeT] forSome { val _1: EnrichGraph[H.type] } which looks pretty complex. But the important part is that this type still contains the knowledge that the G in _1.G.NodeT has type H.type because that information is stored in val _1: EnrichGraph[H.type].
With Set you can't use G#NodeT to give you the simpler type signatures, because G.NodeT is a subtype of G#NodeT and Set is unfortunately invariant. In our usage those type will actually always be equivalent (as I explained above), but the compiler cannot know that.
I need to write a generic method to get all fields of an object and it's value, the class of this object may contains ClassTag, so we should find a way to get it as well, is any way good way ? the difficulty is we don't know the class ahead, it may contains ClassTag (zero to many), It may not.
For example,
class A(x : Int) {}
a = new A(1)
We should output x => 1
class B[T: ClassTag]() {}
b = new B[Float]()
We should output _$1 = Float
def fields(obj: AnyRef) = obj.getClass.getDeclaredFields.map(field => (field.getName, field.get(obj))
will give you an array of pairs of field names and corresponding values, which you can massage into the format you want. You can test for types and do something depending on whether you have a ClassTag or not.
But for your specific examples: neither x in A nor the ClassTag in B are fields, they are just constructor parameters which aren't stored anywhere in the instance. To change this, you can declare it as a val:
class A(private val x: Int)
class B[T]()(private val tag: ClassTag[T])
or make sure they are used somewhere in the body outside the constructor.
I've been lately working on the DSL-style library wrapper over Apache POI functionality and faced a challenge which I can't seem to good solution for.
One of the goals of the library is to provide user with ability to build a spreadsheet model as a collection of immutable objects, i.e.
val headerStyle = CellStyle(fillPattern = CellFill.Solid, fillForegroundColor = Color.AquaMarine, font = Font(bold = true))
val italicStyle = CellStyle(font = Font(italic = true))
with the following assumptions:
User can optionally specify any parameter (that means, that you can create CellStyle with no parameters as well as with the full list of explicitly specified parameters);
If the parameter hasn't been specified explicitly by the user it is considered undefined and the default environment value (default value for the format we're converting to) will be used;
The 2nd point is important, as I want to convert this data model into multiple formats and i.e. the default font in Excel doesn't have to be the same as default font in HTML browser (and if user doesn't define the font family explicitly I'd like him to see the data using those defaults).
To deal with the requirements I've used the variation of the null pattern described here: Pattern for optional-parameters in Scala using null and also suggested here Scala default parameters and null (below a simplified example).
object ModelObject {
def apply(modelParam : String = null) : ModelObject = ModelObject(
modelParam = Option(modelParam)
)
}
case class ModelObject private(modelParam : Option[String])
Since null is used only internally in the companion object and very localized I decided to accept the null-sacrifice for the sake of the simplicity of the solution. The pattern works well with all the reference classes.
However for Scala primitive types wrappers null cannot be specified. This is especially a huge problem with Boolean for which I effectively consider 3 states (true, false and undefined). Wanting to provide the interface, where user still be able to write bold = true I decided to reach to Java wrappers which accept nulls.
object ModelObject {
def apply(boolParam : java.lang.Boolean = null) : ModelObject = ModelObject(
boolParam = Option(boolParam).map(_.booleanValue)
)
}
case class ModelObject private(boolParam : Option[Boolean])
This however doesn't right and I've been wondering whether there is a better approach to the problem. I've been thinking about defining the union types (with additional object denoting undefined value): How to define "type disjunction" (union types)?, however since the undefined state shouldn't be used explicitly the parameter type exposed by IDE to the user, it is going to be very confusing (ideally I'd like it to be Boolean).
Is there any better approach to the problem?
Further information:
More DSL API examples: https://github.com/norbert-radyk/spoiwo/blob/master/examples/com/norbitltd/spoiwo/examples/quickguide/SpoiwoExamples.scala
Sample implementation of the full class: https://github.com/norbert-radyk/spoiwo/blob/master/src/main/scala/com/norbitltd/spoiwo/model/CellStyle.scala
You can use a variation of the pattern I described here: How to provide helper methods to build a Map
To sum it up, you can use some helper generic class to represent optional arguments (much like an Option).
abstract sealed class OptArg[+T] {
def toOption: Option[T]
}
object OptArg{
implicit def autoWrap[T]( value: T ): OptArg[T] = SomeArg(value)
implicit def toOption[T]( arg: OptArg[T] ): Option[T] = arg.toOption
}
case class SomeArg[+T]( value: T ) extends OptArg[T] {
def toOption = Some( value )
}
case object NoArg extends OptArg[Nothing] {
val toOption = None
}
You can simply use it like this:
scala>case class ModelObject(boolParam: OptArg[Boolean] = NoArg)
defined class ModelObject
scala> ModelObject(true)
res12: ModelObject = ModelObject(SomeArg(true))
scala> ModelObject()
res13: ModelObject = ModelObject(NoArg)
However as you can see the OptArg now leaks in the ModelObject class itself (boolParam is typed as OptArg[Boolean] instead of Option[Boolean].
Fixing this (if it is important to you) just requires to define a separate factory as you have done yourself:
scala> :paste
// Entering paste mode (ctrl-D to finish)
case class ModelObject private(boolParam: Option[Boolean])
object ModelObject {
def apply(boolParam: OptArg[Boolean] = NoArg): ModelObject = new ModelObject(
boolParam = boolParam.toOption
)
}
// Exiting paste mode, now interpreting.
defined class ModelObject
defined module ModelObject
scala> ModelObject(true)
res22: ModelObject = ModelObject(Some(true))
scala> ModelObject()
res23: ModelObject = ModelObject(None)
UPDATE The advantage of using this pattern, over simply defining several overloaded apply methods as shown by #drexin is that in the latter case the number of overloads grows very fast with the number of arguments(2^N). If ModelObject had 4 parameters, that would mean 16 overloads to write by hand!
For purposes of my app I need to be able to find out a list of fields of a type (not an instance) and types of those fields in runtime. So far I was only able to get a list of methods of a case class containing getters with classOf[MyCaseClass].getMethods and nothing useful from a simple class. Am I missing something? Are there any reflection libraries for that kinda purposes? How's that done correctly?
Using Scala 2.10 reflection:
scala> import reflect.runtime.{universe => ru}
import reflect.runtime.{universe=>ru}
scala> trait A { val field1: Int; val field2: Char; def meth1: Int }
defined trait A
scala> val fieldSymbols = ru.typeOf[A].members.collect{ case m: ru.MethodSymbol if m.isGetter => m }
fieldSymbols: Iterable[reflect.runtime.universe.MethodSymbol] = List(value field2, value field1)
The returned symbols contain all the type information, e.g.:
scala> fieldSymbols.map(_.typeSignature)
res16: Iterable[reflect.runtime.universe.Type] = List(=> scala.Char, => scala.Int)
You may want to take a look at this document on reflecting scala. getMethods is a method from Java reflection. What can't you find there? From the Javadoc:
String getName(): Returns the name of the method represented by this Method object, as a String.
Class[] getParameterTypes(): Returns an array of Class objects that represent the formal parameter types, in declaration order, of the method represented by this Method object.
Class getReturnType(): Returns a Class object that represents the formal return type of the method represented by this Method object.
You could read more about Java reflection.
Note that not all type information will be available at runtime because of erasure.