I would like an explanation of difference for example between this declaration:
class Clazz(param1: String, param2: Integer)
and this one:
class Clazz(param1: String)(param2: Integer)
Does second declaration affect just the way of instantiating the objects or is there any deeper reason I don't know about.
One reason I thought about would be multiple variable length of parameters for example:
class Clazz(param1: String*)(param2: Integer*)
So are there any others?
#1 Type inference. It goes from left to right and is done per parameter list.
scala> class Foo[A](x: A, y: A => Unit)
defined class Foo
scala> new Foo(2, x => println(x))
<console>:24: error: missing parameter type
new Foo(2, x => println(x))
^
scala> class Foo[A](x: A)(y: A => Unit)
defined class Foo
scala> new Foo(2)(x => println(x))
res22: Foo[Int] = Foo#4dc1e4
#2 Implicit parameter list.
scala> class Foo[A](x: A)(implicit ord: scala.Ordering[A]) {
| def compare(y: A) = ord.compare(x, y)
| }
defined class Foo
scala> new Foo(3)
res23: Foo[Int] = Foo#965701
scala> res23 compare 7
res24: Int = -1
scala> new Foo(new {})
<console>:24: error: No implicit Ordering defined for java.lang.Object.
new Foo(new {})
^
In the second version you are declaring a curried primary constructor for Clazz. So the difference between the two versions is the same as difference between "normal" and curried functions in Scala, i.e.
def foo(param1: String, param2: Int)
def foo(param1: String)(param2: Int)
Most of the time both declarations can be used interchangeably but if you often need to curry function then it makes more sense to declare it in curried form. Note you can also convert a normal function or even constructor into a curried form, for e.g you could transform your normal Clazz constructor into curried form using this:
(new Clazz(_, _)).curried
You also need multiple parameter lists if you are passing an implicit value (as the keyword implicit applies to the complete parameter list)
Related
It is a question followed by this.
Now we have proved that Array[Int] could be implicitly converted into Int => Int, but where is the place that happens?
Obviously, Int => Int is a Function:
scala> var fun = (i: Int) => Array(1,2,3)(i)
fun: Int => Int = <function1>
And Array[Int] is a class:
scala> var arr = Array(1,2,3)
arr: Array[Int] = Array(1, 2, 3)
But arr can be assigned to fun:
scala> fun = arr
fun: Int => Int = WrappedArray(1, 2, 3)
Conversely, it will not work:
scala> arr = fun
<console>:34: error: type mismatch;
found : Int => Int
required: Array[Int]
arr = fun
^
Now, Where is the place that implicit conversion happens?
Since you have WrappedArray I'm guessing you have Scala 2.12 or earlier - Scala 2.13 has ArrayOps.
There is an implicit conversion (actually a whole family o conversions) from Array to WrappedArray. So you have an implicit conversion from Array[T] to WrappedArray[T].
Now, WrappedArray implements all sorts of Scala's collection traits. One of them is Seq implementing PartialFunction (which in turn extends Function). That means that all of collections in Scala are functions from some key (Int in case of sequences, possibly something else in case of Maps). This includes WrappedArray.
So your conversion to WrappedArray automatically turns an Array[T] into a (subtype of a) function Int => T.
As a matter of the fact, this is a reason why you should avoid putting any collection (or things implicitly convertible to collections like Array before 2.13) into implicit scope: a collection in implicit scope automatically becomes implicit conversion itself. This problem was solved only in Scala 3 where implicit conversions became a separate subtype of Function interface.
but where is the place that happens?
Implicit conversions happen when necessary, in this case in the assignment itself.
Look at my reify block in my other answer:
val foo: Function1[Int, Int] = Predef.wrapIntArray(Array.apply(1, 2, 3));
That is the conversion right there, calling the method: wrapIntArray in the Predef object.
Obviously, Int => Int is a Function:
And Array[Int] is a class:
But arr can be assigned to fun:
Conversely, it will not work:
It seems you don't know / understand that in Scala functions are also objects (almost everything in Scala is an object, anything that is a value will be an object).
So we can't directly assign an Array[Int] to a value that should be an Int => Int; because Array[Int] is not a subtype of Int => Int (as shown in my previous answer).
But we can convert an Array[Int] into an ArraySeq[Int] which is a subtype of an Int => Int and thus we can do the assignment.
The opposite won't work because an Int => Int is not an Array[Int] nor a ArraySeq[Int]
Note this is just basic subtyping, a Dog is a subtype of Pet but a Pet is NOT a subtype of Dog
Which one is the class/trait related to Function[Int, Int]? I have checked the link you mentioned: sealed abstract class ArraySeq[+A] extends AbstractSeq[A] with IndexedSeq[A] with IndexedSeqOps[A, ArraySeq, ArraySeq[A]] with StrictOptimizedSeqOps[A, ArraySeq, ArraySeq[A]] with EvidenceIterableFactoryDefaults[A, ArraySeq, ClassTag] with Serializable
Those are the classes / traits ArraySeq directly extends, in the Scaladoc link there is a button to see all supper types ad in those you will see (Int) => A
Edit
The last question is which super type does the (Int) => A come from? There are too many types ArraySeq extended.
You need to traverse the hierarchy:
immutable.ArraySeq extends immutable.AbstractSeq
immutable.AbstractSeq extends immutable.Seq
immutable.Seq extends collection.Seq
collection.Seq extends PartialFunction[Int, A]
PartialFunction[Int, A] extends Function1[Int, A]
When you assign Array(1) to a val of type Int => Int, the implicit conversion scala.LowPriorityImplicits#wrapIntArray is called.
It wraps the Array into ArraySeq and the later implements Function1 or Int => Int
i'm trying to understand the behavior of the compiler in this situation
object ImplicitTest extends App {
def foo[T](implicit x: (String => T)): T = ???
implicit val bar = (x: String) => x.toInt
foo
}
the code above does not compile and gives the following error:
ambiguous implicit values: both method $conforms in object Predef of
type [A]⇒ <:<[A,A] and value bar in object ImplicitTest of type ⇒ String ⇒
Int match expected type String ⇒ T
as the error says my implicit value is conflicting with another implicit defined in Predef... based on this it seems there is no way to declare an implicit parameter to a function converting a value from a known type to an unknown (generic) type.
Is this due to some technical limitation on the compiler or is just the way it is supposed to work, and i'm violating some constraints i'm not aware of?
You're not providing a type parameter to foo when you call it (and there is no other way to infer it, for the following reason), so the compiler is having trouble finding the right one, and the right implicit.
You have the implicit bar: String => Int in scope, but you also have implicits in Predef that create instances of =:= and <:< which both extend A => B, and create implicit String => As. The compiler is looking for some implicit function String => T for foo, but it's not sure which one, and you have multiple in scope. bar will not take precedence because you haven't specified the specific String => T it's looking for.
This will work:
def foo[T](implicit x: (String => T)): T = ???
implicit val bar = (x: String) => x.toInt
foo[Int]
While dealing with some Java code, I wanted to find a way to reduce a Raw Set to include its parameterized type.
I also wanted it to work for Scala sets as well, so I did the following
implicit class Harden[S <% mutable.Set[_]](val set: S) extends AnyVal {
def cast[T] = set.map(_.asInstanceOf[T])
}
That resulted in a compiler error that I didn't expect
Error:(27, 27) field definition is not allowed in value class
implicit class Harden[S <% mutable.Set[_]](val set: S) extends AnyVal {
I didn't find any mention of this type of restriction in the Scala View Bounds or Value Class documentation.
Why is this not allowed? I'm using Scala 2.10.3.
As you can see from this sbt console output:
scala> :type implicit class Harden[S <% mutable.Set[_]](val set: S)
[S]AnyRef {
val set: S
private[this] val set: S
implicit private[this] val evidence$1: S => scala.collection.mutable.Set[_]
def <init>(set: S)(implicit evidence$1: S => scala.collection.mutable.Set[_]): Harden[S]
}
... the actual constructor of Harden desugars behind the scenes to:
def <init>(set: S)(implicit evidence$1: S => scala.collection.mutable.Set[_]): Harden[S]
... (i.e. takes set in one argument list and implicit evidence$1 in another).
As described in value classes limitations here:
must have only a primary constructor with exactly one public, val parameter whose type is not a value class.
... whitch means that Harden violaties this limitation.
You can achieve something similar, howerver. Try to convert your view bound defined on class to implicit evidence on method instead.
Something like this:
scala> implicit class Harden[S](val set: S) extends AnyVal {
| def cast[T](implicit ev: S => scala.collection.mutable.Set[_]) = set.map(_.asInstanceOf[T])
| }
defined class Harden
This will compile:
scala> Set(1,2,3).cast[Any]
res17: scala.collection.mutable.Set[Any] = Set(1, 2, 3)
And this will fail, as expected:
scala> List(1,2,3).cast[Any]
<console>:24: error: No implicit view available from List[Int] => scala.collection.mutable.Set[_].
List(1,2,3).cast[Any]
^
It is not allowed because as structured now, value classes must have exactly one parameter, but
implicit class Foo[A <% B](val a: A)
desugars to
implicit class Foo[A,B](val a: A)(implicit evidence$1: A => B)
which no longer has just a single parameter.
Sometime I stumble into the semi-mysterious notation of
def f[T](..) = new T[({type l[A]=SomeType[A,..]})#l] {..}
in Scala blog posts, which give it a "we used that type-lambda trick" handwave.
While I have some intutition about this (we gain an anonymous type parameter A without having to pollute the definition with it?), I found no clear source describing what the type lambda trick is, and what are its benefits. Is it just syntactic sugar, or does it open some new dimensions?
Type lambdas are vital quite a bit of the time when you are working with higher-kinded types.
Consider a simple example of defining a monad for the right projection of Either[A, B]. The monad typeclass looks like this:
trait Monad[M[_]] {
def point[A](a: A): M[A]
def bind[A, B](m: M[A])(f: A => M[B]): M[B]
}
Now, Either is a type constructor of two arguments, but to implement Monad, you need to give it a type constructor of one argument. The solution to this is to use a type lambda:
class EitherMonad[A] extends Monad[({type λ[α] = Either[A, α]})#λ] {
def point[B](b: B): Either[A, B]
def bind[B, C](m: Either[A, B])(f: B => Either[A, C]): Either[A, C]
}
This is an example of currying in the type system - you have curried the type of Either, such that when you want to create an instance of EitherMonad, you have to specify one of the types; the other of course is supplied at the time you call point or bind.
The type lambda trick exploits the fact that an empty block in a type position creates an anonymous structural type. We then use the # syntax to get a type member.
In some cases, you may need more sophisticated type lambdas that are a pain to write out inline. Here's an example from my code from today:
// types X and E are defined in an enclosing scope
private[iteratee] class FG[F[_[_], _], G[_]] {
type FGA[A] = F[G, A]
type IterateeM[A] = IterateeT[X, E, FGA, A]
}
This class exists exclusively so that I can use a name like FG[F, G]#IterateeM to refer to the type of the IterateeT monad specialized to some transformer version of a second monad which is specialized to some third monad. When you start to stack, these kinds of constructs become very necessary. I never instantiate an FG, of course; it's just there as a hack to let me express what I want in the type system.
The benefits are exactly the same as those conferred by anonymous functions.
def inc(a: Int) = a + 1; List(1, 2, 3).map(inc)
List(1, 2, 3).map(a => a + 1)
An example usage, with Scalaz 7. We want to use a Functor that can map a function over the second element in a Tuple2.
type IntTuple[+A]=(Int, A)
Functor[IntTuple].map((1, 2))(a => a + 1)) // (1, 3)
Functor[({type l[a] = (Int, a)})#l].map((1, 2))(a => a + 1)) // (1, 3)
Scalaz provides some implicit conversions that can infer the type argument to Functor, so we often avoid writing these altogether. The previous line can be rewritten as:
(1, 2).map(a => a + 1) // (1, 3)
If you use IntelliJ, you can enable Settings, Code Style, Scala, Folding, Type Lambdas. This then hides the crufty parts of the syntax, and presents the more palatable:
Functor[[a]=(Int, a)].map((1, 2))(a => a + 1)) // (1, 3)
A future version of Scala might directly support such a syntax.
To put things in context: This answer was originally posted in another thread. You are seeing it here because the two threads have been merged. The question statement in the said thread was as follows:
How to resolve this type definition: Pure[({type ?[a]=(R, a)})#?] ?
What are the reasons of using such construction?
Snipped comes from scalaz library:
trait Pure[P[_]] {
def pure[A](a: => A): P[A]
}
object Pure {
import Scalaz._
//...
implicit def Tuple2Pure[R: Zero]: Pure[({type ?[a]=(R, a)})#?] = new Pure[({type ?[a]=(R, a)})#?] {
def pure[A](a: => A) = (Ø, a)
}
//...
}
Answer:
trait Pure[P[_]] {
def pure[A](a: => A): P[A]
}
The one underscore in the boxes after P implies that it is a type constructor takes one type and returns another type. Examples of type constructors with this kind: List, Option.
Give List an Int, a concrete type, and it gives you List[Int], another concrete type. Give List a String and it gives you List[String]. Etc.
So, List, Option can be considered to be type level functions of arity 1. Formally we say, they have a kind * -> *. The asterisk denotes a type.
Now Tuple2[_, _] is a type constructor with kind (*, *) -> * i.e. you need to give it two types to get a new type.
Since their signatures do not match, you cannot substitute Tuple2 for P. What you need to do is partially apply Tuple2 on one of its arguments, which will give us a type constructor with kind * -> *, and we can substitue it for P.
Unfortunately Scala has no special syntax for partial application of type constructors, and so we have to resort to the monstrosity called type lambdas. (What you have in your example.) They are called that because they are analogous to lambda expressions that exist at value level.
The following example might help:
// VALUE LEVEL
// foo has signature: (String, String) => String
scala> def foo(x: String, y: String): String = x + " " + y
foo: (x: String, y: String)String
// world wants a parameter of type String => String
scala> def world(f: String => String): String = f("world")
world: (f: String => String)String
// So we use a lambda expression that partially applies foo on one parameter
// to yield a value of type String => String
scala> world(x => foo("hello", x))
res0: String = hello world
// TYPE LEVEL
// Foo has a kind (*, *) -> *
scala> type Foo[A, B] = Map[A, B]
defined type alias Foo
// World wants a parameter of kind * -> *
scala> type World[M[_]] = M[Int]
defined type alias World
// So we use a lambda lambda that partially applies Foo on one parameter
// to yield a type of kind * -> *
scala> type X[A] = World[({ type M[A] = Foo[String, A] })#M]
defined type alias X
// Test the equality of two types. (If this compiles, it means they're equal.)
scala> implicitly[X[Int] =:= Foo[String, Int]]
res2: =:=[X[Int],Foo[String,Int]] = <function1>
Edit:
More value level and type level parallels.
// VALUE LEVEL
// Instead of a lambda, you can define a named function beforehand...
scala> val g: String => String = x => foo("hello", x)
g: String => String = <function1>
// ...and use it.
scala> world(g)
res3: String = hello world
// TYPE LEVEL
// Same applies at type level too.
scala> type G[A] = Foo[String, A]
defined type alias G
scala> implicitly[X =:= Foo[String, Int]]
res5: =:=[X,Foo[String,Int]] = <function1>
scala> type T = World[G]
defined type alias T
scala> implicitly[T =:= Foo[String, Int]]
res6: =:=[T,Foo[String,Int]] = <function1>
In the case you have presented, the type parameter R is local to function Tuple2Pure and so you cannot simply define type PartialTuple2[A] = Tuple2[R, A], because there is simply no place where you can put that synonym.
To deal with such a case, I use the following trick that makes use of type members. (Hopefully the example is self-explanatory.)
scala> type Partial2[F[_, _], A] = {
| type Get[B] = F[A, B]
| }
defined type alias Partial2
scala> implicit def Tuple2Pure[R]: Pure[Partial2[Tuple2, R]#Get] = sys.error("")
Tuple2Pure: [R]=> Pure[[B](R, B)]
Sometime I stumble into the semi-mysterious notation of
def f[T](..) = new T[({type l[A]=SomeType[A,..]})#l] {..}
in Scala blog posts, which give it a "we used that type-lambda trick" handwave.
While I have some intutition about this (we gain an anonymous type parameter A without having to pollute the definition with it?), I found no clear source describing what the type lambda trick is, and what are its benefits. Is it just syntactic sugar, or does it open some new dimensions?
Type lambdas are vital quite a bit of the time when you are working with higher-kinded types.
Consider a simple example of defining a monad for the right projection of Either[A, B]. The monad typeclass looks like this:
trait Monad[M[_]] {
def point[A](a: A): M[A]
def bind[A, B](m: M[A])(f: A => M[B]): M[B]
}
Now, Either is a type constructor of two arguments, but to implement Monad, you need to give it a type constructor of one argument. The solution to this is to use a type lambda:
class EitherMonad[A] extends Monad[({type λ[α] = Either[A, α]})#λ] {
def point[B](b: B): Either[A, B]
def bind[B, C](m: Either[A, B])(f: B => Either[A, C]): Either[A, C]
}
This is an example of currying in the type system - you have curried the type of Either, such that when you want to create an instance of EitherMonad, you have to specify one of the types; the other of course is supplied at the time you call point or bind.
The type lambda trick exploits the fact that an empty block in a type position creates an anonymous structural type. We then use the # syntax to get a type member.
In some cases, you may need more sophisticated type lambdas that are a pain to write out inline. Here's an example from my code from today:
// types X and E are defined in an enclosing scope
private[iteratee] class FG[F[_[_], _], G[_]] {
type FGA[A] = F[G, A]
type IterateeM[A] = IterateeT[X, E, FGA, A]
}
This class exists exclusively so that I can use a name like FG[F, G]#IterateeM to refer to the type of the IterateeT monad specialized to some transformer version of a second monad which is specialized to some third monad. When you start to stack, these kinds of constructs become very necessary. I never instantiate an FG, of course; it's just there as a hack to let me express what I want in the type system.
The benefits are exactly the same as those conferred by anonymous functions.
def inc(a: Int) = a + 1; List(1, 2, 3).map(inc)
List(1, 2, 3).map(a => a + 1)
An example usage, with Scalaz 7. We want to use a Functor that can map a function over the second element in a Tuple2.
type IntTuple[+A]=(Int, A)
Functor[IntTuple].map((1, 2))(a => a + 1)) // (1, 3)
Functor[({type l[a] = (Int, a)})#l].map((1, 2))(a => a + 1)) // (1, 3)
Scalaz provides some implicit conversions that can infer the type argument to Functor, so we often avoid writing these altogether. The previous line can be rewritten as:
(1, 2).map(a => a + 1) // (1, 3)
If you use IntelliJ, you can enable Settings, Code Style, Scala, Folding, Type Lambdas. This then hides the crufty parts of the syntax, and presents the more palatable:
Functor[[a]=(Int, a)].map((1, 2))(a => a + 1)) // (1, 3)
A future version of Scala might directly support such a syntax.
To put things in context: This answer was originally posted in another thread. You are seeing it here because the two threads have been merged. The question statement in the said thread was as follows:
How to resolve this type definition: Pure[({type ?[a]=(R, a)})#?] ?
What are the reasons of using such construction?
Snipped comes from scalaz library:
trait Pure[P[_]] {
def pure[A](a: => A): P[A]
}
object Pure {
import Scalaz._
//...
implicit def Tuple2Pure[R: Zero]: Pure[({type ?[a]=(R, a)})#?] = new Pure[({type ?[a]=(R, a)})#?] {
def pure[A](a: => A) = (Ø, a)
}
//...
}
Answer:
trait Pure[P[_]] {
def pure[A](a: => A): P[A]
}
The one underscore in the boxes after P implies that it is a type constructor takes one type and returns another type. Examples of type constructors with this kind: List, Option.
Give List an Int, a concrete type, and it gives you List[Int], another concrete type. Give List a String and it gives you List[String]. Etc.
So, List, Option can be considered to be type level functions of arity 1. Formally we say, they have a kind * -> *. The asterisk denotes a type.
Now Tuple2[_, _] is a type constructor with kind (*, *) -> * i.e. you need to give it two types to get a new type.
Since their signatures do not match, you cannot substitute Tuple2 for P. What you need to do is partially apply Tuple2 on one of its arguments, which will give us a type constructor with kind * -> *, and we can substitue it for P.
Unfortunately Scala has no special syntax for partial application of type constructors, and so we have to resort to the monstrosity called type lambdas. (What you have in your example.) They are called that because they are analogous to lambda expressions that exist at value level.
The following example might help:
// VALUE LEVEL
// foo has signature: (String, String) => String
scala> def foo(x: String, y: String): String = x + " " + y
foo: (x: String, y: String)String
// world wants a parameter of type String => String
scala> def world(f: String => String): String = f("world")
world: (f: String => String)String
// So we use a lambda expression that partially applies foo on one parameter
// to yield a value of type String => String
scala> world(x => foo("hello", x))
res0: String = hello world
// TYPE LEVEL
// Foo has a kind (*, *) -> *
scala> type Foo[A, B] = Map[A, B]
defined type alias Foo
// World wants a parameter of kind * -> *
scala> type World[M[_]] = M[Int]
defined type alias World
// So we use a lambda lambda that partially applies Foo on one parameter
// to yield a type of kind * -> *
scala> type X[A] = World[({ type M[A] = Foo[String, A] })#M]
defined type alias X
// Test the equality of two types. (If this compiles, it means they're equal.)
scala> implicitly[X[Int] =:= Foo[String, Int]]
res2: =:=[X[Int],Foo[String,Int]] = <function1>
Edit:
More value level and type level parallels.
// VALUE LEVEL
// Instead of a lambda, you can define a named function beforehand...
scala> val g: String => String = x => foo("hello", x)
g: String => String = <function1>
// ...and use it.
scala> world(g)
res3: String = hello world
// TYPE LEVEL
// Same applies at type level too.
scala> type G[A] = Foo[String, A]
defined type alias G
scala> implicitly[X =:= Foo[String, Int]]
res5: =:=[X,Foo[String,Int]] = <function1>
scala> type T = World[G]
defined type alias T
scala> implicitly[T =:= Foo[String, Int]]
res6: =:=[T,Foo[String,Int]] = <function1>
In the case you have presented, the type parameter R is local to function Tuple2Pure and so you cannot simply define type PartialTuple2[A] = Tuple2[R, A], because there is simply no place where you can put that synonym.
To deal with such a case, I use the following trick that makes use of type members. (Hopefully the example is self-explanatory.)
scala> type Partial2[F[_, _], A] = {
| type Get[B] = F[A, B]
| }
defined type alias Partial2
scala> implicit def Tuple2Pure[R]: Pure[Partial2[Tuple2, R]#Get] = sys.error("")
Tuple2Pure: [R]=> Pure[[B](R, B)]