Scala: implicitly to implicit class - scala

Given:
implicit class Foo(val i: Int) {
def addValue(v: Int): Int = i + v
}
is it possible apply to it any implicitly?
I get an error here:
<console>:14: error: could not find implicit value for parameter e: Foo
implicitly[Foo]

An implicit class Foo(val i: Int) means that there is an implicit conversion from Int to Foo. So implicitly[Int => Foo] should work.
Think about it like this: if you could summon a Foo with implicitly[Foo], which Foo would you expect to get? A Foo(0)? A Foo(1)? A Foo(2)?

For further details,
implcitly key word can be explained as following
implitly[T] means return implicit value of type T in the context
Which means, to get Foo implicitly you need to create an implicit value in the scope
For example,
implicit class Foo(val i: Int) {
def addValue(v: Int): Int = i + v
}
implicit val foo:Foo = Foo(1)
val fooImplicitly = implicitly[Foo] // Foo(1)
Also, note that Foo itself is only a class,
But by putting implicit key word in front of class definition,
Compiler creates an implicit function of type Int => Foo

Related

Making Typeclass Instance with `implicit def`?

Given:
scala> trait Resource[A] { def f: String }
defined trait Resource
scala> case class Foo(x: String)
defined class Foo
And then an implicit:
scala> implicit def fooToResource(foo: Foo): Resource[Foo] =
new Resource[Foo] { def f = foo.x }
The following works:
scala> implicitly[Resource[Foo]](Foo("foo")).f
res2: String = foo
I defined a function:
scala> def f[A](x: A)(implicit ev: Resource[A]): String = ev.f
f: [A](x: A)(implicit ev: Resource[A])String
However, the following code fails to compile:
scala> f(Foo("foo"))
<console>:17: error: could not find implicit value for parameter ev: Resource[Foo]
f(Foo("foo"))
Secondly, then I tried:
scala> f2(Foo("bippy"))
<console>:17: error: could not find implicit value for parameter ev: Resource[Foo]
f2(Foo("bippy"))
^
Lastly, I attempted:
scala> def g(foo: Foo)(implicit ev: Resource[Foo]): String = ev.f
g: (foo: Foo)(implicit ev: Resource[Foo])String
scala> g(Foo("5"))
<console>:17: error: could not find implicit value for parameter ev: Resource[Foo]
g(Foo("5"))
^
However, it failed too. How can I fix f?
Ok with Peter Neyens' answer, this is not a typeclass, this is an implicit conversion, which you should avoid - there should have been some warning, asking that you import scala.language.implicitConversions.
As a complement, here is why the first implicitly works:
Implicitly is just:
def implicitly[T](implicit ev: T): T = e
When you write implicitly[T] without supplying a parameter, it will look for an implicit of type T in scope and return it. However, you call implicitly with a parameter (I believe there is no legitimate reason to do that, ever), so it would just return your parameter, Foo("foo"), an instance of Foo. Except that you explicitly stated that T should be Resource[Foo]. If you had written a type ascription, such as (Foo("foo"): Resource[Foo]), it would have worked the same way. implicitly is not relevant here.
The point is that Foo("foo") is not of the expected type Resource[Foo], but just a Foo. The compiler would reject that, except that at this point, the implicit conversion you defined above kicks in, and your Foo instance is transformed into a Resource[Foo]. Then, you can call f.
Next, you call your f(Foo("foo")). There is an implicit parameter, however this time, you don't supply it. So the compiler looks for one (while it did no such thing the first time), and as there is no such instance, fails.
The implicit def fooToResource is not a type class instance, but does return one if you supply a Foo, that's the reason the following line works :
implicitly[Resource[Foo]](Foo("foo")).f
A solution would be to change the Resource.f function to take a parameter of type A :
trait Resource[A] {
def f(a: A): String
}
You then could define a Resource type class instance for Foo as follows:
case class Foo(x: String)
implicit val fooResource = new Resource[Foo] {
def f(foo: Foo) = foo.x
}
We can rewrite f to use the changed Resource :
def f[A](a: A)(implicit resA: Resource[A]): String = resA.f(a)
Which does what (I think) you need :
f(Foo("hello world")) // String = hello world

Alternative Syntax When Defining Implicit?

Given the following:
scala> trait Foo { def get: String = "get" }
defined trait Foo
I implemented it and made an implicit:
scala> case class FooImpl(x: String) extends Foo {
| override def get = s"got $x"
| }
defined class FooImpl
scala> implicit val fooImpl = FooImpl("yo")
fooImpl: FooImpl = FooImpl(yo)
Lastly, I tried to write a method that implicitly resolves a Foo, returning get on that implicitly resolved class.
scala> def f[A: Foo](x: A) = x.get
<console>:11: error: Foo does not take type parameters
def f[A: Foo](x: A) = x.get
^
<console>:11: error: value get is not a member of type parameter A
def f[A: Foo](x: A) = x.get
^
But I got the above errors.
So I re-wrote it using the implicit keyword:
scala> def f(implicit x: Foo): String = x.get
f: (implicit x: Foo)String
scala> f
res0: String = got yo
Is it possible to re-write this example to not explicitly specify the implicit keyword?
Note - it's possible that I'm confusing this notation with TypeTag under the section, Using a Context bound of a Type Parameter.
You are using the wrong syntax, what you want is an upper bound:
scala> def f[A <: Foo](x: A) = x.get
f: [A <: Foo](x: A)String
Where you're saying that A is a subtype of Foo and that tells the compiler that A does have a method called get.
The syntax you're using (:) means that there's an implicit conversion from A to Foo[A], the problem is that Foo doesn't take a type parameter, you can also check it into the REPL where the column syntax is translated to an implicit parameter:
scala> trait Foo2[T]
defined trait Foo2
scala> def g[T: Foo2](x: Int): Int = x
g: [T](x: Int)(implicit evidence$1: Foo2[T])Int

passing a function with type parameter as argument

I want to create a function g that takes a function f as a parameter, where f has a type parameter. I can't get a signature for g that compiles. One attempt is like this:
scala> def mock1[A](): A = null.asInstanceOf[A] // STUB
mock1: [A]()A
scala> def mock2[A](): A = null.asInstanceOf[A] // STUB
mock2: [A]()A
scala> def g0(f: [A]() => A): Int = f[Int]()
<console>:1: error: identifier expected but '[' found.
def g0(f: [A]() => A): Int = f[Int]()
^
I can get it to work if I wrap the function that takes a type parameter in a trait, like so:
scala> trait FWrapper { def f[A](): A }
defined trait FWrapper
scala> class mock1wrapper extends FWrapper { def f[A]() = mock1[A]() }
defined class mock1wrapper
scala> class mock2wrapper extends FWrapper { def f[A]() = mock2[A]() }
defined class mock2wrapper
scala> def g(wrapper: FWrapper): Int = wrapper.f[Int]()
g: (wrapper: FWrapper)Int
scala> g(new mock1wrapper)
res8: Int = 0
Is there a way I can accomplish this without introducing the wrapper class?
Scala does (currently) not have support for polymorphic function values. You have two options:
Stick with your wrapper trait (probably easiest to understand)
Use polymorphic function values from Shapeless (fancy, but maybe a bit complicated)
How about this:
def mock[A](): A = null.asInstanceOf[A]
def g[A](f:() => A): A = f()
g(mock[Int])

How to use Scala 2.10 implicit classes

I thought this would be correct usage of the new implicit classes of Scala 2.10:
implicit case class IntOps(i: Int) extends AnyVal {
def twice = i * 2
}
11.twice
Apparently not:
<console>:13: error: value twice is not a member of Int
11.twice
^
Am I missing something (Scala 2.10.0-M6)?
A clue is the desugaring of implicit classes, explained in the SIP-13:
implicit class RichInt(n: Int) extends Ordered[Int] {
def min(m: Int): Int = if (n <= m) n else m
...
}
will be transformed by the compiler as follows:
class RichInt(n: Int) extends Ordered[Int] {
def min(m: Int): Int = if (n <= m) n else m
...
}
implicit final def RichInt(n: Int): RichInt = new RichInt(n)
As you can see, the implicit function that is created has the same name as the original classes. I guess it's done like this to make implicit classes easier to import.
Thus in your case, when you create an implicit case class, there is a conflict between the method name created by the implicit keyword and the companion object created by the case keyword.
This shows there is an ambiguity:
val ops: IntOps = 11
<console>:11: error: ambiguous reference to overloaded definition,
both method IntOps of type (i: Int)IntOps
and object IntOps of type IntOps.type
match argument types (Int) and expected result type IntOps
val ops: IntOps = 11
^
Not sure what exactly happens. But when not using a case class it seems fine:
implicit class IntOps(val i: Int) extends AnyVal {
def twice = i * 2
}
11.twice // ok

What is the Scala identifier "implicitly"?

I have seen a function named implicitly used in Scala examples. What is it, and how is it used?
Example here:
scala> sealed trait Foo[T] { def apply(list : List[T]) : Unit }; object Foo {
| implicit def stringImpl = new Foo[String] {
| def apply(list : List[String]) = println("String")
| }
| implicit def intImpl = new Foo[Int] {
| def apply(list : List[Int]) = println("Int")
| }
| } ; def foo[A : Foo](x : List[A]) = implicitly[Foo[A]].apply(x)
defined trait Foo
defined module Foo
foo: [A](x: List[A])(implicit evidence$1: Foo[A])Unit
scala> foo(1)
<console>:8: error: type mismatch;
found : Int(1)
required: List[?]
foo(1)
^
scala> foo(List(1,2,3))
Int
scala> foo(List("a","b","c"))
String
scala> foo(List(1.0))
<console>:8: error: could not find implicit value for evidence parameter of type
Foo[Double]
foo(List(1.0))
^
Note that we have to write implicitly[Foo[A]].apply(x) since the compiler thinks that implicitly[Foo[A]](x) means that we call implicitly with parameters.
Also see How to investigate objects/types/etc. from Scala REPL? and Where does Scala look for implicits?
implicitly is avaliable in Scala 2.8 and is defined in Predef as:
def implicitly[T](implicit e: T): T = e
It is commonly used to check if an implicit value of type T is available and return it if such is the case.
Simple example from retronym's presentation:
scala> implicit val a = "test" // define an implicit value of type String
a: java.lang.String = test
scala> val b = implicitly[String] // search for an implicit value of type String and assign it to b
b: String = test
scala> val c = implicitly[Int] // search for an implicit value of type Int and assign it to c
<console>:6: error: could not find implicit value for parameter e: Int
val c = implicitly[Int]
^
Here are a few reasons to use the delightfully simple method implicitly.
To understand/troubleshoot Implicit Views
An Implicit View can be triggered when the prefix of a selection (consider for example, the.prefix.selection(args) does not contain a member selection that is applicable to args (even after trying to convert args with Implicit Views). In this case, the compiler looks for implicit members, locally defined in the current or enclosing scopes, inherited, or imported, that are either Functions from the type of that the.prefix to a type with selection defined, or equivalent implicit methods.
scala> 1.min(2) // Int doesn't have min defined, where did that come from?
res21: Int = 1
scala> implicitly[Int => { def min(i: Int): Any }]
res22: (Int) => AnyRef{def min(i: Int): Any} = <function1>
scala> res22(1) //
res23: AnyRef{def min(i: Int): Int} = 1
scala> .getClass
res24: java.lang.Class[_] = class scala.runtime.RichInt
Implicit Views can also be triggered when an expression does not conform to the Expected Type, as below:
scala> 1: scala.runtime.RichInt
res25: scala.runtime.RichInt = 1
Here the compiler looks for this function:
scala> implicitly[Int => scala.runtime.RichInt]
res26: (Int) => scala.runtime.RichInt = <function1>
Accessing an Implicit Parameter Introduced by a Context Bound
Implicit parameters are arguably a more important feature of Scala than Implicit Views. They support the type class pattern. The standard library uses this in a few places -- see scala.Ordering and how it is used in SeqLike#sorted. Implicit Parameters are also used to pass Array manifests, and CanBuildFrom instances.
Scala 2.8 allows a shorthand syntax for implicit parameters, called Context Bounds. Briefly, a method with a type parameter A that requires an implicit parameter of type M[A]:
def foo[A](implicit ma: M[A])
can be rewritten as:
def foo[A: M]
But what's the point of passing the implicit parameter but not naming it? How can this be useful when implementing the method foo?
Often, the implicit parameter need not be referred to directly, it will be tunneled through as an implicit argument to another method that is called. If it is needed, you can still retain the terse method signature with the Context Bound, and call implicitly to materialize the value:
def foo[A: M] = {
val ma = implicitly[M[A]]
}
Passing a subset of implicit parameters explicitly
Suppose you are calling a method that pretty prints a person, using a type class based approach:
trait Show[T] { def show(t: T): String }
object Show {
implicit def IntShow: Show[Int] = new Show[Int] { def show(i: Int) = i.toString }
implicit def StringShow: Show[String] = new Show[String] { def show(s: String) = s }
def ShoutyStringShow: Show[String] = new Show[String] { def show(s: String) = s.toUpperCase }
}
case class Person(name: String, age: Int)
object Person {
implicit def PersonShow(implicit si: Show[Int], ss: Show[String]): Show[Person] = new Show[Person] {
def show(p: Person) = "Person(name=" + ss.show(p.name) + ", age=" + si.show(p.age) + ")"
}
}
val p = Person("bob", 25)
implicitly[Show[Person]].show(p)
What if we want to change the way that the name is output? We can explicitly call PersonShow, explicitly pass an alternative Show[String], but we want the compiler to pass the Show[Int].
Person.PersonShow(si = implicitly, ss = Show.ShoutyStringShow).show(p)
Starting Scala 3 implicitly has been replaced with improved summon which has the advantage of being able to return a more precise type than asked for
The summon method corresponds to implicitly in Scala 2. It is
precisely the same as the the method in Shapeless. The difference
between summon (or the) and implicitly is that summon can return a
more precise type than the type that was asked for.
For example given the following type
trait F[In]:
type Out
def f(v: Int): Out
given F[Int] with
type Out = String
def f(v: Int): String = v.toString
implicitly method would summon a term with erased type member Out
scala> implicitly[F[Int]]
val res5: F[Int] = given_F_Int$#7d0e5fbb
scala> implicitly[res5.Out =:= String]
1 |implicitly[res5.Out =:= String]
| ^
| Cannot prove that res5.Out =:= String.
scala> val x: res5.Out = ""
1 |val x: res5.Out = ""
| ^^
| Found: ("" : String)
| Required: res5.Out
In order to recover the type member we would have to refer to it explicitly which defeats the purpose of having the type member instead of type parameter
scala> implicitly[F[Int] { type Out = String }]
val res6: F[Int]{Out = String} = given_F_Int$#7d0e5fbb
scala> implicitly[res6.Out =:= String]
val res7: res6.Out =:= String = generalized constraint
However summon defined as
def summon[T](using inline x: T): x.type = x
does not suffer from this problem
scala> summon[F[Int]]
val res8: given_F_Int.type = given_F_Int$#7d0e5fbb
scala> summon[res8.Out =:= String]
val res9: String =:= String = generalized constraint
scala> val x: res8.Out = ""
val x: res8.Out = ""
where we see type member type Out = String did not get erased even though we only asked for F[Int] and not F[Int] { type Out = String }. This can prove particularly relevant when chaining dependently typed functions:
The type summoned by implicitly has no Out type member. For this
reason, we should avoid implicitly when working with dependently typed
functions.
A "teach you to fish" answer is to use the alphabetic member index currently available in the Scaladoc nightlies. The letters (and the #, for non-alphabetic names) at the top of the package / class pane are links to the index for member names beginning with that letter (across all classes). If you choose I, e.g., you'll find the implicitly entry with one occurrence, in Predef, which you can visit from the link there.