I have read the answer to my question about scala.math.Integral but I do not understand what happens when Integral[T] is passed as an implicit parameter. (I think I understand the implicit parameters concept in general).
Let's consider this function
import scala.math._
def foo[T](t: T)(implicit integral: Integral[T]) { println(integral) }
Now I call foo in REPL:
scala> foo(0)
scala.math.Numeric$IntIsIntegral$#581ea2
scala> foo(0L)
scala.math.Numeric$LongIsIntegral$#17fe89
How does the integral argument become scala.math.Numeric$IntIsIntegral and scala.math.Numeric$LongIsIntegral ?
The short answer is that Scala finds IntIsIntegral and LongIsIntegral inside the object Numeric, which is the companion object of the class Numeric, which is a super class of Integral.
Read on for the long answer.
Types of Implicits
Implicits in Scala refers to either a value that can be passed "automatically", so to speak, or a conversion from one type to another that is made automatically.
Implicit Conversion
Speaking very briefly about the latter type, if one calls a method m on an object o of a class C, and that class does not support method m, then Scala will look for an implicit conversion from C to something that does support m. A simple example would be the method map on String:
"abc".map(_.toInt)
String does not support the method map, but StringOps does, and there's an implicit conversion from String to StringOps available (see implicit def augmentString on Predef).
Implicit Parameters
The other kind of implicit is the implicit parameter. These are passed to method calls like any other parameter, but the compiler tries to fill them in automatically. If it can't, it will complain. One can pass these parameters explicitly, which is how one uses breakOut, for example (see question about breakOut, on a day you are feeling up for a challenge).
In this case, one has to declare the need for an implicit, such as the foo method declaration:
def foo[T](t: T)(implicit integral: Integral[T]) {println(integral)}
View Bounds
There's one situation where an implicit is both an implicit conversion and an implicit parameter. For example:
def getIndex[T, CC](seq: CC, value: T)(implicit conv: CC => Seq[T]) = seq.indexOf(value)
getIndex("abc", 'a')
The method getIndex can receive any object, as long as there is an implicit conversion available from its class to Seq[T]. Because of that, I can pass a String to getIndex, and it will work.
Behind the scenes, the compile changes seq.IndexOf(value) to conv(seq).indexOf(value).
This is so useful that there is a syntactic sugar to write them. Using this syntactic sugar, getIndex can be defined like this:
def getIndex[T, CC <% Seq[T]](seq: CC, value: T) = seq.indexOf(value)
This syntactic sugar is described as a view bound, akin to an upper bound (CC <: Seq[Int]) or a lower bound (T >: Null).
Please be aware that view bounds are deprecated from 2.11, you should avoid them.
Context Bounds
Another common pattern in implicit parameters is the type class pattern. This pattern enables the provision of common interfaces to classes which did not declare them. It can both serve as a bridge pattern -- gaining separation of concerns -- and as an adapter pattern.
The Integral class you mentioned is a classic example of type class pattern. Another example on Scala's standard library is Ordering. There's a library that makes heavy use of this pattern, called Scalaz.
This is an example of its use:
def sum[T](list: List[T])(implicit integral: Integral[T]): T = {
import integral._ // get the implicits in question into scope
list.foldLeft(integral.zero)(_ + _)
}
There is also a syntactic sugar for it, called a context bound, which is made less useful by the need to refer to the implicit. A straight conversion of that method looks like this:
def sum[T : Integral](list: List[T]): T = {
val integral = implicitly[Integral[T]]
import integral._ // get the implicits in question into scope
list.foldLeft(integral.zero)(_ + _)
}
Context bounds are more useful when you just need to pass them to other methods that use them. For example, the method sorted on Seq needs an implicit Ordering. To create a method reverseSort, one could write:
def reverseSort[T : Ordering](seq: Seq[T]) = seq.reverse.sorted
Because Ordering[T] was implicitly passed to reverseSort, it can then pass it implicitly to sorted.
Where do Implicits Come From?
When the compiler sees the need for an implicit, either because you are calling a method which does not exist on the object's class, or because you are calling a method that requires an implicit parameter, it will search for an implicit that will fit the need.
This search obey certain rules that define which implicits are visible and which are not. The following table showing where the compiler will search for implicits was taken from an excellent presentation about implicits by Josh Suereth, which I heartily recommend to anyone wanting to improve their Scala knowledge.
First look in current scope
Implicits defined in current scope
Explicit imports
wildcard imports
Same scope in other files
Now look at associated types in
Companion objects of a type
Companion objects of type parameters types
Outer objects for nested types
Other dimensions
Let's give examples for them.
Implicits Defined in Current Scope
implicit val n: Int = 5
def add(x: Int)(implicit y: Int) = x + y
add(5) // takes n from the current scope
Explicit Imports
import scala.collection.JavaConversions.mapAsScalaMap
def env = System.getenv() // Java map
val term = env("TERM") // implicit conversion from Java Map to Scala Map
Wildcard Imports
def sum[T : Integral](list: List[T]): T = {
val integral = implicitly[Integral[T]]
import integral._ // get the implicits in question into scope
list.foldLeft(integral.zero)(_ + _)
}
Same Scope in Other Files
This is like the first example, but assuming the implicit definition is in a different file than its usage. See also how package objects might be used in to bring in implicits.
Companion Objects of a Type
There are two object companions of note here. First, the object companion of the "source" type is looked into. For instance, inside the object Option there is an implicit conversion to Iterable, so one can call Iterable methods on Option, or pass Option to something expecting an Iterable. For example:
for {
x <- List(1, 2, 3)
y <- Some('x')
} yield, (x, y)
That expression is translated by the compile into
List(1, 2, 3).flatMap(x => Some('x').map(y => (x, y)))
However, List.flatMap expects a TraversableOnce, which Option is not. The compiler then looks inside Option's object companion and finds the conversion to Iterable, which is a TraversableOnce, making this expression correct.
Second, the companion object of the expected type:
List(1, 2, 3).sorted
The method sorted takes an implicit Ordering. In this case, it looks inside the object Ordering, companion to the class Ordering, and finds an implicit Ordering[Int] there.
Note that companion objects of super classes are also looked into. For example:
class A(val n: Int)
object A {
implicit def str(a: A) = "A: %d" format a.n
}
class B(val x: Int, y: Int) extends A(y)
val b = new B(5, 2)
val s: String = b // s == "A: 2"
This is how Scala found the implicit Numeric[Int] and Numeric[Long] in your question, by the way, as they are found inside Numeric, not Integral.
Companion Objects of Type Parameters Types
This is required to make the type class pattern really work. Consider Ordering, for instance... it comes with some implicits in its companion object, but you can't add stuff to it. So how can you make an Ordering for your own class that is automatically found?
Let's start with the implementation:
class A(val n: Int)
object A {
implicit val ord = new Ordering[A] {
def compare(x: A, y: A) = implicitly[Ordering[Int]].compare(x.n, y.n)
}
}
So, consider what happens when you call
List(new A(5), new A(2)).sorted
As we saw, the method sorted expects an Ordering[A] (actually, it expects an Ordering[B], where B >: A). There isn't any such thing inside Ordering, and there is no "source" type on which to look. Obviously, it is finding it inside A, which is a type parameter of Ordering.
This is also how various collection methods expecting CanBuildFrom work: the implicits are found inside companion objects to the type parameters of CanBuildFrom.
Outer Objects for Nested Types
I haven't actually seen examples of this. I'd be grateful if someone could share one. The principle is simple:
class A(val n: Int) {
class B(val m: Int) { require(m < n) }
}
object A {
implicit def bToString(b: A#B) = "B: %d" format b.m
}
val a = new A(5)
val b = new a.B(3)
val s: String = b // s == "B: 3"
Other Dimensions
I'm pretty sure this was a joke. I hope. :-)
EDIT
Related questions of interest:
Context and view bounds
Chaining implicits
The parameter is implicit, which means that the Scala compiler will look if it can find an implicit object somewhere that it can automatically fill in for the parameter.
When you pass in an Int, it's going to look for an implicit object that is an Integral[Int] and it finds it in scala.math.Numeric. You can look at the source code of scala.math.Numeric, where you will find this:
object Numeric {
// ...
trait IntIsIntegral extends Integral[Int] {
// ...
}
// This is the implicit object that the compiler finds
implicit object IntIsIntegral extends IntIsIntegral with Ordering.IntOrdering
}
Likewise, there is a different implicit object for Long that works the same way.
Related
I am learning Context bound in Scala.
In the below code, I am invoking multiplication operator on the integer parameter. But it errors out. 'a' is considered as type parameter; but it is actually not as per my understanding. Can someone please help.
scala> class Sample[T]
defined class Sample
scala> def method[Int:Sample](a:Int) = a * a
<console>:12: error: value * is not a member of type parameter Int
def method[Int:Sample](a:Int) = a * a
Thanks!
Context bounds are syntactic sugar for implicit generic parameters that are parameterized by some type you're using. This concept is also known as "type class". You define some generic trait, such as your Sample[T], and then you provide implicit(!) instances of that trait for various concrete values of T. We call them "type class instances".
Why implicit? It's an implementation detail that Scala uses to achieve the type class mechanism; type classes also exist in e.g. Haskell, but the mechanism itself is a bit different. Anyways, you can then define a method such as your def method which requires a type class instance for some type. And you can do this with context bound syntax, or with a more verbose and more explicit standard syntax for implicit parameters.
Your definition is using the context bound. But there is something wrong with your example, as indicated by the compilation error. Let's first see a proper example that uses the type class concept correctly.
// type class definition:
trait Sample[T] {
def getSample: T
}
// type class instance(s):
object Sample {
implicit val sampleInt: Sample[Int] =
new Sample[Int] { def getSample = 42 }
}
And now the usage:
import Sample._
// using the context bound syntax
def method1[T : Sample](t: T) = t.getSample
// not using the context bound syntax
def method2(t: T)(implicit ev: Sample[T]) = t.getSample
What we're doing is saying - there is some value t of type T, we don't know much about it, but what we do know is that there is a Sample type class instance available for it. This allows us to do t.getSample.
And now, to finally provide the answer to your problem:
In your code, you are mixing things up. Your T is actually called Int. You intended to use the Int type, but what you did instead is that you named your generic parameter Int. I could have answered this with way less text, but I figured perhaps you would find the bigger picture interesting, rather than just pointing out the mistake.
The type parameter named Int does not represent concrete integer type scala.Int. Instead it is just a confusing coincidence that the type parameter was given the same name Int as the concrete type. If you give it some other name such as T
def method[T: Sample](a: T): T = a * a
the error message should make more sense. Now we see * is not defined for T since Sample type class does not yet provide such capability. Here is an example of how correct syntactic usage might look usage
trait Sample[T] {
def mult(a: T, b: T): T
}
def method[T: Sample](a: T): T = implicitly[Sample[T]].mult(a, a)
def method[T](a: T)(implicit ev: Sample[T]): T = ev.mult(a, a)
You could also have a look at Numeric type class which provides such functionality out of the box
def method[T](a: T)(implicit num: Numeric[T]): T = num.times(a, a)
Your method has a type parameter called Int which shadows the actual Int, just like defining a normal variable would shadow something from an outer scope. The same would happen if you remove the context bound.
What you are probably trying to do is something closer to the following:
trait Sample[A] {
def someOperation(a1: A, a2: A): A
}
implicit object IntSample extends Sample[Int] {
override def someOperation(a1: Int, a2: Int): Int = a1 * a2
}
def method[T: Sample](t: T) = implicitly[Sample[T]].someOperation(t, t)
method(4) // compiles and returns 16
//method("4") // doesn't compile, no implicit instance of Sample[String] in scope
You can play around with this code here on Scastie.
I was watching John De Goes "FP to the Max" video. In the code he does something like this to get the implicit object:
object Program {
def apply[F[_]](implicit F: Program[F]): Program[F] = F
}
Does this imply that the variable name F (the first one in implicit F: Program[F]) is actually a different F? It is very confusing. Does he mean to do:
object Program {
def apply[F[_]](implicit ev: Program[F]): Program[F] = ev
}
How does the compile know which F he is referring to while returning the F? The type constructor or the variable in scope?
Indeed the function parameter T is different from type parameter T, for example
def f[T](T: T): T = T
f(42) // res0: Int = 42
Compiler does not get confused because values exist in a different universe from types:
...there exist two separate universes, the universe of types and the
universe of values. In the universe of values, we have methods which
take values as arguments in round parentheses (or occasionally curly
braces). In the universe of types, we have type constructors, which take types
as arguments in square brackets.
This convention is sometimes used when dealing with typeclasses. It is meant to communicate that we want to simply return the typeclass instance resolved for F. To avoid confusion you could use ev approach you already suggested in question, or even
object Program {
def apply[F[_]: Program]: Program[F] = implicitly[Program[F]]
}
As a side-note, this trick with apply method in companion object of typeclass allows us to avoid having to use implicitly, for example, given
trait Foo[T]
trait Bar[T]
trait Program[F[_]]
implicit val fooProgram: Program[Foo] = ???
implicit val barProgram: Program[Bar] = ???
object Program {
def apply[F[_]: Program]: Program[F] = implicitly
}
then we can write
Program[Bar]
instead of
implicitly[Program[Bar]]
How does the compile know which F he is referring to while returning the F? The type constructor or the variable in scope?
The compiler knows that a method/function can only return a value. So what follows the = has a type but is not a type.
At a high-level, I'm curious to know how implicits defined inside a class body are resolved? I assumed they would be considered part of the current scope (as per the awesome answer here), but I am wrong.
For a specific example, why doesn't the following work:
trait Convert[T, U] {
final def value(record: T)(implicit f: T => U): U = f(record)
}
case class ConvertMap(key: String)
extends Convert[Map[String, String], Boolean] {
implicit def a2b(m: Map[String, String]): Boolean = m.contains(key)
}
I can instantiate the class ConvertMap, but when I call the value method I get an error stating that the view from Map[String, String] => Boolean can't be found.
scala> val c = ConvertMap("key")
c: ConvertMap = ConvertMap(key)
scala> c.value(Map("key" -> "value"))
<console>:13: error: No implicit view available from Map[String,String] => Boolean.
If you re-read the answer you provided for implicit resolution scope, you see that there are a couple of things which happen here causing the implicit you've defined not to be found.
First, if the implicit isn't found in the local scope of the call-site, we defer to "category 2" lookup for implicits which involves the types underlying the implicit. We're searching for an implicit conversion of type Map[String, String] => Boolean. According to the implicit scoping rules, the following is applicable to our use case:
If T is a parameterized type S[T1, ..., Tn], the union of the parts of S and T1, ..., Tn.
Our S[T1.. Tn] is a Map[String, String] (and it's base classes), so we have both Map and String as a candidate type for implicit lookup, but neither of them hold the conversion. Further, the return type is also considered, meaning Boolean is also in scope, but again, it doesn't hold the implicit and hence why the compiler complains.
The simplest thing that can be done to help the compiler find it is to place the implicit inside the companion object of ConvertMap, but that means we can no longer accept a value key in the constructor, which makes your example a bit contrived.
It needs to be resolvable at the call site (where c.value is called).
At that time, the only thing you have in scope c. I am not sure why you would think it makes sense for something defined inside a class to be considered in scope at that point.
BTW, your example doesn't really seem like a good use for implicits to begin with. Why not just make f a member method in the trait?
I'm reading through and working my way through using type classes and I came across this way of defining type classes from the Shapeless guide:
So here goes the example:
object CsvEncoder {
// "Summoner" method
def apply[A](implicit enc: CsvEncoder[A]): CsvEncoder[A] =
enc
// "Constructor" method
def instance[A](func: A => List[String]): CsvEncoder[A] =
new CsvEncoder[A] {
def encode(value: A): List[String] =
func(value)
}
// Globally visible type class instances
}
What I do not understand is the need for the apply method? What is it doing in this context above?
Later on, the guide describes how I could create a type class instance:
implicit val booleanEncoder: CsvEncoder[Boolean] =
new CsvEncoder[Boolean] {
def encode(b: Boolean): List[String] =
if(b) List("yes") else List("no")
}
is actually shortened to:
implicit val booleanEncoder: CsvEncoder[Boolean] =
instance(b => if(b) List("yes") else List("no"))
So my question now is, how does this work? What I do not get is the need for the apply method?
EDIT: I came across a blog post that describes the steps in creating type classes as below:
Define typeclass contract trait Foo.
Define a companion object Foo with a helper method apply that acts like implicitly, and a way of defining Foo instances typically from a function.
Define FooOps class that defines unary or binary operators.
Define FooSyntax trait that implicitly provides FooOps from a Foo instance.
So what is the deal with point number 2, 3 and 4?
Most of those practices came from Haskell (basically an intention to mimic Haskell's type-classes is a reason for so much boilerplate), some of it is just for convenience. So,
2) As #Alexey Romanov mentioned, companion object with apply is just for convenience, so instead of implicitly[CsvEncoder[IceCream]] you could write just CsvEncoder[IceCream] (aka CsvEncoder.apply[IceCream]()), which will return you a required type-class instance.
3) FooOps provides convenience methods for DSLs. For instance you could have something like:
trait Semigroup[A] {
...
def append(a: A, b: A)
}
import implicits._ //you should import actual instances for `Semigroup[Int]` etc.
implicitly[Semigroup[Int]].append(2,2)
But sometimes it's inconvenient to call append(2,2) method, so it's a good practice to provide a symbolic alias:
trait Ops[A] {
def typeClassInstance: Semigroup[A]
def self: A
def |+|(y: A): A = typeClassInstance.append(self, y)
}
trait ToSemigroupOps {
implicit def toSemigroupOps[A](target: A)(implicit tc: Semigroup[A]): Ops[A] = new Ops[A] {
val self = target
val typeClassInstance = tc
}
}
object SemiSyntax extends ToSemigroupOps
4) You can use it as follows:
import SemiSyntax._
import implicits._ //you should also import actual instances for `Semigroup[Int]` etc.
2 |+| 2
If you wonder why so much boilerplate, and why scala's implicit class syntax doesn't provide this functionality from scratch - the answer is that implicit class actually provides a way to create DSL's - it's just less powerful - it's (subjectively) harder to provide operation aliases, deal with more complex dispatching (when required) etc.
However, there is a macro solution that generates boilerplate automatically for you: https://github.com/mpilquist/simulacrum.
One another important point about your CsvEncoder example is that instance is convenience method for creating type-class instances, but apply is a shortcut for "summoning" (requiring) those instances. So, first one is for library extender (a way to implement interface), another one is for a user (a way to call a particular operation provided for that interface).
Another thing to note is that in shapeless the apply method is not only for cuter syntax.
Take for instance this simplified version of shapeless' Generic and some case class Foo.
trait Generic[T] {
type Repr
}
object Generic {
def apply[T](implicit gen: Generic[T]): Generic[T] { type Repr = gen.Repr } = gen
/* lots of macros to generate implicit instances omitted */
}
case class Foo(a: Int, b: String)
Now when I call Generic[Foo] I will get an instance that is typed as Generic[Foo] { type Repr = Int :: String :: HNil }. But if I call implicitly[Generic[Foo]] all the compiler knows about the result is that it's a Generic[Foo]. In other words: the concrete type of Repr is lost and I can't do anything useful with it. The reason is that implicitly is implemented as follows:
def implicitly[T](implicit e: T): T = e
That method declaration basically says: if you ask for a T I promise to give you a T, if I find one, and nothing more. So that means you'd have to ask implicitly[Generic[Foo] { type Repr = Int :: String :: HNil }] and that defeats the purpose of having automatic derivation.
Quoting the guide immediately after the object CsvEncoder definition:
The apply method ... allows us to summon a type class instance given a target type:
CsvEncoder[IceCream]
// res9: CsvEncoder[IceCream] = ...
An implicit question to newcomers to Scala seems to be: where does the compiler look for implicits? I mean implicit because the question never seems to get fully formed, as if there weren't words for it. :-) For example, where do the values for integral below come from?
scala> import scala.math._
import scala.math._
scala> def foo[T](t: T)(implicit integral: Integral[T]) {println(integral)}
foo: [T](t: T)(implicit integral: scala.math.Integral[T])Unit
scala> foo(0)
scala.math.Numeric$IntIsIntegral$#3dbea611
scala> foo(0L)
scala.math.Numeric$LongIsIntegral$#48c610af
Another question that does follow up to those who decide to learn the answer to the first question is how does the compiler choose which implicit to use, in certain situations of apparent ambiguity (but that compile anyway)?
For instance, scala.Predef defines two conversions from String: one to WrappedString and another to StringOps. Both classes, however, share a lot of methods, so why doesn't Scala complain about ambiguity when, say, calling map?
Note: this question was inspired by this other question, in the hopes of stating the problem in a more general manner. The example was copied from there, because it is referred to in the answer.
Types of Implicits
Implicits in Scala refers to either a value that can be passed "automatically", so to speak, or a conversion from one type to another that is made automatically.
Implicit Conversion
Speaking very briefly about the latter type, if one calls a method m on an object o of a class C, and that class does not support method m, then Scala will look for an implicit conversion from C to something that does support m. A simple example would be the method map on String:
"abc".map(_.toInt)
String does not support the method map, but StringOps does, and there's an implicit conversion from String to StringOps available (see implicit def augmentString on Predef).
Implicit Parameters
The other kind of implicit is the implicit parameter. These are passed to method calls like any other parameter, but the compiler tries to fill them in automatically. If it can't, it will complain. One can pass these parameters explicitly, which is how one uses breakOut, for example (see question about breakOut, on a day you are feeling up for a challenge).
In this case, one has to declare the need for an implicit, such as the foo method declaration:
def foo[T](t: T)(implicit integral: Integral[T]) {println(integral)}
View Bounds
There's one situation where an implicit is both an implicit conversion and an implicit parameter. For example:
def getIndex[T, CC](seq: CC, value: T)(implicit conv: CC => Seq[T]) = seq.indexOf(value)
getIndex("abc", 'a')
The method getIndex can receive any object, as long as there is an implicit conversion available from its class to Seq[T]. Because of that, I can pass a String to getIndex, and it will work.
Behind the scenes, the compiler changes seq.IndexOf(value) to conv(seq).indexOf(value).
This is so useful that there is syntactic sugar to write them. Using this syntactic sugar, getIndex can be defined like this:
def getIndex[T, CC <% Seq[T]](seq: CC, value: T) = seq.indexOf(value)
This syntactic sugar is described as a view bound, akin to an upper bound (CC <: Seq[Int]) or a lower bound (T >: Null).
Context Bounds
Another common pattern in implicit parameters is the type class pattern. This pattern enables the provision of common interfaces to classes which did not declare them. It can both serve as a bridge pattern -- gaining separation of concerns -- and as an adapter pattern.
The Integral class you mentioned is a classic example of type class pattern. Another example on Scala's standard library is Ordering. There's a library that makes heavy use of this pattern, called Scalaz.
This is an example of its use:
def sum[T](list: List[T])(implicit integral: Integral[T]): T = {
import integral._ // get the implicits in question into scope
list.foldLeft(integral.zero)(_ + _)
}
There is also syntactic sugar for it, called a context bound, which is made less useful by the need to refer to the implicit. A straight conversion of that method looks like this:
def sum[T : Integral](list: List[T]): T = {
val integral = implicitly[Integral[T]]
import integral._ // get the implicits in question into scope
list.foldLeft(integral.zero)(_ + _)
}
Context bounds are more useful when you just need to pass them to other methods that use them. For example, the method sorted on Seq needs an implicit Ordering. To create a method reverseSort, one could write:
def reverseSort[T : Ordering](seq: Seq[T]) = seq.sorted.reverse
Because Ordering[T] was implicitly passed to reverseSort, it can then pass it implicitly to sorted.
Where do Implicits come from?
When the compiler sees the need for an implicit, either because you are calling a method which does not exist on the object's class, or because you are calling a method that requires an implicit parameter, it will search for an implicit that will fit the need.
This search obey certain rules that define which implicits are visible and which are not. The following table showing where the compiler will search for implicits was taken from an excellent presentation (timestamp 20:20) about implicits by Josh Suereth, which I heartily recommend to anyone wanting to improve their Scala knowledge. It has been complemented since then with feedback and updates.
The implicits available under number 1 below has precedence over the ones under number 2. Other than that, if there are several eligible arguments which match the implicit parameter’s type, a most specific one will be chosen using the rules of static overloading resolution (see Scala Specification §6.26.3). More detailed information can be found in a question I link to at the end of this answer.
First look in current scope
Implicits defined in current scope
Explicit imports
wildcard imports
Same scope in other files
Now look at associated types in
Companion objects of a type
Implicit scope of an argument's type (2.9.1)
Implicit scope of type arguments (2.8.0)
Outer objects for nested types
Other dimensions
Let's give some examples for them:
Implicits Defined in Current Scope
implicit val n: Int = 5
def add(x: Int)(implicit y: Int) = x + y
add(5) // takes n from the current scope
Explicit Imports
import scala.collection.JavaConversions.mapAsScalaMap
def env = System.getenv() // Java map
val term = env("TERM") // implicit conversion from Java Map to Scala Map
Wildcard Imports
def sum[T : Integral](list: List[T]): T = {
val integral = implicitly[Integral[T]]
import integral._ // get the implicits in question into scope
list.foldLeft(integral.zero)(_ + _)
}
Same Scope in Other Files
Edit: It seems this does not have a different precedence. If you have some example that demonstrates a precedence distinction, please make a comment. Otherwise, don't rely on this one.
This is like the first example, but assuming the implicit definition is in a different file than its usage. See also how package objects might be used in to bring in implicits.
Companion Objects of a Type
There are two object companions of note here. First, the object companion of the "source" type is looked into. For instance, inside the object Option there is an implicit conversion to Iterable, so one can call Iterable methods on Option, or pass Option to something expecting an Iterable. For example:
for {
x <- List(1, 2, 3)
y <- Some('x')
} yield (x, y)
That expression is translated by the compiler to
List(1, 2, 3).flatMap(x => Some('x').map(y => (x, y)))
However, List.flatMap expects a TraversableOnce, which Option is not. The compiler then looks inside Option's object companion and finds the conversion to Iterable, which is a TraversableOnce, making this expression correct.
Second, the companion object of the expected type:
List(1, 2, 3).sorted
The method sorted takes an implicit Ordering. In this case, it looks inside the object Ordering, companion to the class Ordering, and finds an implicit Ordering[Int] there.
Note that companion objects of super classes are also looked into. For example:
class A(val n: Int)
object A {
implicit def str(a: A) = "A: %d" format a.n
}
class B(val x: Int, y: Int) extends A(y)
val b = new B(5, 2)
val s: String = b // s == "A: 2"
This is how Scala found the implicit Numeric[Int] and Numeric[Long] in your question, by the way, as they are found inside Numeric, not Integral.
Implicit Scope of an Argument's Type
If you have a method with an argument type A, then the implicit scope of type A will also be considered. By "implicit scope" I mean that all these rules will be applied recursively -- for example, the companion object of A will be searched for implicits, as per the rule above.
Note that this does not mean the implicit scope of A will be searched for conversions of that parameter, but of the whole expression. For example:
class A(val n: Int) {
def +(other: A) = new A(n + other.n)
}
object A {
implicit def fromInt(n: Int) = new A(n)
}
// This becomes possible:
1 + new A(1)
// because it is converted into this:
A.fromInt(1) + new A(1)
This is available since Scala 2.9.1.
Implicit Scope of Type Arguments
This is required to make the type class pattern really work. Consider Ordering, for instance: It comes with some implicits in its companion object, but you can't add stuff to it. So how can you make an Ordering for your own class that is automatically found?
Let's start with the implementation:
class A(val n: Int)
object A {
implicit val ord = new Ordering[A] {
def compare(x: A, y: A) = implicitly[Ordering[Int]].compare(x.n, y.n)
}
}
So, consider what happens when you call
List(new A(5), new A(2)).sorted
As we saw, the method sorted expects an Ordering[A] (actually, it expects an Ordering[B], where B >: A). There isn't any such thing inside Ordering, and there is no "source" type on which to look. Obviously, it is finding it inside A, which is a type argument of Ordering.
This is also how various collection methods expecting CanBuildFrom work: the implicits are found inside companion objects to the type parameters of CanBuildFrom.
Note: Ordering is defined as trait Ordering[T], where T is a type parameter. Previously, I said that Scala looked inside type parameters, which doesn't make much sense. The implicit looked for above is Ordering[A], where A is an actual type, not type parameter: it is a type argument to Ordering. See section 7.2 of the Scala specification.
This is available since Scala 2.8.0.
Outer Objects for Nested Types
I haven't actually seen examples of this. I'd be grateful if someone could share one. The principle is simple:
class A(val n: Int) {
class B(val m: Int) { require(m < n) }
}
object A {
implicit def bToString(b: A#B) = "B: %d" format b.m
}
val a = new A(5)
val b = new a.B(3)
val s: String = b // s == "B: 3"
Other Dimensions
I'm pretty sure this was a joke, but this answer might not be up-to-date. So don't take this question as being the final arbiter of what is happening, and if you do noticed it has gotten out-of-date, please inform me so that I can fix it.
EDIT
Related questions of interest:
Context and view bounds
Chaining implicits
Scala: Implicit parameter resolution precedence
I wanted to find out the precedence of the implicit parameter resolution, not just where it looks for, so I wrote a blog post revisiting implicits without import tax (and implicit parameter precedence again after some feedback).
Here's the list:
1) implicits visible to current invocation scope via local declaration, imports, outer scope, inheritance, package object that are accessible without prefix.
2) implicit scope, which contains all sort of companion objects and package object that bear some relation to the implicit's type which we search for (i.e. package object of the type, companion object of the type itself, of its type constructor if any, of its parameters if any, and also of its supertype and supertraits).
If at either stage we find more than one implicit, static overloading rule is used to resolve it.