Following the awesomely enlightening question by #TravisBrown concerning the enumeration of ADTs using shapeless, I am left with the following code snippet:
implicitly[EnumerableAdt[Foo]].values
I would like to encapsulate this within a method so that I don't have to .values after each invocation (It seems a cleaner API to me, that way). But I can't seem to get it right. Whenever I try to encapsulate the implicitly[EnumerableAdt[Foo]] I get implicit resolution errors.
What I had tried, that made most sense to me, was, for example:
def imply[T](implicit ev: T):Set[T] = implicitly[EnumerableAdt[T]].values
certainly without the ev made even less sense to me.
I am no expert in type level programming.
If you look at the definition of implicitly[X], you can see that is requires an implicit argument of type X in scope. In your example, you have implicit ev: T in scope, which is not enough to call implicitly[EnumerableAdt[T]]! Try the following definition instead:
def imply[T](implicit ev: EnumerableAdt[T]):Set[T] = ev.values
I have a function with a type parameter and I want to find out whether the type parameter is an Option or not. I have read some blogposts, i.e. this one, about type classes in scala recently, so I came up with this solution:
case class OptionFinder[A](isOption: Boolean)
implicit def notOption[A]: OptionFinder[A] = OptionFinder(false)
implicit def hitOption[A]: OptionFinder[Option[A]] = OptionFinder(true)
def myFunction[A](value: A)(implicit optionFinder: OptionFinder[A]): String = {
if (optionFinder.isOption) {"Found Option!"} else {"Found something else."}
}
This works seemingly as desired:
scala> val x: Option[Int] = Some(3)
scala> myFunction(x)
res0: String = Found Option!
scala> val y: String = "abc"
scala> myFunction(y)
res1: String = Found something else.
In the case of Some(3) hitOption is the implicit parameter, even though notOption would match as well (with A = Option[Int]). Obviously the more specific is type chosen. But am I guaranteed that the compiler always chooses the more specific type? And how does that work in the compiler anyway? I did not find a documentation of this behavior yet.
Note: Maybe the title for this question is not best, I'll happily change it for a better one.
There is already a question about this: Scala: Implicit parameter resolution precedence. Which answers itself through a complicated blog post. I think the most important piece of information is in Martin Odersky's comment on the blog post:
Here's a more high-level explanation what goes on with implicit search
in Scala, and which corresponds to how the spec explains it, but in
slightly less formalistic language.
First, we look for implicits that are visible either as locals or as members of enclosing classes and packages or as imports - the
precise rule is that we should be able to access them using their name
only, without any prefix.
If no implicits are found in step 1, we look in the "implicit scope", which contains all sort of companion objects that bear some
relation to the type which we search for (i.e. companion object of the
type itself, of its parameters if any are given, and also of its
supertype and supertraits; the importance is to be as general as
possible without reverting to whole program analysis like Haskell
does).
If at either stage we find more than one implicit, disambiguation
kicks in. Disambiguation is exactly the same as for overloading
resolution. Static overloading resolution resolution rules are a bit
involved, and I won't repeat them here. If it's any consolation:
Java's rules and C#'s rules are considerably more complex than Scala's
in this area.
Now according to this explanation it are "the rules of static overloading resolution" which will disambiguate between notOption and hitOption. To be honest, I fail to see how.
This answer explains that indeed methods with more specific arguments have priority, but I don't know if or how that is related to the overloading rules.
If I were you I would not depend on this behavior too much, but use the easier to understand concept of implicit priority through inheritance. It's a good idea to put your implicits in the companion object anyway.
It boils down to the fact that implicits that are inherited have lower priority. So it's safe to put the implicit you fall back to if hitOption doesn't match in a trait that the companion object extends.
case class OptionFinder[A](isOption: Boolean)
object OptionFinder extends LowerPriority {
implicit def hitOption[A]: OptionFinder[Option[A]] = OptionFinder(true)
}
trait LowerPriority {
implicit def notOption[A]: OptionFinder[A] = OptionFinder(false)
}
def myFunction[A](value: A)(implicit optionFinder: OptionFinder[A]): String = {
if (optionFinder.isOption) {"Found Option!"} else {"Found something else."}
}
This should also work if you put your implicits in a non companion object MyImplicits and import them with import MyImplicits._.
Now that scala has iterated towards a JVM type erasure fix with the ClassTag typeclass, why is it an opt-in, rather than having the compiler always capture the type signature for runtime inspection. Having it an implicit parametrized type constraint would make it possible to call classTag[T] regardless of the generic parameter declaration.
EDIT: I should clarify that I don't mean that scala should change the signature behind the scenes to always inlcude ClassTag. Rather I mean that since ClassTag shows that scala can capture runtime Type information and therefore avoid type erasure limitations, why can't that capture be implicit as part of the compiler so that that information is always available in scala code?
My suspicion is that it's backwards compatibility, java ecosystem compatibility, binary size or runtime overhead related, but those are just speculation.
Backwards compatibility would be completely destroyed, really. If you have a simple method like:
def foo[A](a: A)(implicit something: SomeType) = ???
Then suppose that in the next version of Scala, the compiler suddenly added implicit ClassTags to the signature of all methods with type parameters. This method would be broken. Anywhere it was being explicitly called like foo(a)(someTypeValue) wouldn't work anymore. Binary and source compatibility would be gone.
Java interoperability would be ugly. Supposing our method now looks like this:
def foo[A : ClassTag](a: A) = ???
Because ClassTags are generated by the Scala compiler, using this method from Java would prove more difficult. You'd have to create the ClassTag yourself.
ClassTag<MyClass> tag = scala.reflect.ClassTag$.MODULE$.apply(MyClass.class);
foo(a, tag);
My Java might not be 100% correct, but you get the idea. Anything parameterized would become very ugly. Well, it already is if it requires an implicit ClassTag, but the class of methods where this would be necessary would increase dramatically.
Moreover, type erasure isn't that much of a problem in most parameterized methods we (I, at least) use. I think automatically requiring a ClassTag for each type parameter would be far more trouble than it would help, for the above reasons.
Certainly this would add more compiler overhead, as it would need to generate more ClassTags than it usually would. I don't think it would add much more runtime overhead unless the ClassTag makes a difference. For example, in a simple method like below, the ClassTag doesn't really do anything:
def foo[A : ClassTag](a: A): A = a
We should also note that they're not perfect, either. So adding them isn't an end-all solution to erasure problems.
val list = List(1, "abc", List(1, 2, 3), List("a", "b"))
def find[A: ClassTag](l: List[Any]): Option[A] =
l collectFirst { case a: A => a }
scala> find[List[String]]
res2: Option[List[String]] = Some(List(1, 2, 3)) // Not quite! And no warnings, either.
Adding a ClassTag to every single class instance would add overhead, and surely also break compatibility. It's also in many places not possible. We can't just infuse java.lang.String with a ClassTag. Furthermore, we'd still be just as susceptible to erasure. Having a ClassTag field in each class is really no better than using getClass. We could make comparisons like
case a if(a.getClass == classOf[String]) => a.asInstanceOf[String]
But this is horribly ugly, requires a cast, and isn't necessarily what the ClassTag is meant to fix. If I tried something like this with my find method, it wouldn't work--at all.
// Can't compile
def find[A](l: List[Any]): Option[A] =
l collectFirst { case a if(a.getClass == classOf[A]) => a.asInstanceOf[A] }
Even if I were to fashion this to somehow work with ClassTag, where would it come from? I could not say a.classTag == classTag[A], because A has already been erased. I need the ClassTag at the method call site.
Yes, you would penalise any generic method or class for a use case that is quite seldom (e.g. requiring array construction or heterogeneous map value recovery). With this idea of "always class tags" you would also effectively destroy the possibility to call Scala code from Java. In summery, it simply doesn't make any sense to require a class tag to be always present, neither from compatibility, performance or class size point of view.
For example, Exception.allCatch is defined as
def allCatch[T]: Catch[T]
Why not just
val allCatch: Catch[Nothing]
when Catch is covariant in its argument?
Or, why PartialFunction object defines
def empty[A, B]: PartialFunction[A, B]
instead of just
val empty: PartialFunction[Any,Nothing]
?
Update: So far it seems that the answers miss the point. So please include a specific examples in your answer that really target the question. For example: Show a piece of code that works with def empty[A, B]: PartialFunction[A, B] but doesn't work (or is less convenient) with val empty: PartialFunction[Any,Nothing].
This saves the need for casting later and allows to treat the args type as T instead of Any, which is usually more convenient.
Here is an example:
scala> def func1[T](arg : T) : T = { arg }
func1: [T](arg : T)T
scala> def func2(arg : Any) : Any = { arg }
func2: (arg: Any)Any
scala> func1(4)
res4: Int = 4
scala> func2(4)
res7: Any = 4
Looking at the source of PartialFunction available here we can see that it in fact calls a private method on the PartialFunction object:
private[this] val empty_pf: PartialFunction[Any, Nothing]
So the return type of empty will always be PartialFunction[Any, Nothing]. As for the reasoning behind I have no idea. Maybe someone else have better insight as to why. You could try the language mailing list as well...
If you hard-code the type, like PartialFunction[Any,Nothing], you cannot restrict your function to take a more specific parameter than Any.
By using a generic type parameter, you can end up with a more flexible satisfying all cases and especially making the function safe.
Let's assume you want a function aiming to take an Animal as parameter and returning an Integer.
Let's assume that function is declared as being:
def myFunction: PartialFunction[Any,Nothing]
Firstly, PartialFunction would not be specialized to Animal at parameter side but to Any. What about if I pass a Human as parameter...., it would pass.. What about safety?
Secondly, If this function is declared as returning Nothing, you can't return from it any value but Nothing! Indeed, Nothing subclasses all classes in Scala.
This leads to the known rule that return type parameter must always be covariant in order to make a function interesting, not the case with Nothing.
In fact, Nothing is interesting only when dealing with the empty method of PartialFunction. Logic since an empty PartialFunction by definition involves nothing to return and should be forced to do it :)
You would ask: "So why don't we change the return type to Any?"
Answer: Because you'd lose all the benefit of generic erasure time making compiler to add needed casts automatically => You wouldn't retrieve directly the Integer value, but Any. annoying..
Actually, it seems that Scala standard library has some more places where generic type parameter is redundant (because of type variance). For example, see my question about foreach. My guess, based on #Peter's answer, is that these redundant generics make the interface more clear. Thanks to them, we don't have to remember which types are covariant, contravariant and invariant. Also, this makes things way simpler for people who are not familiar with variance, which is rather an advanced feature of Scala.
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So far implicit parameters in Scala do not look good for me -- it is too close to global variables, however since Scala seems like rather strict language I start doubting in my own opinion :-).
Question: could you show a real-life (or close) good example when implicit parameters really work. IOW: something more serious than showPrompt, that would justify such language design.
Or contrary -- could you show reliable language design (can be imaginary) that would make implicit not neccessary. I think that even no mechanism is better than implicits because code is clearer and there is no guessing.
Please note, I am asking about parameters, not implicit functions (conversions)!
Updates
Global variables
Thank you for all great answers. Maybe I clarify my "global variables" objection. Consider such function:
max(x : Int,y : Int) : Int
you call it
max(5,6);
you could (!) do it like this:
max(x:5,y:6);
but in my eyes implicits works like this:
x = 5;
y = 6;
max()
it is not very different from such construct (PHP-like)
max() : Int
{
global x : Int;
global y : Int;
...
}
Derek's answer
This is great example, however if you can think of as flexible usage of sending message not using implicit please post an counter-example. I am really curious about purity in language design ;-).
In a sense, yes, implicits represent global state. However, they are not mutable, which is the true problem with global variables -- you don't see people complaining about global constants, do you? In fact, coding standards usually dictate that you transform any constants in your code into constants or enums, which are usually global.
Note also that implicits are not in a flat namespace, which is also a common problem with globals. They are explicitly tied to types and, therefore, to the package hierarchy of those types.
So, take your globals, make them immutable and initialized at the declaration site, and put them on namespaces. Do they still look like globals? Do they still look problematic?
But let's not stop there. Implicits are tied to types, and they are just as much "global" as types are. Does the fact that types are global bother you?
As for use cases, they are many, but we can do a brief review based on their history. Originally, afaik, Scala did not have implicits. What Scala had were view types, a feature many other languages had. We can still see that today whenever you write something like T <% Ordered[T], which means the type T can be viewed as a type Ordered[T]. View types are a way of making automatic casts available on type parameters (generics).
Scala then generalized that feature with implicits. Automatic casts no longer exist, and, instead, you have implicit conversions -- which are just Function1 values and, therefore, can be passed as parameters. From then on, T <% Ordered[T] meant a value for an implicit conversion would be passed as parameter. Since the cast is automatic, the caller of the function is not required to explicitly pass the parameter -- so those parameters became implicit parameters.
Note that there are two concepts -- implicit conversions and implicit parameters -- that are very close, but do not completely overlap.
Anyway, view types became syntactic sugar for implicit conversions being passed implicitly. They would be rewritten like this:
def max[T <% Ordered[T]](a: T, b: T): T = if (a < b) b else a
def max[T](a: T, b: T)(implicit $ev1: Function1[T, Ordered[T]]): T = if ($ev1(a) < b) b else a
The implicit parameters are simply a generalization of that pattern, making it possible to pass any kind of implicit parameters, instead of just Function1. Actual use for them then followed, and syntactic sugar for those uses came latter.
One of them is Context Bounds, used to implement the type class pattern (pattern because it is not a built-in feature, just a way of using the language that provides similar functionality to Haskell's type class). A context bound is used to provide an adapter that implements functionality that is inherent in a class, but not declared by it. It offers the benefits of inheritance and interfaces without their drawbacks. For example:
def max[T](a: T, b: T)(implicit $ev1: Ordering[T]): T = if ($ev1.lt(a, b)) b else a
// latter followed by the syntactic sugar
def max[T: Ordering](a: T, b: T): T = if (implicitly[Ordering[T]].lt(a, b)) b else a
You have probably used that already -- there's one common use case that people usually don't notice. It is this:
new Array[Int](size)
That uses a context bound of a class manifests, to enable such array initialization. We can see that with this example:
def f[T](size: Int) = new Array[T](size) // won't compile!
You can write it like this:
def f[T: ClassManifest](size: Int) = new Array[T](size)
On the standard library, the context bounds most used are:
Manifest // Provides reflection on a type
ClassManifest // Provides reflection on a type after erasure
Ordering // Total ordering of elements
Numeric // Basic arithmetic of elements
CanBuildFrom // Collection creation
The latter three are mostly used with collections, with methods such as max, sum and map. One library that makes extensive use of context bounds is Scalaz.
Another common usage is to decrease boiler-plate on operations that must share a common parameter. For example, transactions:
def withTransaction(f: Transaction => Unit) = {
val txn = new Transaction
try { f(txn); txn.commit() }
catch { case ex => txn.rollback(); throw ex }
}
withTransaction { txn =>
op1(data)(txn)
op2(data)(txn)
op3(data)(txn)
}
Which is then simplified like this:
withTransaction { implicit txn =>
op1(data)
op2(data)
op3(data)
}
This pattern is used with transactional memory, and I think (but I'm not sure) that the Scala I/O library uses it as well.
The third common usage I can think of is making proofs about the types that are being passed, which makes it possible to detect at compile time things that would, otherwise, result in run time exceptions. For example, see this definition on Option:
def flatten[B](implicit ev: A <:< Option[B]): Option[B]
That makes this possible:
scala> Option(Option(2)).flatten // compiles
res0: Option[Int] = Some(2)
scala> Option(2).flatten // does not compile!
<console>:8: error: Cannot prove that Int <:< Option[B].
Option(2).flatten // does not compile!
^
One library that makes extensive use of that feature is Shapeless.
I don't think the example of the Akka library fits in any of these four categories, but that's the whole point of generic features: people can use it in all sorts of way, instead of ways prescribed by the language designer.
If you like being prescribed to (like, say, Python does), then Scala is just not for you.
Sure. Akka's got a great example of it with respect to its Actors. When you're inside an Actor's receive method, you might want to send a message to another Actor. When you do this, Akka will bundle (by default) the current Actor as the sender of the message, like this:
trait ScalaActorRef { this: ActorRef =>
...
def !(message: Any)(implicit sender: ActorRef = null): Unit
...
}
The sender is implicit. In the Actor there is a definition that looks like:
trait Actor {
...
implicit val self = context.self
...
}
This creates the implicit value within the scope of your own code, and it allows you to do easy things like this:
someOtherActor ! SomeMessage
Now, you can do this as well, if you like:
someOtherActor.!(SomeMessage)(self)
or
someOtherActor.!(SomeMessage)(null)
or
someOtherActor.!(SomeMessage)(anotherActorAltogether)
But normally you don't. You just keep the natural usage that's made possible by the implicit value definition in the Actor trait. There are about a million other examples. The collection classes are a huge one. Try wandering around any non-trivial Scala library and you'll find a truckload.
One example would be the comparison operations on Traversable[A]. E.g. max or sort:
def max[B >: A](implicit cmp: Ordering[B]) : A
These can only be sensibly defined when there is an operation < on A. So, without implicits we’d have to supply the context Ordering[B] every time we’d like to use this function. (Or give up type static checking inside max and risk a runtime cast error.)
If however, an implicit comparison type class is in scope, e.g. some Ordering[Int], we can just use it right away or simply change the comparison method by supplying some other value for the implicit parameter.
Of course, implicits may be shadowed and thus there may be situations in which the actual implicit which is in scope is not clear enough. For simple uses of max or sort it might indeed be sufficient to have a fixed ordering trait on Int and use some syntax to check whether this trait is available. But this would mean that there could be no add-on traits and every piece of code would have to use the traits which were originally defined.
Addition:
Response to the global variable comparison.
I think you’re correct that in a code snipped like
implicit val num = 2
implicit val item = "Orange"
def shopping(implicit num: Int, item: String) = {
"I’m buying "+num+" "+item+(if(num==1) "." else "s.")
}
scala> shopping
res: java.lang.String = I’m buying 2 Oranges.
it may smell of rotten and evil global variables. The crucial point, however, is that there may be only one implicit variable per type in scope. Your example with two Ints is not going to work.
Also, this means that practically, implicit variables are employed only when there is a not necessarily unique yet distinct primary instance for a type. The self reference of an actor is a good example for such a thing. The type class example is another example. There may be dozens of algebraic comparisons for any type but there is one which is special.
(On another level, the actual line number in the code itself might also make for a good implicit variable as long as it uses a very distinctive type.)
You normally don’t use implicits for everyday types. And with specialised types (like Ordering[Int]) there is not too much risk in shadowing them.
Based on my experience there is no real good example for use of implicits parameters or implicits conversion.
The small benefit of using implicits (not needing to explicitly write a parameter or a type) is redundant in compare to the problems they create.
I am a developer for 15 years, and have been working with scala for the last 1.5 years.
I have seen many times bugs that were caused by the developer not aware of the fact that implicits are used, and that a specific function actually return a different type that the one specified. Due to implicit conversion.
I also heard statements saying that if you don't like implicits, don't use them.
This is not practical in the real world since many times external libraries are used, and a lot of them are using implicits, so your code using implicits, and you might not be aware of that.
You can write a code that has either:
import org.some.common.library.{TypeA, TypeB}
or:
import org.some.common.library._
Both codes will compile and run.
But they will not always produce the same results since the second version imports implicits conversion that will make the code behave differently.
The 'bug' that is caused by this can occur a very long time after the code was written, in case some values that are affected by this conversion were not used originally.
Once you encounter the bug, its not an easy task finding the cause.
You have to do some deep investigation.
Even though you feel like an expert in scala once you have found the bug, and fixed it by changing an import statement, you actually wasted a lot of precious time.
Additional reasons why I generally against implicits are:
They make the code hard to understand (there is less code, but you don't know what he is doing)
Compilation time. scala code compiles much slower when implicits are used.
In practice, it changes the language from statically typed, to dynamically typed. Its true that once following very strict coding guidelines you can avoid such situations, but in real world, its not always the case. Even using the IDE 'remove unused imports', can cause your code to still compile and run, but not the same as before you removed 'unused' imports.
There is no option to compile scala without implicits (if there is please correct me), and if there was an option, none of the common community scala libraries would have compile.
For all the above reasons, I think that implicits are one of the worst practices that scala language is using.
Scala has many great features, and many not so great.
When choosing a language for a new project, implicits are one of the reasons against scala, not in favour of it. In my opinion.
Another good general usage of implicit parameters is to make the return type of a method depend on the type of some of the parameters passed to it. A good example, mentioned by Jens, is the collections framework, and methods like map, whose full signature usually is:
def map[B, That](f: (A) ⇒ B)(implicit bf: CanBuildFrom[GenSeq[A], B, That]): That
Note that the return type That is determined by the best fitting CanBuildFrom that the compiler can find.
For another example of this, see that answer. There, the return type of the method Arithmetic.apply is determined according to a certain implicit parameter type (BiConverter).
It's easy, just remember:
to declare the variable to be passed in as implicit too
to declare all the implicit params after the non-implicit params in a separate ()
e.g.
def myFunction(): Int = {
implicit val y: Int = 33
implicit val z: Double = 3.3
functionWithImplicit("foo") // calls functionWithImplicit("foo")(y, z)
}
def functionWithImplicit(foo: String)(implicit x: Int, d: Double) = // blar blar
Implicit parameters are heavily used in the collection API. Many functions get an implicit CanBuildFrom, which ensures that you get the 'best' result collection implementation.
Without implicits you would either pass such a thing all the time, which would make normal usage cumbersome. Or use less specialized collections which would be annoying because it would mean you loose performance/power.
I am commenting on this post a bit late, but I have started learning scala lately.
Daniel and others have given nice background about implicit keyword.
I would provide me two cents on implicit variable from practical usage perspective.
Scala is best suited if used for writing Apache Spark codes. In Spark, we do have spark context and most likely the configuration class that may fetch the configuration keys/values from a configuration file.
Now, If I have an abstract class and if I declare an object of configuration and spark context as follows :-
abstract class myImplicitClass {
implicit val config = new myConfigClass()
val conf = new SparkConf().setMaster().setAppName()
implicit val sc = new SparkContext(conf)
def overrideThisMethod(implicit sc: SparkContext, config: Config) : Unit
}
class MyClass extends myImplicitClass {
override def overrideThisMethod(implicit sc: SparkContext, config: Config){
/*I can provide here n number of methods where I can pass the sc and config
objects, what are implicit*/
def firstFn(firstParam: Int) (implicit sc: SparkContext, config: Config){
/*I can use "sc" and "config" as I wish: making rdd or getting data from cassandra, for e.g.*/
val myRdd = sc.parallelize(List("abc","123"))
}
def secondFn(firstParam: Int) (implicit sc: SparkContext, config: Config){
/*following are the ways we can use "sc" and "config" */
val keyspace = config.getString("keyspace")
val tableName = config.getString("table")
val hostName = config.getString("host")
val userName = config.getString("username")
val pswd = config.getString("password")
implicit val cassandraConnectorObj = CassandraConnector(....)
val cassandraRdd = sc.cassandraTable(keyspace, tableName)
}
}
}
As we can see the code above, I have two implicit objects in my abstract class, and I have passed those two implicit variables as function/method/definition implicit parameters.
I think this is the best use case that we can depict in terms of usage of implicit variables.