I am new to scala and trying to support an application written in scala using ciris package.
I want to understand the is ciris ConfigDecoder and what the below code is trying to do.
#inline implicit def sourceTopicsConfigDecoder(implicit ev: ConfigDecoder[String, NonEmptyString]): ConfigDecoder[String, SourceTopics] =
ev.map(_.value.split(",").toSet.map(NonEmptyString.unsafeFrom)) map SourceTopics.apply
#inline implicit val sourceTopicsShow: Show[SourceTopics] =
_.unMk.mkString(",")
I've never used (or even heard of) Ciris before, but a quick visit to the documentation informs me that ConfigDecoder is the means by which the received configuration type (usually a String) is cast to a more useful type. Something like: env("SIZE_LIMIT").as[Long]
I also learned that, while many useful ConfigDecoders are supplied, you can also make your own for decoding configuration values into application specific types, and that's what sourceTopicsConfigDecoder appears to be doing. It pulls an existing String-to-NonEmptyString decoder from the implicit scope and uses it to build a String-to-SourceTopics decoder. (SourceTopics must be previously defined.)
The new decoder is made implicit so that elsewhere in the code you can do something like: env("SRC_TOPICS").as[SourceTopics]
Introduction
I had created a lovely one-liner:
Option("something").map(_ => Try("something else")).flatten.getOrElse("default")
which actually does not compile, with error:
Error:(15, 31) Cannot prove that scala.util.Try[String] <:< Option[B].
Option("").map(_ => Try("")).flatten.getOrElse("");}
^
so I had found a way around:
Option("something").flatMap(_ => Try("something else").toOption).getOrElse("default")
But, the problem
My colleague warned me, that my construction is actually loosing the error information. This is true, and in real-life application - not acceptable.
After getting rid of all the repetition I had ended up with:
implicit class CoolTry[T](t: Try[T]) extends StrictLogging {
def toOptionSE: Option[T] = t match {
case Success(s) => Some(s)
case Failure(ex) =>
logger.error(ex.getMessage, ex)
None
}
}
using:
Option("something").flatMap(_ => Try(new Exception("error")).toOptionSE).getOrElse("default")
Question
I believe there are many similar cases in every application and I simply do not know if either my approach is bad or Try().toOption is simply done wrong?
I understand that logging is a side effect, but while using Try I guess everyone does expect it if something goes wrong?
Can the implicit class be improved?
Is there any utility library handling Try().toOption my way?
Or what (other) approach should I take here?
Thanks!
Forcing logging every time you change a Try[T] to an Option[T] is an undesired effect IMO. When you do such a transformation you're explicitly admitting that you don't really care about the internals of the failure, if it happened. All you want is to access the result, if it exists. Most of the time you say "well, this is undesirable, I always want to log exceptions", but sometimes you simply only care about the end result, so dealing with an Option[T] can be good enough.
Or what (other) approach should I take here?
Using Either[A, B] is an option here, especially in Scala 2.12 when it became right biased. You can simply map over it and only at the end check if there was an error and then log (created with Scala 2.12.1):
val res: Either[Throwable, String] = Option("something")
.map(_ => Try("something else").toEither)
.getOrElse(Right("default"))
.map(str => s"I got my awesome $str")
Ideally (IMO) deferring the logging side effect to the last possible point would serve you better.
One possible improvement would be to make it crystal clear to the user of what is going on; between the implicit being injected somewhere by the compiler and the apparently "pure" name (toOptionSE) it may not be evident of what is going on for a second developer reading and/or modifying your code. Furthermore, you're fixing how you treat the error case and don't leave the opportunity to handle it differently from logging it.
You can treat errors by leveraging projection, like the failed projection defined over Try. If you really want to do this fluently and on one line, you can leverage implicit classes like this.
implicit class TryErrorHandlingForwarding[A](t: Try[A]) {
def onError(handler: Throwable => Unit): Try[A] = {
t.failed.foreach(handler)
t
}
}
// maybe here you want to have an actual logger
def printStackTrace: Throwable => Unit =
_.printStackTrace
Option("something").
flatMap(_ => Try(???).onError(printStackTrace).toOption).
getOrElse("default")
Also, here I'm assuming that for whatever reason you cannot use Try right from the start (as it's been suggested in a comment).
So usually when we run a method that can both fail and return a value, we can encode our method return type as Either[SomeErrorType, ReturnType]. But many times we're running a method for its side effects, so the return type is Unit.
I could of course return an Either[SomeErrorType, Unit] but that definitely looks odd.
I could also just return an Option[SomeErrorType] but it doesn't really look a lot better (and breaks a possibly existing symmetry with other Either[SomeErrorType, NonUnitReturnType]s.
What's your approach in these cases?
def m(): Unit // and implicitly know that exceptions can be thrown?;
def m(): Either[SomeErrorType, Unit] // this is odd;
def m(): Option[SomeErrorType] // this is odd, as it makes it look as the return type ofm()on a successful run is an error code.
Other that I can't think of?
Thanks
I use Try[Unit] for that case.
It encodes that the result of the method either succeeds or fails with some Exception, which can be further processed.
vs T => Unit Try lifts errors to the application level, encoding in the signature that some error can be expected and allowing the application to handle it as a value.
vs. Option[T] => Option is only able to encode that the operation had a value or not
vs. Either[SomeErrorType, Unit] => Try It's easier to work with using monadic constructions.
I've used something like this to implement checks. (imaginary example)
for {
entity <- receiveEntity // Try[Entity]
_ <- isRelational(entity)
_ <- isComplete(entity)
_ <- isStable(entity)
} yield entity
where each check is of the form: Entity => Try[Unit]
This will return the entity if all checks pass of the first error that failed the check.
One more option that hasn't been mentioned yet is Validated from cats. All the options mentioned so far (Try, Either, Option) are monads, while Validated is an applicative functor. In practice this means you can accumulate errors from multiple methods returning Validated, and you can do several validations in parallel. This might not be relevant to you, and this is a bit orthogonal to the original question, but I still feel it's worth mentioning in this context.
As for the original question, using Unit return type for a side-effecting function is perfectly fine. The fact this function can also return error shouldn't get in your way when you define the "real" (right, successful, etc.) return type. Therefore, if I were to select from your original options, I'd go for Either[Error, Unit]. It definitely doesn't look odd to me, and if anyone sees any drawbacks in it, I'd like to know them.
I made codes using play framework in scala which look like the following:
object Application extends Controller {
def hoge = Action( implicit request =>
val username = MyCookie.getName.get
Ok("hello " + username)
}
}
object MyCookie {
def getName( implicit request: RequestHeader ) = {
request.cookies.get("name").map(_.value)
}
}
I got a code review from my coworker. He said this code is not readable because of implicit parameter. I couldn't reply to his opinion. So could you tell me what is the best way to use implicit parameters? When should I use implicit parameters?
You should use implicit parameters when there is almost always a "right" way to do things, and you want to ignore those details almost all the time; or when there often is no way to do things, and the implicits provide the functionality for those things that work.
For an example of the first case, in scala.concurrent.Future, almost every method takes an implicit ExecutionContext. You almost never care what your ExecutionContext is from call to call; you just want it to work. But when you need to change the execution context you can supply it as an explicit parameter.
For an example of the second case, look at the CanBuildFroms in the collections library. You cannot build anything from anything; certain capabilities are supplied, and the lack of an implicit that, say, lets you package up a bunch of Vector[Option[String]]s into a HashSet[Char] is one major way to keep the library powerful and flexible yet sane.
You are doing neither: apparently you're just using it to save a little typing in one spot at the expense of the other spot. And, in this case, in doing so you've made it less obvious how things work, as you have to look all over the place to figure out where that implicit request actually gets used. If you want to save typing, you're much better off using short variable names but being explicit about it:
Action{ req => val name = MyCookie.getName(req).get; Ok("hello "+name) }
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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.