softwaremill / scala-common

Tiny independent libraries with a single purpose, often a single class

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scala-common

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Tiny independent libraries with a single purpose, often a single class.

Tagging

Maven Central

Tag instances with arbitrary types. Useful if you'd like to differentiate between instances on the type level without runtime overhead. Tags are only used at compile-time to provide additional type safety.

An instance of type T tagged with type U has type T @@ U (which is sugar for @@[T, U]). You can only use x: T @@ U when an instance of type T @@ V is expected when U is a subtype of V (U <: V).

The tag can be any type, but usually it is just an empty marker trait.

To add tags to existing instances, you can use the taggedWith[_] method, which returns a tagged instance (T.taggedWith[U]: T @@ U). Tagged instances can be used as regular ones, without any constraints.

SBT dependency:

libraryDependencies += "com.softwaremill.common" %% "tagging" % "2.2.1"

Example:

import com.softwaremill.tagging._

class Berry()

trait Black
trait Blue

val berry = new Berry()
val blackBerry: Berry @@ Black = berry.taggedWith[Black]
val blueBerry: Berry @@ Blue = berry.taggedWith[Blue]

// compile error: val anotherBlackBerry: Berry @@ Black = blueBerry

Original idea by Miles Sabin. Similar implementations are also available in Shapeless and Scalaz.

Tagging and typeclasses

Let's consider the following example:

import com.softwaremill.tagging._

// Our typeclass
trait Serializer[T] {
  def doSerialize(t: T): String
}

// Method that leverages typeclass, to transform some T into a String
def serialize[T](t: T)(implicit ser: Serializer[T]): String = {
  ser.doSerialize(t)
}

// Typeclass instance for type `Long`
implicit val longSerializer = new Serializer[Long] {
  override def doSerialize(t: Long): String = "Long number: " + t
}

val longNumber = 30L
serialize(longNumber) // Compiles and returns "Long number: 30"

// Our marker trait to be used as a tag
trait UserId

val id: Long @@ UserId = 1024L.taggedWith[UserId]
serialize(id) // Won't compile: could not find implicit value for parameter ser

Because tagged type T @@ U is considered by the compiler as a different type than T, it will complain about missing implicit typeclass Serializer[_] for T @@ U, even if there is instance of Serializer[T] already in scope.

To solve this problem just either mix-in TypeclassTaggingCompat[M[_]]/AnyTypeclassTaggingCompat trait or import contents of the AnyTypeclassTaggingCompat object:

import com.softwaremill.tagging.AnyTypeclassTaggingCompat._

serialize(id) // Compiles and returns "Long number: 1024"

TypeclassTaggingCompat brings implicit conversion, that can adapt any implicit M[T] to be used as M[T @@ U].

Future Try extensions

Maven Central

Provides two utility methods for extending Future:

  • tried: Future[Try[T]] - reifying the Future's result.
  • transformTry(f: Try[T] => Try[S]): Future[S] - corresponds to 2.12's new transform variant, allowing to supply a single function (if, for example, you already have one handy), instead of two. Note: unfortunately, it was not possible to name this method transform, due to how scalac handles implicit resolution.

SBT depedency:

libraryDependencies += "com.softwaremill.common" %% "futuretry" % "1.0.0"

Example:

val myFuture: Future[Foo] = ...
val myUsefulTransformer: Try[Foo] => Try[Bar] = ...

def someWeirdApiMethod(future: Future[Try[Foo]])
 
import com.softwaremill.futuretry._

someWeirdApiMethod(myFuture.tried)

val myBetterFuture: Future[Bar] = myFuture.transformTry(myUsefulTransformer)

Future Squash

Maven Central

Goal

Monad stacks are not easy to compose (e.g. in a for comprehension), and if you don't want to use Monad transformers you can use this Future additional methods.

FutureSquash.fromEither

import com.softwaremill.futuresquash.FutureSquash

FutureSquash.fromEither(Right("a")) //Future("a")
FutureSquash.fromEither(Left(BoomError)) //Future(BoomError)

squash Future[Either[Throwable, A]] and Future[Try[A]]

You can use squash on a Future[Either[Throwable, A]] to get a Future[A].

import FutureSquash._

abstract class Error(message: String) extends Exception(message)
case object BoomError extends Error("Boom")

val fea: Future[Either[Error, String]] = Future(Right("a"))
val feb: Future[Either[Error, String]] = Future(Left(BoomError))

fea.squash //Future("a")
feb.squash //Future(BoomError)

You can also squash on a Future[Try[A]] to get a Future[A] in much the same way:

import FutureSquash._

val fta: Future[Try[String]] = Future(Success("a"))
val ftb: Future[Try[String]] = Future(Failure(new Exception("Boom")))

fta.squash //Future("a")
ftb.squash //Future(Exception("Boom"))

It can also be useful to compose several Future[Either[Throwable, _]] without monad transformers :

def fea: Future[Either[Throwable, Int]] = Future(Right(1))
def feb(a: Int): Future[Either[Throwable, Int]] = Future(Right(a + 2))

val composedAB: Future[Int] = for {
  a <- fea.squash
  ab <- feb(a).squash
} yield ab

composedAB // Future("ab")

val error: Either[Throwable, Int] = Left(BoomError)
val composedABWithError: Future[Int] = for {
  a <- Future.successful(error).squash
  ab <- feb(a).squash
} yield ab

composedABWithError //Future(Failure(BoomError))

Composing several Future[Try[_]]s without monad transformers is also possible:

def fta: Future[Try[Int]] = Future(Success(1))
def ftb(a: Int): Future[Try[Int]] = Future(Success(a + 2))

val composedAB: Future[Int] = for {
  a <- fta.squash
  ab <- ftb(a).squash
} yield ab

composedAB // Future("ab")

Options

Same operations can be used with options : FutureSquash.fromOption and squash on Future[Option[A]]. For empty options, an EmptyValueError will be raised.

Either additional operations (EitherOps)

Maven Central

This small util methods allow to use Either for multiple values validation, to avoid for comprehension fail-fast behavior and accumulate errors.

Example :

import com.softwaremill.eitherops._

case class Person(firstName: String, lastName: String, age: Int)

sealed trait Error
case class NumericError(message: String) extends Error
case class OtherError(message: String) extends Error

def validateAge(intValue: Int): Either[NumericError, Int] = ??? 
def validateFirstName(stringValue: String): Either[OtherError, String] = ???
def validateLastName(stringValue: String): Either[OtherError, String] = ???

val errors: Seq[Error] = EitherOps.collectLefts(
  validateFirstName("john"),
  validateLastName("doe"),
  validateAge(40)
)

if (errors.isEmpty) ???// use for comprehension here to build a Person 
else ??? // handle errors here

EitherOpscollectRights symmetric method is provided for convenience.

Simple benchmarking utilities

Maven Central

Provides utilities for benchmarking.

  • Timed.runTests(tests: List[(String, () => String)], repetitions: Int): runs specified number of repetitions of given code blocks and collects results (mean and standard deviation). A warmup round of all tests will be executed before measuring any statistics. Tests will be executed in random order.

  • Timed.runTests(tests, repetitions, warmup) runs multiple repetitions of shuffled tests provided as a list of PerfTest instances. Each PerfTest should define a name, body (synchronous code block) and, optionally, an additional code block that specifies a "warmup" that will be run before each test execution. The Timed object can also consume an optional warmup argument which specified code block that should be executed before all tests. If omitted, the default global warmup will run all the provided tests once (without collecting metrics).

Example:

val simpleTests: List[(String, () => String)] = List(
  ("test1", () => {
    // do some calculation
    "Ok"
  }),
  ("test2", () => {
    // do some other calculation
    "Ok"
  })
)

Timed.runTests(simpleTests, repetitions = 50)

or

case class MyTest(name: String, param: Int) extends PerfTest {

  override def warmup(): Unit = {
    // prepare some resources
  }

  override def run(): Try[String] = {
    // do some calculations
    Success(String)
  }
}
  
val tests: List[MyTest] = List(
  MyTest("test1", param = 665), MyTest("test2", param = 777)    
)
  
Timed.runTests(tests, repetitions = 50, warmup = (tests: List[MyTest]) => {
  // global warmup body
})