Porting of the Raise DSL from the Arrow Kt Kotlin library

The library is available on Maven Central. To use it, add the following dependency to your build.sbt files:

libraryDependencies += "in.rcard.raise4s" %% "core" % "0.4.0"

The library is only available for Scala 3.

The Raise DSL is a new way to handle typed errors in Scala. Instead of using a wrapper type to address both the happy path and errors, the Raise[E] type describes the possibility that a function can raise a logical error of type E. A function that can raise an error of type E must execute in a scope that can also handle the error. In recent Scala, it's something that is referred to direct style.

The easiest way to define a function that can raise an error of type E is to create a context function using the Raise[E] the implicit parameter:

case class User(id: String, name: String)

sealed trait Error

case class UserNotFound(id: String) extends Error

def findUserById(id: String): Raise[Error] ?=> User = User(id, "Alice")

We can do better than that, using the infix type raises:

def findUserById(id: String): User raises Error = User(id, "Alice")

How do we read the above syntax? The function findUserById returns a User and can raise an error of type Error or any of its subtypes.

The above function let us short-circuit an execution and raise an error of type UserNotFound using the raise function:

def findUserById(id: String): User raises Error =
  if (id == "42") User(id, "Alice") else Raise.raise(UserNotFound(id))

The type of error a function can raise is checked at compile time. If we try to raise an error of a different type, the compiler will complain:

def findUserById(id: String): User raises Error =
  if (id == "42") User(id, "Alice") else Raise.raise("User not found")

The above code will not compile with the following error:

[error] 9 |  if (id == "42") User(id) else Raise.raise("User not found")
[error]   |                                                       ^
[error]   |No given instance of type in.rcard.raise4s.Raise[String] was found for parameter raise of method raise in package in.rcard.raise4s
[error] one error found

It's also possible to lift a value to the context Raise[E] ?=> A. If we want to lift a value of type A to the context Raise[Nothing], we can use the succeed function:

def aliceUser: User raises UserNotFound = Raise.succeed(User("42", "Alice"))

More useful is to lift a value of type E as the error in the context Raise[E]:

def userNotFound: User raises UserNotFound = UserNotFound("42").raise[User]

We can always rewrite che last function as follows:

def userNotFound: User raises UserNotFound = {
  Raise.raise(UserNotFound("42"))
}

We may have noticed that one advantage of using the Raise[E] context is that the return type of the function listed only the happy path. As we’ll see in a moment, this is a huge advantage when we want to compose functions that can raise errors.

As you might guess from the previous compiler error, the Raise DSL is using implicit resolutions under the hood. In fact, to execute a function that uses the Raise DSL we need to provide an instance of the Raise[E] type class for the error type E. The most generic way to execute a function that can raise an error of type E and that is defined in the context of a Raise[E] is the fold function:

Raise.fold(
  block = {
    findUserById("43")
  },
  catchBlock = ex => println(s"Error: $ex"),
  recover = error => println(s"User not found: $error"),
  transform = user => println(s"User found: $user")
)

Let’s split the above function into parts. The block parameter is the function that we want to execute. The catchBlock parameter is a function that is executed when the block function throws an exception. Don't worry: The lambda handles only NonFatal exceptions. The recover parameter is a function that is executed when the block function raises a logical typed error of type E. Finally, the transform parameter is a function that is executed when the block function returns a value of type A, which is the happy path. All the handling blocks return the exact value of type B.

The fold function “consumes” the context, creating a concrete instance of a Raise[E] type and executing the block lambda in the context of that instance.

There are other flavors of the fold function. So, please, be sure to check them in the documentation.

For those who are not interested in handling possible exceptions raised by a function, there is a more straightforward function available, called run:

val maybeUser: Error | User = Raise.run {
  findUserById("42")
}

Please be aware that any exception thrown inside the Raise[E] context will bubble up and not be transformed automatically into a logical typed error. What if we want to convert the exception into a typed error? For example, we want to convert the IllegalArgumentException into a UserNotFound. Well, we can do it using a function called catching:

def findUserByIdWithEx(id: String): User =
  if (id == "42") User(id, "Alice") else throw new IllegalArgumentException(s"User not found with id: $id")

val maybeUser: Either[Error, User] =
  Raise.either:
    Raise.catching[User](() => findUserByIdWithEx("42")) {
      case _: IllegalArgumentException => Raise.raise(UserNotFound("42"))
    }

There is also a version of the catching function defined as an extension method of any A type. The above code can be rewritten as follows:

findUserByIdWithEx("42").catching {
  case _: IllegalArgumentException => Raise.raise(UserNotFound("42"))
}

We will see the either function in a moment. As we can see, there’s nothing special with the catching function. It just catches the exception and calls the catch lambda with the exception. The catching function lets the fatal exception bubble up.

It’s a different story if we want to recover or react to a typed error. In this case, we can use the recover function:

case class NegativeAmount(amount: Double) extends Error

def convertToUsd(amount: Double, currency: String): Double raises NegativeAmount =
  if (amount < 0) Raise.raise(NegativeAmount(amount))
  else amount * 1.2

val usdAmount: Double =
  Raise.recover({
    convertToUsd(-1, "EUR")
  }) { case NegativeAmount(amount) => 0.0D }

If you don't care about the error type, and you want to recover from any error with a fixed value, you can use the withDefault function:

val usdAmount: Double =
  Raise.withDefault(0.0D) {
    convertToUsd(-1, "EUR")
  }

What if we want to accumulate more than one error in a dedicated data structure? For example, say we have a list of ids, and we want to retrieve all the associated users.

def findUsersByIds(ids: List[String]): List[User] raises List[UserNotFound]

As you might guess, the library gives us a dedicated function to execute a transformation on a list of values and accumulate the errors in a List[Error]. The function is called mapOrAccumulate:

def findUsersByIds(ids: List[String]): List[User] raises List[UserNotFound] =
  Raise.mapOrAccumulate(ids) { id =>
    findUserById(id)
  }

If at least one error is raised, the mapOrAccumulate function will accumulate all the errors in a List[Error]. If no error is raised, the function will return a List[User]. There is also a version of the mapOrAccumulate function defined as extension method of any Iterable[A] type:

import in.rcard.raise4s.RaiseIterableDef.mapOrAccumulate

def findUsersByIds(ids: List[String]): List[User] raises List[UserNotFound] =
  ids.mapOrAccumulate { id =>
    findUserById(id)
  }

We can obtain the same result using the values extension function:

import in.rcard.raise4s.RaiseIterableDef.values

def findUsersByIds(ids: List[String]): List[User] raises List[UserNotFound] =
  val usersOrErrors: List[User raises UserNotFound] = ids.map(id => findUserById(id))
  usersOrErrors.values

Did anyone say traverse?

As we said, the mapOrAccumulate function allows the combination of the results of a transformation applied to a collection of elements of the same type. What if we want to combine transformations applied to objects of different types?

A classic example is the validation during the creation of an object. Say we want a Salary type with amount and currency information:

case class Salary(amount: Double, currency: String)

Now, we need to create a hierarchy of the possible logical typed errors we can have while creating a Salary object. We’ll check for the following two errors:

1. The amount must be greater than zero
2. The currency must be made of three capital letters

We define the following hierarchy of types to represent the above errors:

sealed trait SalaryError

case object NegativeAmount extends SalaryError

case class InvalidCurrency(message: String) extends SalaryError

In general, we want to avoid the creation of invalid objects. To do so, we can define what we call a smart constructor. Smart constructors are factories that look like regular constructors but perform validations and generally return the valid object or some typed error. The smart constructor must perform all the needed validation on input data before creating a concrete instance of the object.

We can’t use the mapOrAccumulate function we previously saw because we don’t have a list of objects of the same type as input. Fortunately, the library provides the zipOrAccumulate function, which we need.

object Salary {
  def apply(amount: Double, currency: String): Salary raises List[SalaryError] = {
    Raise.zipOrAccumulate(
      {
        Raise.ensure(amount >= 0.0)(NegativeAmount)
      },
      {
        Raise.ensure(currency != null && currency.matches("[A-Z]{3}")) {
          InvalidCurrency("Currency must be not empty and valid")
        }
      }
    ) { (_, _) =>
      Salary(amount, currency)
    }
  }
}

Many different versions of the function differ in the number of input parameters. The maximum number of single input parameters is 9. If we need more, we must apply the function recursively multiple times.

Recently, we added an experimental DSL for error accumulation inspired by the new Arrow 2.0 library. We decided to try a more user-friendly DSL instead of the exoteric mapOrAccumulate and zipAccumulate functions, called accumulate:

object Salary {
  def apply(amount: Double, currency: String): Salary raises List[SalaryError] = 
    accumulate {
      val validatedAmount = accumulating {
        ensure[SalaryError](amount >= 0.0)(NegativeAmount)
        amount
      }
      val validatedCurrency = accumulating {
        ensure[SalaryError](currency != null && currency.matches("[A-Z]{3}")) {
          InvalidCurrency("Currency must be not empty and valid")
        }
        currency
      }
      Salary(validatedAmount, validatedCurrency)
    }
}

The accumulate and accumulating functions are defined in the in.rcard.raise4s.Accumulation object. As you can see, no more lambda tuples are needed. Every raised error is intercepted and accumulated in a list of errors inside the accumulating block. The accumulate function will return the happy path or a list of errors.

The accumulating function returns an instance of the Value[A] type and not an instance of the validated A object itself. The first time the Value[A] instance is used, the library performs an implicit conversion to the A object under the hood. The implicit conversion will raise the accumulated errors if there is an error during the validation.

We can also use the accumulate API to mimic the behavior of the mapOrAccumulate function. For example, we can rewrite the findUsersByIds function as follows:

import in.rcard.raise4s.Accumulation.*

def findUsersByIds(ids: List[String]): List[User] raises List[Error] = accumulate {
  val users = ids.map(id => accumulating { findUserById(id) })
  users
}

The above code uses another implicit conversion under the hood. It's converting from a List[Value[Error, User]] to a List[User] raises List[Error]. In case you want to remove the extra users variable, you need to help the compiler understand the return type of the map function:

def findUsersByIds(ids: List[String]): List[User] raises List[Error] = accumulate {
  ids.map[Value[Error, User]](id => accumulating { findUserById(id) })
}

If we don't force the map function to return a Value[Error, User] instance, the compiler will infer the return type as List[User] and the implicit conversion for the single Value[Error, User] will be applied, instead of the one for the List[Value[Error, User]].

What if we want to convert a computation in the Raise[E] context to a function returning an Either[E, A], a Try[A], an Option[A]? Well, nothing is more straightforward than that.

Let’s start with Either[E, A]. The either builder is what we're searching for. We can translate the result of the findUserById function to an Either[Error, User] quite easily:

val maybeUser: Either[Error, User] =
  Raise.either:
    findUserById("42")

If we want to retrieve more information of a user using her name, we can just use the User instance directly:

val maybeUserNameInUpperCase: Either[Error, String] =
  Raise.either:
    val user: User = findUserById("42")
    user.name.toUpperCase

Please praise the simplicity and absence of boilerplate code, like calls to map functions or when expressions. This is the power of Scala direct style.

It’s also possible to make the backward conversion from an Either[E, A] to a Raise[E] using the value function:

val userNameInUpperCaseRaiseLambda: Raise[Error] ?=> String = maybeUserNameInUpperCase.value

The value function is very handful when we need to compose functions that return an Either[E, A]:

val one = Right(1)
val two = Right(2)
val three = Raise.either {
  val oneValue = one.value
  val twoValue = two.value
  oneValue + twoValue
}

The value function calls the raise function if the Either instance is a Left; otherwise, it returns the value wrapped by the Right instance. Despite the trivial logic implemented in the above example, it's a good example of how to compose functions that return an Either[E, A] using the Raise DSL without the use of any flatMap function.

A useful shortcut is available when we need to transform a List[Either[E, A]] into a List[A] raises E. The eventual raised error E is the first error found in the list of Either[E, A]:

val eitherList: List[Either[String, Int]] = List(Right(1), Left("error"), Right(3))
val raiseList: List[Int] raises String = listWithError.value

Be aware that before version 0.0.5, the value function was called bind().

We can do the same with Try[A] and Option[A] using the asTry and option builders, respectively. Let's start with the asTry builder. In this case, the only available type of error is Throwable:

val maybeUserWithTry: Try[User] =
  Raise.asTry:
    findUserByIdWithEx("42")

As you might guess, any fatal exception thrown inside the asTry context will bubble up and not handled.

Last but not least, the option builder:

def findUserByIdWithNone(id: String): User raises None.type =
  if (id == "42") User(id, "Alice") else Raise.raise(None)

val maybeUserWithOpt: Option[User] =
  Raise.option:
    findUserByIdWithNone("42")

The bind function is available for Try[A] and Option[A] as well.

By the way, there are more feature in the Raise DSL. Please, check the documentation for more information.

We can call a strategy the way we want to handle the errors raised by a function. The whole raise4s library give you many predefined ways to deal with errors. For example, you can fold on a function, run it, either it, asTry it, option it, and so on.

However, the library gives you the possibility to define your own strategy. To do so, you need to define a new instance of the Raise[E] type class. The Raise[E] type class is a type class that defines how to handle errors of type E. The Raise[E] type class is defined as follows:

trait Raise[-Error]:
  def raise(e: Error): Nothing

For example, you can define a strategy that simply throw an exception for every error raised by a function:

def loadConfiguration(file: String): Configuration raises ConfigurationError = ???

given Raise[Any] = error => throw new RuntimeException(error.toString)
val configuration = loadConfiguration(args(0))

The library gives you a type alias to define strategies on all the errors:

type anyRaised = Raise[Any]

Then, we can rewrite the above example as follows:

given anyRaised = error => throw new RuntimeException(error.toString)

We can define a strategy for a single error as well:

given Raise[ConfigurationError] = error => exit(1)

In the above example we're saying that a configuration error must stop the execution of the program.

The library defines a withError function to map an error raised by a function to an error of a different type:

val actual = either {
  Raise.withError[Int, String, Int](s => s.length) {
    raise("error")
  }
}
actual should be(Left(5))

In the above example, we map the error of type String raised by the given lambda into an error of type Int. The withError function does its job quite well. However, the function's ergonomics are not so good due to the pair of lambdas we need to provide: The first one is the function that maps the error, and the second one is the function that raises the error.

We can achieve the same result using a dedicated strategy. In detail, the library defines a MapError strategy that maps an error of type E into an error of type F. The MapError strategy is defined as follows:

trait MapError[From, To] extends Raise[From] {
  def map(error: From): To

  def raise(error: From): Nothing = throw Raised(map(error))
}

Then, we can map an error of type String into an error of type Int by implementing the MapError strategy as follows:

val finalLambda: String raises Int = {
  given MapError[String, Int] = error => error.length

  raise("Oops!")
}
val result: Int | String = Raise.run(finalLambda)
result shouldBe 5

As you can see, we're entirely focused on the happy path, and we can define how to handle the errors in a dedicated place.

We've already seen how to recover from a typed error using the Raise.recover function. The function works quite well, but it's not so ergonomic. We need to provide two lambdas as input for the function. The first is the block we want to execute, and the second is the lambda to apply in case of recovery.

We can obtain the same result by applying the RecoverWith strategy and by using the recoverable DSL:

case class NegativeAmount(amount: Double) extends Error

def convertToUsd(amount: Double, currency: String): Double raises NegativeAmount =
  if (amount < 0) Raise.raise(NegativeAmount(amount))
  else amount * 1.2

given RecoverWith[NegativeAmount, Double] = {
  case NegativeAmount => 0.0D
}
val usdAmount: Double = Raise.recoverable {
  convertToUsd(-1, "EUR")
}

The strategy lets us define how to recover from a typed error in a dedicated place, leaving the happy logic in the main block.

If you want to contribute to the project, please do it! Any help is welcome.

This project is inspired by the Arrow Kt library's Raise DSL. I want to thank the Arrow Kt team for their outstanding work. In detail, thanks to Simon Vergauwen for the great discussions we had on Slack. A lot of thanks also to Daniel Ciocîrlan, my mentor and friend.