LeftBias

This object contains utilities for defining an environment in which Either instances are "left-biased".

Left-biased means instances of Either[A,B] can be operated upon via methods that render thme quite analogous to an Option[A], except that failures are represented by a Right containing information about the problem rathar than by uninformative None. In particular, a left-biased Either can be used directly in a for comprehension. Unlike a LeftProjection, a left-biased Either supports all features of a for comprehension, including pattern matching, filtering, and variable assignment.

Left-biasing decorates vanilla Either[A,B] with the following API.

def flatMap[BB >: B, Z]( f : A => Either[Z,BB] ) : Either[Z,BB]
def map[Z]( f : A => Z )                         : Either[Z,B]
def withFilter( p : A => Boolean )               : Either[A,B]
def exists( f : A => Boolean )                   : Boolean
def forall( f : A => Boolean )                   : Boolean
def foreach[U]( f : A => U )                     : Any
def get                                          : A
def getOrElse[AA >: A ]( or : =>AA )             : AA
def toOption                                     : Option[A]
def toSeq                                        : collection.Seq[A]
def xget                                         : B
def xgetOrElse[BB>:B]( or : =>BB )               : BB
def xmap[Z]( f : B => Z )                        : Either[A,Z]
def replaceIfEmpty[BB>:B]( replacement : =>BB )  : Either[A,BB]
def isEmpty                                      : Boolean
def isLeftBiased                                 : Boolean
def isRightBiased                                : Boolean
def conformsToBias                               : Boolean

For the full documentation, plase see Ops.

To enable left-biasing, you first define a bias, typically via the Either.LeftBias.withEmptyToken factory.

// will impart a left-bias suitable to Either[A,String]
val LeftBias = Either.LeftBias.withEmptyToken("EMPTY")
import LeftBias._

The specified "empty token" will define the value in the Right object that will be used to indicate when a filter or pattern-match of a left-side object fails.

If you don't wish to specify an empty token, you may use the more convenient

import Either.LeftBias._

However, if you do not specify an empty token, filter operations or pattern match failures that would leave the left-side empty will result in the raising of a java.util.NoSuchElementException.

You may also create a left-biased environment within a trait, class, object, or package by having the entity extend the trait LeftBias or the abstract base class LeftBias.Base.

A Detailed example

Consider a hypothetical process that must read some binary data from a URL, parse the data into a Record, ensure that record's year is no older than 2011, and then operate upon the data via one of several external services, depending on the opCode, ultimately producing a Result. Things that can go wrong include network errors on loading, parsing problems, failure of the timestamp condition, and failures with the external service.

With Option, one might define a process like:

case class Record( opCode : Int, data : Seq[Byte], year : Int )
trait Result

def loadURL( url : String )                   : Option[Seq[Byte]] = ???
def parseRecord( bytes : Seq[Byte] )          : Option[Record] = ???
def process( opCode : Int, data : Seq[Byte] ) : Option[Result] = ???

def processURL( url : String ) : Option[Result] = {
  for {
    rawBytes                     <- loadURL( url )
    Record( opCode, data, year ) <- parseRecord( rawBytes )
    result                       <- process( opCode, data ) if year >= 2011
  } yield result
}

That's fine, but if anything goes wrong, clients will receive a value None, and have no specific information about what the problem was. Instead of option, we might try to use Either, defining error codes for things that can go wrong. Unconventionally (since we are considering left-biased Either here), we'll let good values arrive as Left values and embed our error codes in Right objects.

object Error {
  case object Empty extends Error;
  case class IO( message : String, stackTrace : List[StackTraceElement] = Nil ) extends Error;
  case class NoParse( message : String, stackTrace : List[StackTraceElement] = Nil ) extends Error;
  case object BadYear extends Error;
  case class ProcessorFailure( message : String, stackTrace : List[StackTraceElement] = Nil ) extends Error;
}
sealed trait Error;

case class Record( opCode : Int, data : Seq[Byte], year : Int )
trait Result

def loadURL( url : String )                   : Either[Seq[Byte],Error] = ??? // fails w/Error.IO
def parseRecord( bytes : Seq[Byte] )          : Either[Record,Error]    = ??? // fails w/Error.NoParse
def process( opCode : Int, data : Seq[Byte] ) : Either[Result,Error]    = ??? // fails w/Error.ProcessorFailure

// we try to use the left projection but the
// for comprehension fails to compile
def processURL( url : String ) : Either[Result,Error] = {
  for {
    rawBytes                     <- loadURL( url ).left
    Record( opCode, data, year ) <- parseRecord( rawBytes ).left
    result                       <- process( opCode, data ).left if year >= 2011
  } yield result
}

Unfortunately, this fails to compile, because scala.util.Either.LeftProjection does not support pattern-matching (used to extract opCode, data, and year) or filtering (which we perform based on year).

Instead, we can left-bias the Either:

object Error {
  case object Empty extends Error;
  case class IO( message : String, stackTrace : List[StackTraceElement] = Nil ) extends Error;
  case class NoParse( message : String, stackTrace : List[StackTraceElement] = Nil ) extends Error;
  case object BadYear extends Error;
  case class ProcessorFailure( message : String, stackTrace : List[StackTraceElement] = Nil ) extends Error;
}
sealed trait Error;

// left-bias Either
val LeftBias = Either.LeftBias.withEmptyToken( Error.Empty )
import LeftBias._

case class Record( opCode : Int, data : Seq[Byte], year : Int )
trait Result

def loadURL( url : String )                   : Either[Seq[Byte],Error] = ??? // fails w/Error.IO
def parseRecord( bytes : Seq[Byte] )          : Either[Record,Error]    = ??? // fails w/Error.NoParse
def process( opCode : Int, data : Seq[Byte] ) : Either[Result,Error]    = ??? // fails w/Error.ProcessorFailure

// use Either directly, rather than a projection, in the for comprehension
def processURL( url : String ) : Either[Result,Error] = {
  for {
    rawBytes                     <- loadURL( url )
    Record( opCode, data, year ) <- parseRecord( rawBytes )
    result                       <- process( opCode, data ) if year >= 2011
  } yield result
}

There is a problem: Error.Empty, would be the result of the year filter failing, when it should be Error.BadYear.

The most general and straightforward wat to fix this is to replace empty tokens as they arrive with more informative errors. For this purpose, biased Either offers a method called LeftBias.Ops.replaceIfEmpty:

// use Either directly, rather than a projection, in the for comprehension
def processURL( url : String ) : Either[Result,Error] = {
  val processed = {
    for {
      rawBytes                     <- loadURL( url )
      Record( opCode, data, year ) <- parseRecord( rawBytes )
      result                       <- process( opCode, data ) if year >= 2011
    } yield result
  }
  processed.replaceIfEmpty( Error.BadYear )
}

The scope of the left-bias is the scope of the import. An alternative way to ensure empty tokens yield meaningful information is to define separate biases for separate contexts.

val ProcessURLBias       = Either.LeftBias.withEmptyToken( Error.BadYear )
val EnableHyperdriveBias = Either.LeftBias.withEmptyToken( Error.SpaceTimeRupture )
...

def processURL( url : String ) : Either[Result,Int] = {
  import ProcessURLBias._
  for {
    rawBytes                     <- loadURL( url )
    Record( opCode, data, year ) <- parseRecord( rawBytes )
    result                       <- process( opCode, data ) if year >= 2011
  } yield result
}

Source: Either.scala

Linear Supertypes

AnyRef, Any

Type Members

abstract class Base[R] extends LeftBias[R]

May be extended by classes, objects, traits, and packages (via package objects).
May be extended by classes, objects, traits, and packages (via package objects).
Within entities that extend this class objects of type Either will be left-biased, and have the full-suite of left-biased operations (including operations suitable to for comprehensions) automatically made available.
Empty Either values will be represented by Right values containing the token of type R passed to the superclass constructor.
If you are using polymorphic error types, be sure to specify the type argument to Base, as too narrow a type may be inferred from a token of a derived type. In English, this means you should extend Either.LeftBias.Base[Error]( Error.Empty ), not just Either.LeftBias.Base( Error.Empty ).
Note: Entities that do not need to extend another class can extend the abstract class Either.LeftBias.Base, which allows defining the Empty token in the superclass constructor rather than overriding EmptyTokenDefinition.
For more information, please see LeftBias
```
class Clown extends Either.LeftBias.Base[Clown.Error]( Clown.Error.Empty ) {
  import Clown.{Cash,Laugh,Error};

  // use Either values directly in for comprehensions
  def perform : Either[Cash,Error] = {
     for {
       laugh <- makeFunny
       money <- begForCash( laugh ) if laugh.loudness > 2
     } yield money
  }

  def makeFunny : Either[Laugh,Error] = ???
  def begForCash( laugh : Laugh )  : Either[Cash,Error] = ???
}
object Clown {
  final object Error {
    case object Empty extends Error;
    case object JokeBombed extends Error;
    case object NoAudience extends Error;
    case object Cheapskates extends Error;
  }
  sealed trait Error;

  case class Laugh( loudness : Int );
  case class Cash( quantity : Int, currencyCode : String );
}
```
implicit final class Ops[A, B] extends AnyVal

A value class implementing the operations of a left-biased Either, which throws a java.util.NoSuchElementException if a filter operation or pattern match results in empty.
A value class implementing the operations of a left-biased Either, which throws a java.util.NoSuchElementException if a filter operation or pattern match results in empty.
Typical use
```
import Either.LeftBias._
```
For documentation of the operations, please see LeftBias.withEmptyToken.Ops
For more, please see LeftBias.
final class withEmptyToken[+E] extends Generic[E]

A typeclass instance which implements the operations of a left-biased Either.
A typeclass instance which implements the operations of a left-biased Either. An unsuccessful filter or pattern match will result in a Right containing a specified empty token of type E.
Typical use
```
val LeftBias = Either.LeftBias.withEmptyToken(-1); //suitable for Either[A,Int]
import LeftBias._
```
For more, please see LeftBias.

Value Members

final def !=(arg0: Any): Boolean

Test two objects for inequality.
Test two objects for inequality.
returns
true if !(this == that), false otherwise.

Definition Classes
AnyRef → Any
final def ##(): Int

Equivalent to x.hashCode except for boxed numeric types and null.
Equivalent to x.hashCode except for boxed numeric types and null. For numerics, it returns a hash value which is consistent with value equality: if two value type instances compare as true, then ## will produce the same hash value for each of them. For null returns a hashcode where null.hashCode throws a NullPointerException.
returns
a hash value consistent with ==

Definition Classes
AnyRef → Any
final def ==(arg0: Any): Boolean

The expression x == that is equivalent to if (x eq null) that eq null else x.equals(that).
The expression x == that is equivalent to if (x eq null) that eq null else x.equals(that).
arg0
the object to compare against this object for equality.
returns
true if the receiver object is equivalent to the argument; false otherwise.

Definition Classes
AnyRef → Any
final def asInstanceOf[T0]: T0

Cast the receiver object to be of type T0.
Cast the receiver object to be of type T0.
Note that the success of a cast at runtime is modulo Scala's erasure semantics. Therefore the expression 1.asInstanceOf[String] will throw a ClassCastException at runtime, while the expression List(1).asInstanceOf[List[String]] will not. In the latter example, because the type argument is erased as part of compilation it is not possible to check whether the contents of the list are of the requested type.
returns
the receiver object.

Definition Classes
Any
Exceptions thrown
ClassCastException if the receiver object is not an instance of the erasure of type T0.
def clone(): AnyRef

Create a copy of the receiver object.
Create a copy of the receiver object.
The default implementation of the clone method is platform dependent.
returns
a copy of the receiver object.

Attributes
protected[java.lang]
Definition Classes
AnyRef
Annotations
@throws( ... )
Note
not specified by SLS as a member of AnyRef
final def eq(arg0: AnyRef): Boolean

Tests whether the argument (arg0) is a reference to the receiver object (this).
Tests whether the argument (arg0) is a reference to the receiver object (this).
The eq method implements an equivalence relation on non-null instances of AnyRef, and has three additional properties:
- It is consistent: for any non-null instances x and y of type AnyRef, multiple invocations of x.eq(y) consistently returns true or consistently returns false.
- For any non-null instance x of type AnyRef, x.eq(null) and null.eq(x) returns false.
- null.eq(null) returns true.
When overriding the equals or hashCode methods, it is important to ensure that their behavior is consistent with reference equality. Therefore, if two objects are references to each other (o1 eq o2), they should be equal to each other (o1 == o2) and they should hash to the same value (o1.hashCode == o2.hashCode).
returns
true if the argument is a reference to the receiver object; false otherwise.

Definition Classes
AnyRef
def equals(arg0: Any): Boolean

The equality method for reference types.
The equality method for reference types. Default implementation delegates to eq.
See also equals in scala.Any.
returns
true if the receiver object is equivalent to the argument; false otherwise.

Definition Classes
AnyRef → Any
def finalize(): Unit

Called by the garbage collector on the receiver object when there are no more references to the object.
Called by the garbage collector on the receiver object when there are no more references to the object.
The details of when and if the finalize method is invoked, as well as the interaction between finalize and non-local returns and exceptions, are all platform dependent.

Attributes
protected[java.lang]
Definition Classes
AnyRef
Annotations
@throws( classOf[java.lang.Throwable] )
Note
not specified by SLS as a member of AnyRef
final def getClass(): Class[_]

A representation that corresponds to the dynamic class of the receiver object.
A representation that corresponds to the dynamic class of the receiver object.
The nature of the representation is platform dependent.
returns
a representation that corresponds to the dynamic class of the receiver object.

Definition Classes
AnyRef → Any
Note
not specified by SLS as a member of AnyRef
def hashCode(): Int

The hashCode method for reference types.
The hashCode method for reference types. See hashCode in scala.Any.
returns
the hash code value for this object.

Definition Classes
AnyRef → Any
final def isInstanceOf[T0]: Boolean

Test whether the dynamic type of the receiver object is T0.
Test whether the dynamic type of the receiver object is T0.
Note that the result of the test is modulo Scala's erasure semantics. Therefore the expression 1.isInstanceOf[String] will return false, while the expression List(1).isInstanceOf[List[String]] will return true. In the latter example, because the type argument is erased as part of compilation it is not possible to check whether the contents of the list are of the specified type.
returns
true if the receiver object is an instance of erasure of type T0; false otherwise.

Definition Classes
Any
final def ne(arg0: AnyRef): Boolean

Equivalent to !(this eq that).
Equivalent to !(this eq that).
returns
true if the argument is not a reference to the receiver object; false otherwise.

Definition Classes
AnyRef
final def notify(): Unit

Wakes up a single thread that is waiting on the receiver object's monitor.
Wakes up a single thread that is waiting on the receiver object's monitor.

Definition Classes
AnyRef
Note
not specified by SLS as a member of AnyRef
final def notifyAll(): Unit

Wakes up all threads that are waiting on the receiver object's monitor.
Wakes up all threads that are waiting on the receiver object's monitor.

Definition Classes
AnyRef
Note
not specified by SLS as a member of AnyRef
final def synchronized[T0](arg0: ⇒ T0): T0

Definition Classes
AnyRef
def toString(): String

Creates a String representation of this object.
Creates a String representation of this object. The default representation is platform dependent. On the java platform it is the concatenation of the class name, "@", and the object's hashcode in hexadecimal.
returns
a String representation of the object.

Definition Classes
AnyRef → Any
final def wait(): Unit

Definition Classes
AnyRef
Annotations
@throws( ... )
final def wait(arg0: Long, arg1: Int): Unit

Definition Classes
AnyRef
Annotations
@throws( ... )
final def wait(arg0: Long): Unit

Definition Classes
AnyRef
Annotations
@throws( ... )
object withEmptyToken

A factory for a typeclass instance which implements the operations of a left-biased Either.
A factory for a typeclass instance which implements the operations of a left-biased Either. An unsuccessful filter or pattern match will result in a Right containing a specified empty token of type E.
Typical use
```
val LeftBias = Either.LeftBias.withEmptyToken(-1); //suitable for Either[A,Int]
import LeftBias._
```
For more, please see LeftBias.

Related Docs: trait LeftBias | package Either

object LeftBias

Type Members

abstract class Base[R] extends LeftBias[R]

implicit final class Ops[A, B] extends AnyVal

final class withEmptyToken[+E] extends Generic[E]

Value Members

final def !=(arg0: Any): Boolean

final def ##(): Int

final def ==(arg0: Any): Boolean

final def asInstanceOf[T0]: T0

def clone(): AnyRef

final def eq(arg0: AnyRef): Boolean

def equals(arg0: Any): Boolean

def finalize(): Unit

final def getClass(): Class[_]

def hashCode(): Int

final def isInstanceOf[T0]: Boolean

final def ne(arg0: AnyRef): Boolean

final def notify(): Unit

final def notifyAll(): Unit

final def synchronized[T0](arg0: ⇒ T0): T0

def toString(): String

final def wait(): Unit

final def wait(arg0: Long, arg1: Int): Unit

final def wait(arg0: Long): Unit

object withEmptyToken

Inherited from AnyRef

Inherited from Any

Ungrouped