swaldman / leftright   0.0.1


Implicit enrichment of scala.util.Either to support monadic bias

Scala versions: 2.11 2.10

leftright — monadicize Either

This library tries to address shortcomings (4,5) of scala.util.Either in its core use-case as a monadic container for failable operations that carry information about failures through a sequence of operations. The existing projections API does not support for loops that make use of pattern matching, conditional guards, or viariable assignment, rendering projections crippled scala monads.

The code in this library was originally proposed to be included within scala.util.Either. It is fully documented in a fork of that class, here.

However, while it remains separate, you will need to include a dependency, currently

"com.mchange" %% "leftright" % "0.0.1"


import com.mchange.leftright.BiasedEither

and instead of referring to Either.LeftBias, you must refer to BiasedEither.LeftBias.


Here is a simple example:

import com.mchange.leftright.BiasedEither

val RightBias = BiasedEither.RightBias.withEmptyToken[String]("EMPTY")
import RightBias._

val a : Either[String,Int] = Right(1)
val b : Either[String,Int] = Right(99)

for( v <- a; w <- b ) yield v+w          // Right(100)
for( v <- a; w <- b if v > 10) yield v+w // Left(EMPTY)

For more complete examples, see the documentation in the scala fork

  1. Pull Request: Implicit enrichment as alternative to broken Either projection APIs
  2. SLIP Proposal - Implicit enrichment of scala.util.Either to support monadic bias
  3. Extensively updated Either API documentation
  4. SI-7222: Pattern match typing fail in for comprehension
  5. SI-5589: For-comprehension on Either.RightProjection with Tuple2 extractor in generator fails to compile
  6. Proposed Either/Or/Xor/Validation SLIP expert group