Using Eithers with Scala "for" syntax
either, for-comprehension, monads, scala, typeclass
Solution
It doesn't work in scala 2.11 and earlier because `Either` is not a monad. Though there's talk of right-biasing it, you can't use it in a for-comprehension: you have to get a `LeftProject` or `RightProjection`, like below:
for {
foo <- Right[String,Int](1).right
bar <- Left[String,Int]("nope").right
} yield (foo + bar)
That returns `Left("nope")`, by the way.
On Scalaz, you'd replace `Either` with `Validation`. Fun fact: `Either`'s original author is Tony Morris, one of Scalaz authors. He wanted to make `Either` right-biased, but was convinced otherwise by a colleague.
Problem
As I understand it, Scala "for" syntax is extremely similar to Haskell's monadic "do" syntax. In Scala, "for" syntax is often used for `List`s and `Option`s. I'd like to use it with `Either`s, but the necessary methods are not present in the default imports. ``` for { foo <- Right(1) bar <- Left("nope") } yield (foo + bar) // expected result: Left("nope") // instead I get "error: value flatMap is not a member..." ``` Is this functionality available through some import? There is a slight hitch: ``` for { foo <- Right(1) if foo > 3 } yield foo // expected result: Left(???) ``` For a List, it would be `List()`. For `Option`, it would be `None`. Do the Scala standard libraries provide a solution to this? (Or perhaps `scalaz`?) How? Suppose I wanted to provide my own "monad instance" for Either, how could I do that?