CODEWITHSUNDEEP
scalascalaz
Yann Moisan
Yann Moisan

Reputation: 8281

lift a value into a ValidationNel

Given a value and a predicate, how to create a ValidationNel ?

Here is my code :

def toValidationNel[A, E](a: A)(f: A => Boolean, fail: => E) : ValidationNel[E, A] =
  if (f(a)) fail.failureNel[A] else a.successNel[E]

Is there an existing combinator in scalaz ?

Upvotes: 1

Views: 105

Answers (2)

Travis Brown
Travis Brown

Reputation: 139058

Scalaz includes an either extension method for booleans that's similar to Either.cond but arguably a little nicer:

import scalaz._, Scalaz._

def toValNel[A, E](a: A)(f: A => Boolean, fail: => E) : ValidationNel[E, A] =
  f(a).either(a).or(fail.wrapNel).validation

I've never really liked the fact that the right side for Either.cond comes on the left, and Scalaz's version makes the either-or-ness a little clearer to my eye, but it's also potentially harder for people who aren't familiar with Scalaz to understand at a glance. I don't see any strong reason to prefer one over the other, though.

Upvotes: 1

Peter Neyens
Peter Neyens

Reputation: 9820

You could use Either.cond from scala.util.Either :

import scalaz.ValidationNel
import scalaz.syntax.std.either._
import scalaz.syntax.nel._

def toValidationNel[A, E](a: A)(f: A => Boolean, fail: => E) : ValidationNel[E, A] =
  Either.cond(f(a), a, fail.wrapNel).validation

Upvotes: 1

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