CODEWITHSUNDEEP
scalascalaz
chrsan
chrsan

Reputation: 3400

Compose Scalaz validations

I would like to use Scalaz for validations and like to be able to reuse the validation functions in different contexts. I'm totally new to Scalaz btw.

Let's say I have these simple checks:

def checkDefined(xs: Option[String]): Validation[String, String] =
  xs.map(_.success).getOrElse("empty".fail)

def nonEmpty(str: String): Validation[String, String] =
  if (str.nonEmpty) str.success else "empty".fail

def int(str: String): Validation[String, Int] = ...

I like to be able to compose validations where output from one is fed into the other. I could easily do that with flatMap or via for comprehensions but it feels like there must be a better way than that.

for {
  v1 <- checkDefined(map.get("foo"))
  v2 <- nonEmpty(v1)
  v3 <- int(v2)
  v4 <- ...
} yield SomeCaseClass(v3, v4)

or

val x1 = checkDefined(map get "foo").flatMap(nonEmpty).flatMap(int)
val x2 = check(...)

// How to combine x1 and x2?

Any thoughts from the Scalaz experts out there?

Upvotes: 14

Views: 4685

Answers (5)

oxbow_lakes
oxbow_lakes

Reputation: 134330

You might want to have a look at the Tale of Three Nightclubs which describes validation composition using:

  1. Monads (i.e. flatMap)
  2. Applicative functors two ways (using |@| and traverse)

Basically the rules amount to this: composition via monads is fail-fast. That is, your computation will short-circuit at this point and resolve to a Failure(e). Using applicative functors means that you can accumulate failures (maybe for web-form validation) - which you do by using a collection (which is a Semigroup) as the failure type - the canconical examples use NonEmptyList.

There is other useful stuff on Validation as well:

val1 <+> val2    //Acts like an `orElse`
val1 >>*<< val2  //Accumulates both successes and failures

In your specific example, why do you think there "must be a better way" than doing it via a for-comprehension? It can be improved slightly, though:

def checkDefined(xs: Option[String]) = xs.toSuccess("empty :-(")

In which case, it doesn't really deserve a whole method:

for {
  v1 <- map get "foo" toSuccess "Empty :-("
  v2 <- some(v1) filterNot (_.isEmpty) toSuccess "Empty :-("
  v3 <- (v2.parseInt.fail map (_.getMessage)).validation 
  v4 <- ...
} yield SomeCaseClass(v3, v4)

Upvotes: 14

P. Frolov
P. Frolov

Reputation: 876

In addition to missingfaktor's answer, one may note that scalaz 7 don't have a Monad for Validation due to mismatch of its behavior with Apply instance. So one may define Bind for Validation, along with converters for convenience:

import scalaz.{Bind, Kleisli, Validation, Success, Failure}

implicit def toKleisli[E, A, B](f: A => Validation[E, B]): Kleisli[Validation[E, ?], A, B] =
  Kleisli[Validation[E, ?], A, B](f)

implicit def fromKleisli[E, A, B](f: Kleisli[Validation[E, ?], A, B]): A => Validation[E, B] = f.run

implicit def validationBind[E] = new Bind[Validation[E, ?]] {

  def bind[A, B](fa: Validation[E, A])(f: (A) => Validation[E, B]): Validation[E, B] = {
    import Validation.FlatMap._
    fa.flatMap(f)
  }

  def map[A, B](fa: Validation[E, A])(f: (A) => B): Validation[E, B] = fa.map(f)
}

val parse: Option[String] => Validation[String, Int] = checkDefined _ >=> nonEmpty _ >=> int _

println(parse(None)) // Failure(empty)
println(parse(Some(""))) // Failure(empty)
println(parse(Some("abc"))) // Failure(java.lang.NumberFormatException: For input string: "abc")
println(parse(Some("42"))) // Success(42)

Upvotes: 0

Erik Kaplun
Erik Kaplun

Reputation: 38247

I've recently coded a simple "framework" for declarative validations that are composable. I've initially based my implementation on @missingfaktor's answer, however, on top of what he's come up with, I've added a DSL using Shapeless's Generic for working with tuples of arbitrary size of inputs to be validated that are fed in to functions of matching arity.

Its usage is as follows:

def nonEmpty[A] = (msg: String) => Vali { a: Option[A] =>
  a.toSuccess(msg)
}

def validIso2CountryCode = (msg: String) => Vali { x: String =>
  IsoCountryCodes2to3.get(x).toSuccess(msg)
}

val postal = "12345".some
val country = "GB".some

val params = (
  postal
     |> nonEmpty[String]("postal required"),
  country
     |> nonEmpty[String]("country required")
    >=> validIso2CountryCode("country must be valid")
)

// parameter type inference doesn't work here due to the generic type level nature of the implementation; any improvements are welcome!
validate(params) { (postal: String, country: String) =>
  println(s"postal: $postal, country: $country")
}

The implementation can be found at https://gist.github.com/eallik/eea6b21f8e5154e0c97e.

Upvotes: 0

Eugene Zhulenev
Eugene Zhulenev

Reputation: 9734

Expression

for {
  v1 <- checkDefined(map.get("foo"))
  v2 <- nonEmpty(v1)
  v3 <- int(v2)
  v4 <- someComputation()
} yield SomeCaseClass(v3, v4)

coulde be replaced in such way

(checkDefined(map.get("foo")).liftFailNel |@| nonEmpty(v1)) {(v1, v2) =
    SomeCaseClass(int(v2), someComputation)
}

and the result will be

 Validtion[NonEmptyList[String], SomeCaseClass] which is equal to ValidationNEL[String, SomeCaseClass]

if both validation fails, NonEmptyList will contain both of them

Upvotes: 0

missingfaktor
missingfaktor

Reputation: 92106

In addition to the solutions suggested by @oxbow_lakes, you can also use Kleisli composition. 

scala> import scalaz._, Scalaz._
import scalaz._
import Scalaz._

scala> def f: Int => Validation[String, Int] = i => if(i % 2 == 0) Success(i * 2) else    Failure("Odd!")
f: Int => scalaz.Validation[String,Int]

scala> def g: Int => Validation[String, Int] = i => if(i > 0) Success(i + 1) else Failure("Not positive!")
g: Int => scalaz.Validation[String,Int]

scala> type Va[+A] = Validation[String, A]
defined type alias Va

scala> import Validation.Monad._
import Validation.Monad._

scala> kleisli[Va, Int, Int](f) >=> kleisli[Va, Int, Int](g)
res0: scalaz.Kleisli[Va,Int,Int] = scalaz.Kleislis$$anon$1@4fae3fa6

scala> res0(11)
res1: Va[Int] = Failure(Odd!)

scala> res0(-4)
res2: Va[Int] = Failure(Not positive!)

scala> res0(4)
res3: Va[Int] = Success(9)

A function of type A => M[B] where M : Monad is called a Kleisli arrow.

You can compose two Kleisli arrows A => M[B] and B => M[C] to get an arrow A => M[C] using >=> operator. This is known as Kleisli composition.

The expression kleisli(f) >=> kleisli(g) >=> kleisli(h) is equivalent to x => for(a <- f(x); b <- g(a); c <- h(b)) yield c, minus the unnecessary local bindings.

Upvotes: 17

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