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POScalaz : validating in a for-comprehension and logging
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2012-03-27T19:28:09.607
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2012-08-14T08:33:03.107
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2012-05-23T19:47:35.870
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I admit that the title is not very explicit : sorry for that. Assume I have a for-comprehension : <pre><code>for {v1<-Validation1(input) v2<-Validation2(v1) v3<-Validation3(v2) } yield result </code></pre> Validation1, Validation2 and Validation3 do some checking (e.g "age > 18") and use fail/success ; so if something is wrong, the for-comprehension aborts and I get the reason in the failure part of the result, else I get the expected value in the success part. So far, so good and nothing very difficult. But Validation1, Validation2, Validation3 are successfull if their input satisfies some rules (e.g : "the guy can vote because his age is greater than 18 and his nationality is French") . what I want is to keep trace of the rules that are applied in order to be able to display them at the end. It's clearly a use case of logging. but I hesitate on the way to do it : <ol> <li>Have an object "logger" that is accessible by any function (Validation1, 2 and 3 but also the caller that wants to display the content of the log)</li> <li>Make the logger a parameter of Validation1, 2 and 3</li> <li>Wait for the pertinent chapter of "Functional programming in Scala" :)</li> <li>Other?</li> </ol> Thank for your advices Edited on april 10 So, suppose I want to compute the function : x -> 1/sqrt(x) First, I compute sqrt(x) by checking that x > 0 and then I take the inverse if not zero. with scalaz.Validation, it is simple : <pre><code>val failsquareroot= "Can't take squareroot of negative number" val successsquareroot= "Squareroot ok" val failinverse="Can't take inverse of zero" val successinverse= "Inverse ok" def squareroot(x:Double)=if (x < 0) failsquareroot.fail else sqrt(x).success def inverse(x:Double)= if (x == 0) failinverse.fail else (1/x).success def resultat(x:Double)= for { y <- squareroot(x) z<-inverse(y) } yield z </code></pre> Now, if squareroot successes, I want to log the string successsquaretoot and if inverse sucesses, I want to log the string successinverse so that the function resultat accumulates the both strings in case of success I started with ValidationT as Yo Eight suggested : <pre><code> def squareroot2(x:Double)=ValidationT[({type f[x] = Writer[String,x]})#f, String,Double](Writer(successsquareroot,squareroot(x))) def inverse2(x:Double)=ValidationT[({type f[x] = Writer[String,x]})#f, String,Double](Writer(successinverse,inverse(x))) </code></pre> But I can't find how to combine them in a for-comprehension. Furthermore, to get the result of one of them, I have to write : squareroot2(4).run.run which seems strange and in the way I wrote it, even in case of failure the strings successsquareroot is logged : <pre><code> println(squareroot2(-1).run.run) </code></pre> prints : (Squareroot ok,Failure(Can't take squareroot of negative number)) Thank you! Benoit Edited on april 12 So Yo Eight suggested this snippet : <pre><code> def squareroot(x:Double) = if (x < 0) failureT("Can't take squareroot of negative number") else successT(sqrt(x)) def inverse(x:Double) = if (x == 0) failureT("Can't take inverse of zero ") else successT(1/x) for { y <- squareroot(x).flatMapF(i => Writer("Squareroot ok", i)) z <- inverse(y).flatMapF(i => Writer("Inverse ok", i)) } yield z </code></pre> and he warned me that some type annotations was necessary. Effectivly, the return tpye of squareroot and inverse is rather ugly : it's a ValidationT of something that I had difficulties to understand! So, I had to specify the return type explictly : def inverse(x:Double) : ValidationT[?,E,A] where "E" is String and "A" is Double (that was easy!). But what about the first one? It must be a monad (as far as I understand) and I choosed the simpliest : Id (that is Identity). So now we have : <pre><code> def squareroot(x:Double):ValidationT[Id,String,Double]=if (x < 0) failureT(failsquareroot) else successT(sqrt(x)) def inverse(x:Double):ValidationT[Id,String,Double]=if (x == 0) failureT(failinverse)else successT(1/x) </code></pre> But the for-comprehension doesn't compile because "y" is not a Double but a WriterT[Id, String, Double] Furthermore, the first logged message ("Squareroot ok") is "lost". Eventually, I did like that : <pre><code> def resultat(x:Double) = for { y <- squareroot(x).flatMapF(i => Writer("Squareroot ok", i)) z <- inverse(y.run._2).flatMapF(i => Writer(y.run._1 + ", Inverse ok", i)) } yield z.run //Note that writing "z.run.run" doesn't compile println("0 : " + resultat(0.0).run) println("-1 : " +resultat(-1.0).run) println("4 : " + resultat(4).run) </code></pre> which gives : <pre><code> 0 : Failure(Can't take inverse of zero) -1 : Failure(Can't take squareroot of negative number) 4 : Success((Squareroot ok, Inverse ok,0.5) </code></pre> Cool! I would be better to use a List[String] for the Writer, but I think that I'm on the good way! And now, I can think to my holidays (tomorrow!) :) Edited on may 14 well, the code doesn't compile, but the error is in Yo Eight's last suggestion (Note that it is not an offense again Yo Eight who is a model of kindness!) . I submit you the full code and the error : <pre><code>import scala.math._ import scalaz._ import Scalaz._ object validlog extends ValidationTFunctions { val failsquareroot= "Can't take squareroot of negative number" val successsquareroot= "Squareroot ok" val failinverse="Can't take inverse of zero" val successinverse= "Inverse ok" case class MyId[A]( v: A) implicit val myIdPointed = new Pointed[MyId]{ def point[A](v: => A) = MyId(v) } implicit def unId[A](my: MyId[A]): A = my.v def squareroot(x:Double):ValidationT[({type f[x] = WriterT[MyId,String, x]})#f,String,Double]=if (x < 0) failureT[({type f[x] = WriterT[MyId,String, x]})#f,String,Double](failsquareroot) else successT[({type f[x] = WriterT[MyId,String, x]})#f,String,Double](sqrt(x)) def inverse(x:Double):ValidationT[({type f[x] = WriterT[MyId, String, x]})#f,String,Double]=if (x == 0) failureT[({type f[x] = WriterT[MyId,String, x]})#f,String,Double](failinverse) else successT[({type f[x] = WriterT[MyId,String, x]})#f,String,Double](1/x) /* def resultat(x:Double) = for { y <- squareroot(x).flatMapF(i => Writer(", Squareroot ok", i)) z <- inverse(y).flatMapF(i => Writer(", Inverse ok", i)) } yield z */ def main(args: Array[String]): Unit = { println(inverse(0.0).run) println(inverse(0.5).run) println(squareroot(-1.0).run) println(inverse(4.0).run) } } </code></pre> Here is the terminal's session : <pre><code>benoit@benoit-laptop:~$ cd scala benoit@benoit-laptop:~/scala$ scala -version Scala code runner version 2.9.2 -- Copyright 2002-2011, LAMP/EPFL benoit@benoit-laptop:~/scala$ scala -cp ./scalaz7/scalaz-core_2.9.2-7.0-SNAPSHOT.jar validlog.scala /home/benoit/scala/validlog.scala:15: error: object creation impossible, since method map in trait Functor of type [A, B](fa: Main.MyId[A])(f: A => B)Main.MyId[B] is not defined implicit val myIdPointed = new Pointed[MyId]{ ^ one error found </code></pre> I guess there is something that I've missed from the beginning that could explain why I'm sticked for some weeks! Benoit Edited on may 15 Compiling your code, I have a first error : <pre><code> could not find implicit value for parameter F: scalaz.Pointed[Main.$anon.ValidationTExample.WriterAlias] </code></pre> After some tries, I rewrote the import in this manner : <pre><code>import scalaz.Writer import scalaz.std.string._ import scalaz.Id._ import scalaz.WriterT import scalaz.ValidationT import scala.Math._ </code></pre> There is still one error : <pre><code> error: could not find implicit value for parameter F: scalaz.Monad[[x]scalaz.WriterT[[+X]X,String,x]] y <- squareroot(x).flatMapF(i => Writer("Squareroot ok", i)) ^ one error found </code></pre> This error was present with the code you wrote on may 14. Obviously, it is difficult to understand what to iimport exactly with scalaz-seven. Using the version 6, things looked simpler : one just had to import scalaz._ and Scalaz._ I feel like a "desperate housewriter" :) (yes, I agree, it is not very astute but it's relaxing!) Benoit May 23 Ouf! It effectively works with the last version of scalaz-seven : note that I had to build it instead of downloading a snapshot. that's great! For those who are interested, here is the output : <pre><code> 0 : (Squareroot ok,Failure(Can't take inverse of zero )) -1 : (,Failure(Can't take squareroot of negative number)) 4 : (Squareroot ok, Inverse ok,Success(0.5)) </code></pre> Yo Eight, if by chance we meet one day, i'll pay you a beer! Benoit
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<validation><logging><scalaz>
Title
Scalaz : validating in a for-comprehension and logging
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1. USbhericher
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1. COI think your version of scalaz-seven is not good. The code is running correctly with the last version of scalaz-seven branch
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2. COI missed your post. That is what I guessed. I'm using the snapshot of april 14. I should have got time to test with the last version this evening
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