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copied!<p>In order to log during monadic computation, you have to use an instance of Writer monad. Since monad doesn't compose and you want to keep "Validation" effect, you should use a Validation Monad Transformer. I don't know which version of ScalaZ you're using but Scalaz7 (branch scalaz-seven) provides such monad transformer (namely ValidationT).</p> <p>so we get:</p> <pre><code>ValidationT[({type f[x] = Writer[W, x]})#f, A] </code></pre> <p>with W the type of your logger</p> <p><strong>According to your edit</strong>, here's how I'll do it</p> <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) </code></pre> <p>And now, how to use it in a for-comprehension</p> <pre><code>for { y <- squareroot(x).flatMapF(i => Writer("Squareroot ok", i)) z <- inverse(y).flatMapF(i => Writer("Inverse ok", i)) } yield z </code></pre> <p>Those snippets might need more type annotations</p> <p><strong>Edited on april 13</strong></p> <p>Here's the correct type annotations for your methods:</p> <pre><code> def squareroot(x:Double):ValidationT[({type f[x] = Writer[String, x]})#f,String,Double] def inverse(x:Double):ValidationT[{type f[x] = Writer[String, x]})#f,String,Double] </code></pre> <p>That way, you can define resultat method like this:</p> <pre><code>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 </code></pre> <p>You could also use List[String] as a log type because it's a monoid</p> <p>BTW, I speak French if it can help :-)</p> <p><em><strong>Edit on May 14</em></strong></p> <p>The problem was: The compiler cannot resolve </p> <pre><code>implicitly[Pointed[({ type f[x] = Writer[String, x] })#f]] </code></pre> <p>because WriterT need an instance of Monoid[String] and Pointed[Id]. </p> <pre><code>import std.string._ // this import all string functions and instances import Id._ // this import all Id functions and instances </code></pre> <p>Here is the full executable code</p> <pre><code>import scalaz._ import std.string._ import Id._ import scalaz.WriterT import scalaz.ValidationT import scala.Math._ object ValidationTExample extends Application { type ValidationTWriterAlias[W, A] = ValidationT[({type f[x] = Writer[W, x]})#f, W, A] type WriterAlias[A] = Writer[String, A] def squareroot(x:Double): ValidationTWriterAlias[String, Double] = if (x < 0) ValidationT.failureT[WriterAlias, String, Double]("Can't take squareroot of negative number") else ValidationT.successT[WriterAlias, String, Double](sqrt(x)) def inverse(x:Double): ValidationTWriterAlias[String, Double] = if (x == 0) ValidationT.failureT[WriterAlias, String, Double]("Can't take inverse of zero ") else ValidationT.successT[WriterAlias, 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 println("0 : " + resultat(0.0).run.run) println("-1 : " + resultat(-1.0).run.run) println("4 : " + resultat(4).run.run) } </code></pre> <p><em><strong>Edit on August 14</em></strong> </p> <p>This code is no longer valid in scalaz-seven. ValidationT has been removed since Validation is not a monad. Hopefully, EitherT can be used instead. Besides, a new MonadWriter/ListenableMonadWriter typeclass has been added to alleviate those type annotations.</p> <pre><code>import scalaz._ import std.string._ import syntax.monadwriter._ import scala.Math._ object EitherTExample extends Application { implicit val monadWriter = EitherT.monadWriter[Writer, String, String] def squareroot(x: Double) = if (x < 0) monadWriter.left[Double]("Can't take squareroot of negative number") else monadWriter.right[Double](sqrt(x)) def inverse(x: Double) = if (x == 0) monadWriter.left[Double]("Can't take inverse of zero") else monadWriter.right[Double](1 / x) def resultat(x: Double) = for { y <- squareroot(x) :++> "Squareroot ok" z <- inverse(y) :++> ", Inverse ok" } yield z println("0 : " + resultat(0.0).run.run) println("-1 : " + resultat(-1.0).run.run) println("4 : " + resultat(4).run.run) } </code></pre>

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