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    <p>I'm not claiming that this answer is necessarily any better than <a href="https://stackoverflow.com/a/13963136/334519">drstevens's</a>, but it takes a slightly different approach and wouldn't fit in a comment there.</p> <p>First for our validation methods (note that I've changed the type of <code>toInt</code> a bit, for reasons I'll explain below):</p> <pre><code>import scalaz._, Scalaz._ def allDigits: (String) =&gt; ValidationNEL[String, String] = s =&gt; if (s.forall(_.isDigit)) s.successNel else "Not all digits".failNel def maxSizeOfTen: (String) =&gt; ValidationNEL[String, String] = s =&gt; if (s.size &lt;= 10) s.successNel else "Too big".failNel def toInt(s: String) = try(s.toInt.right) catch { case _: NumberFormatException =&gt; NonEmptyList("Still not an integer").left } </code></pre> <p>I'll define a type alias for the sake of convenience:</p> <pre><code>type ErrorsOr[+A] = NonEmptyList[String] \/ A </code></pre> <p>Now we've just got a couple of Kleisli arrows:</p> <pre><code>val validator = Kleisli[ErrorsOr, String, String]( allDigits.flatMap(x =&gt; maxSizeOfTen.map(x *&gt; _)) andThen (_.disjunction) ) val integerizer = Kleisli[ErrorsOr, String, Int](toInt) </code></pre> <p>Which we can compose:</p> <pre><code>val together = validator &gt;&gt;&gt; integerizer </code></pre> <p>And use like this:</p> <pre><code>scala&gt; together("aaa") res0: ErrorsOr[Int] = -\/(NonEmptyList(Not all digits)) scala&gt; together("12345678900") res1: ErrorsOr[Int] = -\/(NonEmptyList(Too big)) scala&gt; together("12345678900a") res2: ErrorsOr[Int] = -\/(NonEmptyList(Not all digits, Too big)) scala&gt; together("123456789") res3: ErrorsOr[Int] = \/-(123456789) </code></pre> <p>Using <code>flatMap</code> on something that isn't monadic makes me a little uncomfortable, and combining our two <code>ValidationNEL</code> methods into a Kleisli arrow in the <code>\/</code> monad—which also serves as an appropriate model for our string-to-integer conversion—feels a little cleaner to me.</p>
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