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POHow to compose function to applicatives with scalaz
 

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  1. POHow to compose function to applicatives with scalaz
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    <p>While learning <strong>Scalaz 6</strong>, I'm trying to write type-safe readers returning validations. Here are my new types:</p> <pre><code>type ValidReader[S,X] = (S) =&gt; Validation[NonEmptyList[String],X] type MapReader[X] = ValidReader[Map[String,String],X] </code></pre> <p>and I have two functions creating map-readers for ints and strings (*):</p> <pre><code>def readInt( k: String ): MapReader[Int] = ... def readString( k: String ): MapReader[String] = ... </code></pre> <p>Given the following map:</p> <pre><code>val data = Map( "name" -&gt; "Paul", "age" -&gt; "8" ) </code></pre> <p>I can write two readers to retrieve the name and age:</p> <pre><code>val name = readString( "name" ) val age = readInt( "age" ) println( name(data) ) //=&gt; Success("Paul") println( age(data) ) //=&gt; Success(8) </code></pre> <p>Everything works fine, but now I want to compose both readers to build a <code>Boy</code> instance:</p> <pre><code>case class Boy( name: String, age: Int ) </code></pre> <p>My best take is:</p> <pre><code> val boy = ( name |@| age ) { (n,a) =&gt; ( n |@| a ) { Boy(_,_) } } println( boy(data) ) //=&gt; Success(Boy(Paul,8)) </code></pre> <p>It works as expected, but the expression is awkward with two levels of applicative builders. Is there a way, to get the following syntax to work ?</p> <pre><code> val boy = ( name |@| age ) { Boy(_,_) } </code></pre> <p>(*) Full and runnable implementation in: <a href="https://gist.github.com/1891147" rel="noreferrer">https://gist.github.com/1891147</a></p> <hr> <p><strong>Update:</strong> Here is the compiler error message that I get when trying the line above or Daniel suggestion:</p> <pre><code>[error] ***/MapReader.scala:114: type mismatch; [error] found : scalaz.Validation[scalaz.NonEmptyList[String],String] [error] required: String [error] val boy = ( name |@| age ) { Boy(_,_) } [error] ^ </code></pre>
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    1. COI'll post an answer later, but as a hint, remember that `Applicative[G]` and `Applicative[F]` implies `Applicative[[x]F[G[x]]`. In scalaz 7, `Applicative#compose` witnesses this fact. Work directly with the type classes to begin with, rather than using the `|@|` syntax.
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    2. COThanks, but I still don't get it, so I'll wait for your answer. Note that I am using scalaz 6 (question updated).
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    3. CO@paradigmatic Have you tried using `apply` explicitly? Like `( name |@| age) apply { Boy(_, _) }`?
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      1. USDaniel C. Sobral
 

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