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PODSL for safe navigation operator in Scala
 

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  1. PODSL for safe navigation operator in Scala
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    <p>I want to build a Scala DSL to convert from a existing structure of Java POJOs to a structure equivalent to a Map.</p> <p>However the incoming objects structure is very likely to contain a lot of null references, which will result in no value in the output map.</p> <p>The performance is very important in this context so I need to avoid both reflection and throw/catch NPE.</p> <p>I have considered already <a href="https://stackoverflow.com/questions/1163393/best-scala-imitation-of-groovys-safe-dereference-operator">this topic</a> which does not meet with my requirements.</p> <p>I think the answer may lie in the usage of macros to generate some special type but I have no experience in the usage of scala macros.</p> <p>More formally :</p> <p>POJO classes provided by project : (there will be like 50 POJO, nested, so I want a solution which does not require to hand-write and maintain a class or trait for each of them)</p> <pre><code> case class Level1( @BeanProperty var a: String, @BeanProperty var b: Int) case class Level2( @BeanProperty var p: Level1, @BeanProperty var b: Int) </code></pre> <p>expected behaviour :</p> <pre><code> println(convert(null)) // == Map() println(convert(Level2(null, 3))) // == Map("l2.b" -&gt; 3) println(convert(Level2(Level1("a", 2), 3))) // == Map(l2.p.a -&gt; a, l2.p.b -&gt; 2, l2.b -&gt; 3) </code></pre> <p>correct implementation but I want an easier DSL for writing the mappings</p> <pre><code> implicit def toOptionBuilder[T](f: =&gt; T) = new { def ? : Option[T] = Option(f) } def convert(l2: Level2): Map[String, _] = l2? match { case None =&gt; Map() case Some(o2) =&gt; convert(o2.p, "l2.p.") + ("l2.b" -&gt; o2.b) } def convert(l1: Level1, prefix: String = ""): Map[String, _] = l1? match { case None =&gt; Map() case Some(o1) =&gt; Map( prefix + "a" -&gt; o1.a, prefix + "b" -&gt; o1.b) } </code></pre> <p>Here is how I want to write with a DSL :</p> <pre><code> def convertDsl(l2:Level2)={ Map( "l2.b" -&gt; l2?.b, "l2.p.a" -&gt; l2?.l1?.a, "l2.p.b" -&gt; l2?.l1?.b ) } </code></pre> <p>Note that it is perfectly fine for me to specify that the property is optional with '?'. What I want is to generate statically using a macro a method l2.?l1 or l2?.l1 which returns Option[Level1] (so type checking is done correctly in my DSL).</p>
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    1. USMichel Daviot
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    1. COI was hinted on a mailing-list to use macros quasiquotes but it seems a bit tricky to implement in scala 2.10 at the moment. http://docs.scala-lang.org/overviews/macros/quasiquotes.html ; https://gist.github.com/anonymous/7ab617d054f28d68901b ; http://docs.scala-lang.org/overviews/macros/paradise.html#macro_paradise_for_210x
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      1. PODSL for safe navigation operator in Scala
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