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PODefining lift for all mappable types
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  1. PODefining lift for all mappable types
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    copied!<p>For the sake of my own curiosity, I tried to define Haskell's <code>liftM</code> for all types that define <code>map[B](f : A =&gt; B): M[A]</code>, but couldn't get there on my own*. I was hoping that somebody with a deeper insight into Scala's type system might be able to explain it to me.</p> <p>First I attempted to use structural type constraints:</p> <pre><code>import language.higherKinds import language.reflectiveCalls def lift[A, B, F[_] &lt;: {def map(f : A =&gt; B): F[B]}](a : F[A])(g : A =&gt; B) = a map g lift(Some(1)) {_ + 1} // error: inferred type arguments [Int,Nothing,Some] do not // conform to method lift's type parameter bounds // [A,B,F[_] &lt;: AnyRef{def map(f: A =&gt; B): F[B]}] </code></pre> <p>I've attempted a few other things in this vein, but could never get a function to accept a type defining <code>map[B](f : A =&gt; B): F[B]</code>. I also took a whack at it via traits:</p> <pre><code>object Liftable { import language.higherKinds trait Mappable[A, F[_]] { def map[B](f : A =&gt; B): F[B] } implicit class MappableOption[A](opt : Option[A]) extends Mappable[A, Option] { def map[B](f : A =&gt; B) = opt map f } def lift[A, B, F[_]](a : Mappable[A, F])(f : A =&gt; B) = a map f } import Liftable._ lift(Some(1)) {_ + 1} // Some(2) lift(None:Option[Int]) {_ + 1} // None </code></pre> <p><strong>This works</strong>, <em>but</em> it would require defining an implicit class for <em>every</em> type that defines the <code>map</code> function. It loses a lot of it's usefulness there. I'm not sure if there's a way to define an implicit conversion for all types that define <code>map</code> - it brings me back to the problem I had with my first approach.</p> <p>Am I overlooking something? Is the only viable way to do this to follow the second approach, and define an implicit conversion for each mappable type individually?</p> <hr> <p><sub> * I'm aware that the <em>Scalaz</em> library has some form of the <code>lift</code> function defined for its <code>Functor</code> trait, but I wasn't fully able to understand how it works in context with their entire library just by glancing through the source code. </sub></p>
 

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