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POScala: Making implicit conversion A->B work for Option[A] -> Option[B]
 

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  1. POScala: Making implicit conversion A->B work for Option[A] -> Option[B]
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    <p>I'm trying to write a function which re-uses the implicit conversions which I have for Object A -> Object B when they are wrapped in an Option in a generic way so that Option[A] -> Option[B] conversions also work.</p> <p>What I've come up with is:</p> <pre><code>implicit def fromOptionToOption[A, B](from: Option[A])(implicit conversion: (A) =&gt; B): Option[B] = from.map(conversion(_)) </code></pre> <p>This works when I assign a Some(..) to a value but not when I assign an Option val; see the following console output:</p> <pre><code>scala&gt; trait T defined trait T scala&gt; case class Foo(i: Int) extends T defined class Foo scala&gt; case class Bar(i: Int) extends T defined class Bar scala&gt; implicit def fromFooToBar(f: Foo):Bar = Bar(f.i) fromFooToBar: (f: Foo)Bar scala&gt; implicit def fromBarToFoo(b: Bar):Foo = Foo(b.i) fromBarToFoo: (b: Bar)Foo scala&gt; implicit def fromOptionToOption[A, B](from: Option[A])(implicit conversion: (A) =&gt; B): Option[B] = from.map(conversion(_)) fromOptionToOption: [A, B](from: Option[A])(implicit conversion: (A) =&gt; B)Option[B] scala&gt; val foo: Option[Foo] = Some(Bar(1)) foo: Option[Foo] = Some(Foo(1)) // THIS WORKS as expected scala&gt; val fooOpt = Some(Foo(4)) fooOpt: Some[Foo] = Some(Foo(4)) scala&gt; val barOpt2: Option[Bar] = fooOpt &lt;console&gt;:16: error: type mismatch; found : Some[Foo] required: Option[Bar] val barOpt2: Option[Bar] = fooOpt ^ //THIS FAILS. </code></pre> <p>I don't really see the difference between the first and second conversion. Somehow it doesn't invoke the implicit conversion in the latter. I guess it has something to do with the type system, but I can't see how just yet. Any ideas? -Albert (I'm on scala 2.9.1)</p>
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    1. COIn the first case, it's invoking `fromFooToBar` alone (it decides that since it needs an `Option[Bar]`, it should really call `Some(x: Bar)`, and does the conversion to get there); in the second it has to invoke `fromOptionToOption` and doesn't. This explains the difference, but I am not sure how to solve the problem.
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    2. CONote: `val barOpt: Option[Bar] = Some(Foo(4))` works as expected.
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      1. USDan Burton
    3. CO@Dan Burton - Exactly; this is because it is turned into `Some(fromBarToFoo(Foo(4)))`.
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