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POWhy returning a match from macro gives me type errors?
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  1. POWhy returning a match from macro gives me type errors?
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    copied!<p>Consider this example:</p> <pre><code>class A object Macros { def genA(str: String): A[_] = macro _genA def _genA(c: Context)(str: c.Expr[String]): c.Expr[A[_]] = { import c.universe._ c.Expr[A[_]]( reify { new A[Int] }.tree) } // somewhere in code genA("int") // --&gt; new A[Int] </code></pre> <p>It works as expected. However, when I introduce a match, that selects an expression via match, I get into problems:</p> <pre><code>class A object Macros { def genA(str: String): A[_] = macro _genA def _genA(c: Context)(str: c.Expr[String]): c.Expr[A[_]] = { import c.universe._ c.Expr[A[_]]( Match( str.tree.duplicate, List( CaseDef(Literal(Constant("int")), EmptyTree, reify { new A[Int] }.tree)))) } } // again, somewhere far, far away in the code genA("int") // it expands to ("int" match { case "int" =&gt; new A[Int] }) </code></pre> <p>It compiles fine, but after macro expansion I get this error:</p> <pre><code>found : rsf.macros.A[Int] required: rsf.macros.A[_$1] where type _$1 genA("int") ^ </code></pre> <p>The same expression, when entered manually, compiles fine:</p> <pre><code>val a: A[_] = "int" match { case "int" =&gt; new A[Int] } </code></pre> <p>Why does this happen? Is there a way to fix it?</p> <p><strong>EDIT:</strong></p> <p>I now use the following workaround:</p> <pre><code>class A object Macros { def genA(str: String): A[_] = macro _genA def _genA(c: Context)(str: c.Expr[String]): c.Expr[A[_]] = { import c.universe._ val mtch = c.Expr[A[_]]( Match( str.tree.duplicate, List( CaseDef(Literal(Constant("int")), EmptyTree, reify { new A[Int] }.tree)))) reify { mtch.splice.asInstanceOf[A[_]] } } } </code></pre> <p>But I feel that every time this macro executes, somewhere a kitten dies. Maybe there is some way to spare the kittens?</p>
 

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