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Diffstat (limited to 'test/files/run/t7200.scala')
-rw-r--r-- | test/files/run/t7200.scala | 34 |
1 files changed, 34 insertions, 0 deletions
diff --git a/test/files/run/t7200.scala b/test/files/run/t7200.scala new file mode 100644 index 0000000000..ba342df14d --- /dev/null +++ b/test/files/run/t7200.scala @@ -0,0 +1,34 @@ +import language.higherKinds + +object Test extends App { + + // Slice of comonad is where this came up + trait Foo[F[_]] { + def coflatMap[A, B](f: F[A] => B): F[A] => F[B] + } + + // A non-empty list + case class Nel[A](head: A, tail: List[A]) + + object NelFoo extends Foo[Nel] { + + // It appears that the return type for recursive calls is not inferred + // properly, yet no warning is issued. Providing a return type or + // type arguments for the recursive call fixes the problem. + + def coflatMap[A, B](f: Nel[A] => B) = // ok w/ return type + l => Nel(f(l), l.tail match { + case Nil => Nil + case h :: t => { + val r = coflatMap(f)(Nel(h, t)) // ok w/ type args + r.head :: r.tail + } + }) + } + + // Without a recursive call all is well, but with recursion we get a + // ClassCastException from Integer to Nothing + NelFoo.coflatMap[Int, Int](_.head + 1)(Nel(1, Nil)) // Ok + NelFoo.coflatMap[Int, Int](_.head + 1)(Nel(1, List(2))) // CCE + +} |