I have class, parameterized with HList
and some other type. How can I use map
on HList
in one of its methods?
Compilation of this code throws java.lang.AssertionError
:
class Test[L <: HList, P](l: L, p: P) {
type Cont[T] = (P, T)
object generator extends (Id ~> Cont) {
def apply[T](t: T) = p -> t
}
def test(implicit m: Mapper[generator.type, L]) = {
l map generator
}
}
new Test(1 :: HNil, 'a).test // java.lang.AssertionError
My goal is this kind of result:
type Cont[T] = (Symbol, T)
val p = 'a
object generator extends (Id ~> Cont) {
def apply[T](t: T) = p -> t
}
scala> (1 :: 'b' :: HNil) map generator
res0: shapeless.::[(Symbol, Int),shapeless.::[(Symbol, Char),shapeless.HNil]] = ('a,1) :: ('a,b) :: HNil
This is a bug in the Scala compiler (both 2.9.2 and 2.10.0-RC1).
As a workaround, if you split the creation of the instance of Test
and the invocation of the test
method across two expression then it works as expected,
scala> val t = new Test(1 :: HNil, 'a)
t: Test[shapeless.::[Int,shapeless.HNil],Symbol] = Test@4b153b34
scala> t.test
res0: shapeless.::[(Symbol, Int),shapeless.HNil] = ('a,1) :: HNil
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