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Abstract over case classes companion objects
Tue, 2012-01-03, 17:40
Is there a way to abstract over case classes companion objects, for a given arity?
I tried the following with no success:
scala> def caseClass2[A1, A2, R](companion: { def apply: Function2[A1, A2, R]; def unapply: Function1[R, Option[(A1, A2)]] }) = 42caseClass2: [A1, A2, R](companion: AnyRef{def apply: (A1, A2) => R; def unapply: (R) => Option[(A1, A2)]})Int
scala> case class A(a: Int, s: String)defined class A
scala> caseClass2(A)<console>:11: error: type mismatch; found : A.type (with underlying type object A) required: AnyRef{def apply: (?, ?) => ?; def unapply: (?) => Option[(?, ?)]} caseClass2(A) ^
I tried the following with no success:
scala> def caseClass2[A1, A2, R](companion: { def apply: Function2[A1, A2, R]; def unapply: Function1[R, Option[(A1, A2)]] }) = 42caseClass2: [A1, A2, R](companion: AnyRef{def apply: (A1, A2) => R; def unapply: (R) => Option[(A1, A2)]})Int
scala> case class A(a: Int, s: String)defined class A
scala> caseClass2(A)<console>:11: error: type mismatch; found : A.type (with underlying type object A) required: AnyRef{def apply: (?, ?) => ?; def unapply: (?) => Option[(?, ?)]} caseClass2(A) ^
> scala> def caseClass2[A1, A2, R](companion: { def apply: Function2[A1, A2,
> R]; def unapply: Function1[R, Option[(A1, A2)]] }) = 42
This won't work because `apply` and `unapply` in case class companion
objects are not functions but methods.
However, there are some limitations in structural types:
type Companion[A1, A2, R] = { def apply(a1: A1, a2: A2): R; def
unapply(r: R): Option[(A1, A2)] }
:7: error: Parameter type in structural refinement may not
refer to an abstract type defined outside that refinement
I recommend using a common trait:
scala> :paste
// Entering paste mode (ctrl-D to finish)
trait Blubb[T] extends ((Int, String) => T)
object Foo extends Blubb[Foo]
case class Foo(n: Int, s: String)
// Exiting paste mode, now interpreting.
defined trait Blubb
defined module Foo
defined class Foo