在 Scala 中使用通用更新函数在其部件上实现产品类型 [英] Implement product type in Scala with generic update function working on its parts
问题描述
在 Scala 中,我需要创建一个表示复合值的产品类型 &
,例如:
In Scala, I need to create a product type &
that represents a compound value, e.g.:
val and: String & Int & User & ... = ???
即and
应该有一个 String
部分和一个 Int
部分和一个 User
部分.这类似于 Scala with
关键字:
I.e. and
should have a String
part and an Int
part and a User
parts inside. This is similar to Scala with
keyword:
val and: String with Int with User with ... = ???
拥有这样的产品类型,我需要一种方法,拥有一个功能 A =>A
,将其应用到某个产品值,并在 A
部分更改后取回该产品.这意味着产品中的每种类型都必须是唯一的 - 这是可以接受的.
Having such product type I need a way to, having a function A => A
, apply it to some product value and get that product back with A
part altered. It implies that each type in product must be unique - that's acceptable.
一个重要的限制是,当应用一个函数 A =>A
到产品,我只知道产品内部有 A
但没有关于它包含的其他类型的信息.但是作为函数的调用者,我向它传递了一个包含完整类型信息的产品,并希望将此完整类型作为函数签名的一部分返回.
One important limitation is that, when applying a function A => A
to the product, I only know that the product has A
somewhere inside but no information about other types it consists of. But as a caller of the function, I pass it a product with full type information and expect to get this full type back as part of function signature.
在伪代码中:
def update[A, Rest](product: A & Rest, f: A => A): A & Rest
使用 Shapeless 或其他深奥的东西对我来说是可以的.我尝试使用 HList
s 但它们是有序的,而像异构集这样的东西在这里更适合代表 A &休息
部分.
Using Shapeless or other esoteric stuff is okay for me. I tried using HList
s but they are ordered, while something like heterogeneous set would be more appropriate here to represend A & Rest
part.
更新:
以下是解决我的用例的代码,取自下面的 Régis Jean-Gilles 回答,并添加了阅读支持、一些评论和改进的类型安全性:
Here is the code that solves my use case taken from Régis Jean-Gilles answer below wit added read support, some comments, and improved type-safety:
object product {
/** Product of `left` and `right` values. */
case class &[L, R](left: L, right: R)
implicit class AndPimp[L](val left: L) extends AnyVal {
/** Make a product of `this` (as left) and `right`. */
def &[R](right: R): L & R = new &(left, right)
}
/* Updater. */
/** Product updater able to update value of type `A`. */
trait ProductUpdater[P, A] {
/** Update product value of type `A`.
* @return updated product */
def update(product: P, f: A ⇒ A): P
}
trait LowPriorityProductUpdater {
/** Non-product value updater. */
implicit def valueUpdater[A]: ProductUpdater[A, A] = new ProductUpdater[A, A] {
override def update(product: A, f: A ⇒ A): A = f(product)
}
}
object ProductUpdater extends LowPriorityProductUpdater {
/** Left-biased product value updater. */
implicit def leftProductUpdater[L, R, A](implicit leftUpdater: ProductUpdater[L, A]): ProductUpdater[L & R, A] =
new ProductUpdater[L & R, A] {
override def update(product: L & R, f: A ⇒ A): L & R =
leftUpdater.update(product.left, f) & product.right
}
/** Right-biased product value updater. */
implicit def rightProductUpdater[L, R, A](implicit rightUpdater: ProductUpdater[R, A]): ProductUpdater[L & R, A] =
new ProductUpdater[L & R, A] {
override def update(product: L & R, f: A ⇒ A): L & R =
product.left & rightUpdater.update(product.right, f)
}
}
/** Update product value of type `A` with function `f`.
* Won't compile if product contains multiple `A` values.
* @return updated product */
def update[P, A](product: P)(f: A ⇒ A)(implicit updater: ProductUpdater[P, A]): P =
updater.update(product, f)
/* Reader. */
/** Product reader able to read value of type `A`. */
trait ProductReader[P, A] {
/** Read product value of type `A`. */
def read(product: P): A
}
trait LowPriorityProductReader {
/** Non-product value reader. */
implicit def valueReader[A]: ProductReader[A, A] = new ProductReader[A, A] {
override def read(product: A): A = product
}
}
object ProductReader extends LowPriorityProductReader {
/** Left-biased product value reader. */
implicit def leftProductReader[L, R, A](implicit leftReader: ProductReader[L, A]): ProductReader[L & R, A] =
new ProductReader[L & R, A] {
override def read(product: L & R): A =
leftReader.read(product.left)
}
/** Right-biased product value reader. */
implicit def rightProductReader[L, R, A](implicit rightReader: ProductReader[R, A]): ProductReader[L & R, A] =
new ProductReader[L & R, A] {
override def read(product: L & R): A =
rightReader.read(product.right)
}
}
/** Read product value of type `A`.
* Won't compile if product contains multiple `A` values.
* @return value of type `A` */
def read[P, A](product: P)(implicit productReader: ProductReader[P, A]): A =
productReader.read(product)
// let's test it
val p = 1 & 2.0 & "three"
read[Int & Double & String, Int](p) // 1
read[Int & Double & String, Double](p) // 2.0
read[Int & Double & String, String](p) // three
update[Int & Double & String, Int](p)(_ * 2) // 2 & 2.0 & three
update[Int & Double & String, Double](p)(_ * 2) // 1 & 4.0 & three
update[Int & Double & String, String](p)(_ * 2) // 1 & 2.0 & threethree
}
推荐答案
这是一个仅使用纯 Scala 且不需要库的解决方案.它依赖于使用相当标准方法的类型类:
Here's a solution using only pure scala with no required library. It relies on a type class using a rather standard approach:
scala> :paste
// Entering paste mode (ctrl-D to finish)
case class &[L,R](left: L, right: R)
implicit class AndOp[L](val left: L) {
def &[R](right: R): L & R = new &(left, right)
}
trait ProductUpdater[P,A] {
def apply(p: P, f: A => A): P
}
trait LowPriorityProductUpdater {
implicit def noopValueUpdater[P,A]: ProductUpdater[P,A] = {
new ProductUpdater[P,A] {
def apply(p: P, f: A => A): P = p // keep as is
}
}
}
object ProductUpdater extends LowPriorityProductUpdater {
implicit def simpleValueUpdater[A]: ProductUpdater[A,A] = {
new ProductUpdater[A,A] {
def apply(p: A, f: A => A): A = f(p)
}
}
implicit def productUpdater[L, R, A](
implicit leftUpdater: ProductUpdater[L, A], rightUpdater: ProductUpdater[R, A]
): ProductUpdater[L & R, A] = {
new ProductUpdater[L & R, A] {
def apply(p: L & R, f: A => A): L & R = &(leftUpdater(p.left, f), rightUpdater(p.right, f))
}
}
}
def update[A,P](product: P)(f: A => A)(implicit updater: ProductUpdater[P,A]): P = updater(product, f)
// Exiting paste mode, now interpreting.
让我们测试一下:
scala> case class User(name: String, age: Int)
defined class User
scala> val p: String & Int & User & String = "hello" & 123 & User("Elwood", 25) & "bye"
p: &[&[&[String,Int],User],String] = &(&(&(hello,123),User(Elwood,25)),bye)
scala> update(p){ i: Int => i + 1 }
res0: &[&[&[String,Int],User],String] = &(&(&(hello,124),User(Elwood,25)),bye)
scala> update(p){ s: String => s.toUpperCase }
res1: &[&[&[String,Int],User],String] = &(&(&(HELLO,123),User(Elwood,25)),BYE)
scala> update(p){ user: User =>
| user.copy(name = user.name.toUpperCase, age = user.age*2)
| }
res2: &[&[&[String,Int],User],String] = &(&(&(hello,123),User(ELWOOD,50)),bye)
<小时>
更新:回应:
当产品不包含要更新的值时,是否可以使其不编译
Is it possible to make this not compile when a product doesn't contain a value to update
是的,这绝对是可能的.我们可以改变 ProductUpdater
类型类,但在这种情况下,我发现引入一个单独的类型类 ProductContainsType
作为给定产品 P
至少包含一个 A
类型的元素:
Yes it is most definitely possible. We could alter the ProductUpdater
type class but in this case I find it much easier to introduce a separate type class ProductContainsType
as an evidence that a given product P
contains at least one element of type A
:
scala> :paste
// Entering paste mode (ctrl-D to finish)
@annotation.implicitNotFound("Product ${P} does not contain type ${A}")
abstract sealed class ProductContainsType[P,A]
trait LowPriorityProductContainsType {
implicit def compositeProductContainsTypeInRightPart[L, R, A](
implicit rightContainsType: ProductContainsType[R, A]
): ProductContainsType[L & R, A] = null
}
object ProductContainsType extends LowPriorityProductContainsType {
implicit def simpleProductContainsType[A]: ProductContainsType[A,A] = null
implicit def compositeProductContainsTypeInLeftPart[L, R, A](
implicit leftContainsType: ProductContainsType[L, A]
): ProductContainsType[L & R, A] = null
}
// Exiting paste mode, now interpreting.
现在我们可以定义更严格的update
方法:
Now we can define our stricter update
method:
def strictUpdate[A,P](product: P)(f: A => A)(
implicit
updater: ProductUpdater[P,A],
containsType: ProductContainsType[P,A]
): P = updater(product, f)
让我们看看:
scala> strictUpdate(p){ s: String => s.toUpperCase }
res21: &[&[&[String,Int],User],String] = &(&(&(HELLO,123),User(Elwood,25)),BYE)
scala> strictUpdate(p){ s: Symbol => Symbol(s.name.toUpperCase) }
<console>:19: error: Product &[&[&[String,Int],User],String] does not contain type Symbol
strictUpdate(p){ s: Symbol => Symbol(s.name.toUpperCase) }
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