在Scala中，是否存在任何条件无法将隐式视图传播到其他隐式函数? [英] In scala, are there any condition where implicit view won't be able to propagate to other implicit function?
问题描述
假设定义了一个名为"summoner"的类，该类能够从作用域中调用隐式视图:
Assuming that A class called 'summoner' was defined, that is capable of summoning implicit views from the scope:
case class Summoner[R]() {
def summon[T](v: T)(implicit ev: T => R): R = ev(v)
}
我发现它在大多数情况下都有效，但是在某些情况下它不起作用，例如以下是(不是太)使用singleton-ops库的简短案例:
I found that it works most of the time, but there are cases where it doesn't work, e.g. the following is a (not too) short case which uses the singleton-ops library:
import shapeless.Witness
import singleton.ops.+
import singleton.ops.impl.Op
trait Operand {
def +[
X >: this.type <: Operand,
Y <: Operand
](that: Y): Op2[X, Y] = {
Op2[X, Y](this, that)
}
}
object Operand {
abstract class ProvenToBe[O <: Arity]()(implicit val out: O) extends Operand {}
object ProvenToBe {
implicit class Trivial[O <: Arity, T <: ProvenToBe[O]](
val self: T
) extends Proof {
override type Out = O
override def out: Out = self.out
}
}
}
trait Proof extends Serializable {
def self: Operand
type Out <: Arity
def out: Out
}
object Proof {
trait Out_=[+O <: Arity] extends Proof {
type Out <: O
}
trait Invar[S] extends Out_=[Arity.Const[S]] {
type SS = S
}
}
trait Arity extends Operand {}
object Arity {
trait Const[S] extends Arity {
type SS = S
}
object Const {
implicit class Same[S](val self: Const[S]) extends Proof.Invar[S] {
override type Out = Const[S]
override def out: Const[S] = self
}
}
class FromOp[S <: Op]() extends Const[S]
object FromOp {
implicit def summon[S <: Op](implicit s: S): FromOp[S] = new FromOp[S]()
}
class FromLiteral[S <: Int](val w: Witness.Lt[Int]) extends Const[S] {}
object FromLiteral {
implicit def summon[S <: Int](implicit w: Witness.Aux[S]): FromLiteral[S] =
new FromLiteral[S](w)
}
def apply(w: Witness.Lt[Int]): FromLiteral[w.T] = {
FromLiteral.summon[w.T](w) //TODO: IDEA inspection error
}
}
case class Op2[
+A1 <: Operand,
+A2 <: Operand
](
a1: A1,
a2: A2
) extends Operand {}
object Op2 {
implicit class ProveInvar[
A1 <: Operand,
A2 <: Operand,
S1,
S2
](
val self: Op2[A1, A2]
)(
implicit
bound1: A1 => Proof.Invar[S1],
bound2: A2 => Proof.Invar[S2]
) extends Proof.Invar[S1 + S2] {
override type Out = Arity.FromOp[S1 + S2]
override def out: Out = new Arity.FromOp[S1 + S2]()
}
}
尝试按原样使用隐式视图时:
When attempting to use the implicit view as-is:
implicit val a = Arity(3)
implicit val b = Arity(4)
val op = a + b
op: Proof // implicit view works
但是在使用召唤师时:
val summoner = Summoner[Proof]()
summoner.summon(op) // oops
[Error] /home/peng/git/shapesafe/spike/src/main/scala/edu/umontreal/kotlingrad/spike/arity/package.scala:141: No implicit view available from edu.umontreal.kotlingrad.spike.arity.package.Op2[edu.umontreal.kotlingrad.spike.arity.package.Arity.FromLiteral[Int(3)],edu.umontreal.kotlingrad.spike.arity.package.Arity.FromLiteral[Int(4)]] => edu.umontreal.kotlingrad.spike.arity.package.Proof.
one error found
FAILURE: Build failed with an exception.
此错误消息看起来很平淡，几乎类似于常见的隐式类型不匹配错误，但是以前的用法已经消除了这种可能性.所以我的问题是:
This error message looks quite bland, almost resembles a common implicit type mismatch error, but previous usage has already culled out that possibility. So my questions are:
-
此行为是什么原因?
What's the cause of this behaviour?
你怎么知道的?
推荐答案
我告诉过您有关使用reify，-Xlog-implicits调试隐式函数以及在
I told you about debugging implicits with reify, -Xlog-implicits and manual resolution of implicits in In scala 2 or 3, is it possible to debug implicit resolution process in runtime?
如果您打印树
import scala.reflect.runtime.universe._
println(reify{
op: Proof
}.tree)
您将看到如何解决隐式转换
you'll see how implicit conversion is resolved
(App.this.Op2.ProveInvar(App.this.op)(((self) => Arity.this.Const.Same(self)), ((self) => Arity.this.Const.Same(self))): App.this.Proof)
确实是手动解决的
summoner.summon[Op2[Arity.FromLiteral[3], Arity.FromLiteral[4]]](op)(t =>
Op2.ProveInvar(t)(a1 => Arity.Const.Same(a1), a2 => Arity.Const.Same(a2))
)
可以编译，但是编译器本身找不到隐式转换
compiles but compiler itself can't find implicit conversion
summoner.summon[Op2[Arity.FromLiteral[3], Arity.FromLiteral[4]]](op) //doesn't compile
如果打开-Xlog-implicits，您将看到详细信息
If you switch -Xlog-implicits on you'll see details
Information: $conforms is not a valid implicit value for App.Arity.FromLiteral[3] => App.Proof.Invar[Nothing] because:
hasMatchingSymbol reported error: type mismatch;
found : App.Arity.FromLiteral[3] => App.Arity.FromLiteral[3]
required: App.Arity.FromLiteral[3] => App.Proof.Invar[Nothing]
summoner.summon[Op2[Arity.FromLiteral[3], Arity.FromLiteral[4]]](op)
Information: Arity.this.Const.Same is not a valid implicit value for App.Arity.FromLiteral[3] => App.Proof.Invar[Nothing] because:
hasMatchingSymbol reported error: type mismatch;
found : App.Arity.Const[Nothing] => App.Arity.Const.Same[Nothing]
required: App.Arity.FromLiteral[3] => App.Proof.Invar[Nothing]
summoner.summon[Op2[Arity.FromLiteral[3], Arity.FromLiteral[4]]](op)
Information: App.this.Op2.ProveInvar is not a valid implicit value for App.Op2[App.Arity.FromLiteral[3],App.Arity.FromLiteral[4]] => App.Proof because:
hasMatchingSymbol reported error: No implicit view available from App.Arity.FromLiteral[3] => App.Proof.Invar[Nothing].
summoner.summon[Op2[Arity.FromLiteral[3], Arity.FromLiteral[4]]](op)
As I told you in When calling a scala function with compile-time macro, how to failover smoothly when it causes compilation errors? you can not always check existence of implicit conversion with implicit parameter (implicit ev: T => R). Sometimes existence of implicit instance T => R is not the same as existence of implicit conversion T => R (not all implicit conversions are typeclass-based). Try to replace
val summoner = Summoner[Proof]()
summoner.summon(op) //doesn't compile
使用
summonImplicitView[Proof](op) //compiles
def summonImplicitView[B] = new PartiallyAppliedSummonImplicitView[B]
class PartiallyAppliedSummonImplicitView[B] {
def apply[A](a: A): B = macro summonImplicitViewImpl[A, B]
}
def summonImplicitViewImpl[A: c.WeakTypeTag, B: c.WeakTypeTag](c: whitebox.Context)(a: c.Tree): c.Tree = {
import c.universe._
val tpA = weakTypeOf[A]
val tpB = weakTypeOf[B]
val view = c.inferImplicitView(tree = a, from = tpA, to = tpB, silent = false)
q"$view($a)"
}
您还可以尝试从，但是此类型类并非始终有效，因为它是基于类型的，并且并非所有隐式转换都是基于类型的，它在隐式解析期间会忽略v的值.
but this type class will work not always because it's type-based and not all implicit conversions are type-based, it ignores value of v during implicit resolution.
我想我终于找到了这个问题(这样，如果我们修复它，Summoner将可以在没有宏的情况下使用).您再次失去了类型优化功能.
I guess I finally found the issue (such that if we fix it Summoner will work without macros). You again lost type refinement.
case class Summoner[R]() {
def summon[T](v: T)(implicit ev: T => R): R = ev(v)
}
val summoner = Summoner[Proof {type Out <: Arity.FromOp[3 + 4]}]()
// or even
//val summoner = Summoner[Proof {type Out <: Arity.FromOp[3 + 4]; type SS = 3 + 4}]()
summoner.summon(op) //compiles
这就是为什么您在-Xlog-implicits日志中包含Nothing的原因.
That's why you had Nothings in -Xlog-implicits logs.
我想我已经修复了您的代码.在编写逻辑时，您将隐式实例与隐式转换混合在一起.隐式转换是棘手的.我建议仅根据类型类(MyTransform)编写逻辑，然后如果需要转换，请针对这些类型类定义它们(myConversion).
I guess I fixed your code. While writing your logic you mixed implicit instances with implicit conversions. Implicit conversions are tricky. I would recommend to write your logic only in terms of type classes (MyTransform) and then if you need conversions define them (myConversion) with respect to these type classes.
// doesn't extend T => R intentionally
trait MyTransform[-T, +R] {
def transform(v: T): R
}
implicit def myConversion[T, R](v: T)(implicit mt: MyTransform[T, R]): R = mt.transform(v)
case class Summoner[R]() {
def summon[T](v: T)(implicit ev: MyTransform[T, R]): R = ev.transform(v)
}
trait Operand {
def +[
X >: this.type <: Operand,
Y <: Operand
](that: Y): Op2[X, Y] = {
Op2[X, Y](this, that)
}
}
object Operand {
abstract class ProvenToBe[O <: Arity]()(implicit val out: O) extends Operand {}
object ProvenToBe {
implicit def trivial[O <: Arity, T <: ProvenToBe[O]]: MyTransform[T, Trivial[O, T]] = self => new Trivial(self)
/*implicit*/ class Trivial[O <: Arity, T <: ProvenToBe[O]](
val self: T
) extends Proof {
override type Out = O
override def out: Out = self.out
}
}
}
trait Proof extends Serializable {
def self: Operand
type Out <: Arity
def out: Out
}
object Proof {
trait Out_=[+O <: Arity] extends Proof {
type Out <: O
}
trait Invar[S] extends Out_=[Arity.Const[S]] {
type SS = S
}
}
trait Arity extends Operand {}
object Arity {
trait Const[S] extends Arity {
type SS = S
}
object Const {
implicit def same[S]: MyTransform[Const[S], Same[S]] = self => new Same(self)
/*implicit*/ class Same[S](val self: Const[S]) extends Proof.Invar[S] {
override type Out = Const[S]
override def out: Const[S] = self
}
}
class FromOp[S <: Op]() extends Const[S]
object FromOp {
implicit def summon[S <: Op](implicit s: S): FromOp[S] = new FromOp[S]()
}
class FromLiteral[S <: Int](val w: Witness.Lt[Int]) extends Const[S] {}
object FromLiteral {
implicit def summon[S <: Int](implicit w: Witness.Aux[S]): FromLiteral[S] =
new FromLiteral[S](w)
}
def apply(w: Witness.Lt[Int]): FromLiteral[w.T] = {
FromLiteral.summon[w.T](w) //TODO: IDEA inspection error
}
}
case class Op2[
+A1 <: Operand,
+A2 <: Operand
](
a1: A1,
a2: A2
) extends Operand {}
object Op2 {
implicit def proveInvar[A1 <: Operand, A2 <: Operand, S1, S2](implicit
bound1: MyTransform[A1, Proof.Invar[S1]],
bound2: MyTransform[A2, Proof.Invar[S2]]
): MyTransform[Op2[A1, A2], ProveInvar[A1, A2, S1, S2]]
= self => new ProveInvar(self)
/*implicit*/ class ProveInvar[
A1 <: Operand,
A2 <: Operand,
S1,
S2
](
val self: Op2[A1, A2]
)/*(
implicit
bound1: A1 => Proof.Invar[S1],
bound2: A2 => Proof.Invar[S2]
)*/ extends Proof.Invar[S1 + S2] {
override type Out = Arity.FromOp[S1 + S2]
override def out: Out = new Arity.FromOp[S1 + S2]()
}
}
implicit val a = Arity(3)
implicit val b = Arity(4)
val op = a + b
op: Proof // compiles
val summoner = Summoner[Proof]()
summoner.summon(op) // compiles
这篇关于在Scala中，是否存在任何条件无法将隐式视图传播到其他隐式函数?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
