[英]Scala typeclass for providing an instance either from derivation or from an existing implicit value
I'm getting started with generic programming in Scala and I'm trying to design a flexible buildable schema type (a generic description of ADTs) that can be translated into third party serialization (eg, circe, upickle) or schema (eg, tapir) type class instances.我开始使用 Scala 中的通用编程,我正在尝试设计一个灵活的可构建模式类型(ADT 的通用描述),可以转换为第三方序列化(例如,circe、upickle)或模式(例如,tap ) 类型 class 实例。 When building schemas for products and coproducts, it has to be able to resolve schemas for subcomponents.在为产品和副产品构建模式时,它必须能够解析子组件的模式。
The way I would like to achieve this is with a Provider
type class, that will "provide" an instance of a given type either by resolving a Deriver
or by resolving an existing implicit instance.我想实现这一点的方法是使用Provider
类型 class,它将通过解析Deriver
或解析现有的隐式实例来“提供”给定类型的实例。 By making the derivation provider a lower priority method, it should prefer instances to derivations.通过使派生提供程序成为优先级较低的方法,它应该更喜欢实例而不是派生。
Below is a version of my solution applied to a simpler case: generating a NamesFor[T]
instance that contains the extracted field names of type T.下面是我的解决方案的一个版本,它应用于一个更简单的案例:生成一个NamesFor[T]
实例,其中包含提取的 T 类型的字段名称。
import shapeless._
import shapeless.labelled.FieldType
trait NamesFor[ T ] {
type Names
def names : Names
}
object NamesFor {
type Aux[ T, Names0 ] = NamesFor[ T ] { type Names = Names0 }
}
trait Deriver[ From, To ] {
def derive : To
}
object Deriver {
implicit def deriveNameFromSymbol[ S <: Symbol ](
implicit wit : Witness.Aux[ S ],
) : Deriver[ S, String ] =
new Deriver[ S, String ] {
override def derive : String = wit.value.name
}
implicit def deriveNameFromFieldType[ K <: Symbol, T ](
implicit deriver : Deriver[ K, String ],
) : Deriver[ FieldType[ K, T ], String ] = new Deriver[ FieldType[ K, T ], String ] {
override def derive : String = deriver.derive
}
// Deriving HList
implicit val deriveFromHnil : Deriver[ HNil, HNil ] =
new Deriver[ HNil, HNil ] {
override def derive : HNil = HNil
}
implicit def deriveNameHListFromHList[ Head, Tail <: HList, Res <: HList ](
implicit
headDeriver : Lazy[ Deriver[ Head, String ] ],
tailDeriver : Deriver[ Tail, Res ],
) : Deriver[ Head :: Tail, String :: Res ] = new Deriver[ Head :: Tail, String :: Res ] {
override def derive : String :: Res = headDeriver.value.derive :: tailDeriver.derive
}
// Here's the deriver we're interested in
implicit def deriveNamesFromLabelledGeneric[ T, R <: HList, Names0 <: HList ](
implicit
lGenEv : LabelledGeneric.Aux[ T, R ],
rDer : Deriver[ R, Names0 ],
) : Deriver[ T, NamesFor[ T ] ] = new Deriver[ T, NamesFor[ T ] ] {
override def derive : NamesFor.Aux[ T, Names0 ] = new NamesFor[ T ] {
type Names = Names0
override def names : Names = rDer.derive
}
}
}
trait Provider[ T ] {
def provide : T
}
object Provider {
implicit def provideInstance[ T ](
implicit inst : T,
) : Provider[ T ] = new Provider[T] {
override def provide : T = inst
}
}
trait LowPriorityProvider {
implicit def provideDerivation[ From, To ](
implicit
deriver : Deriver[ From, To ],
) : Provider[ To ] = new Provider[ To ] {
override def provide : To = deriver.derive
}
}
I can derive instances in the following two ways:我可以通过以下两种方式派生实例:
1: implicit provider of existing NamesFor[T]
instance 1:现有NamesFor[T]
实例的隐式提供者
case class Test( int : Int )
implicit val namesInst : NamesFor[ Test ] = new NamesFor[Test] {
override type Names = String :: HNil
override def names : String :: HNil = "INT_FIELD" :: HNil
}
val provider = implicitly[ Provider[ NamesFor[ Test ] ] ]
println( provider.provide.names ) // output: INT_FIELD :: HNil
2: implicit deriver of a NamesFor[T]
from T
2: NamesFor[T]
从T
的隐式派生
case class Test( int : Int )
val deriver = implicitly[ Deriver[ Test, NamesFor[ Test ] ] ]
println( deriver.derive.names ) // output: int :: HNil
but it fails when I try to resolve a provider of NamesFor[T]
without an implicit NamesFor[T]
instance in scope.但是当我尝试在NamesFor[T]
中解析没有隐式NamesFor[T]
实例的 NamesFor[T] 提供者时,它失败了。 That is, it fails to resolve an instance by way of provideDerivation
:也就是说,它无法通过provideDerivation
解析实例:
case class Test( int : Int )
val provider = implicitly[ Provider[ NamesFor[ Test ] ] ]
// Err: could not find implicit value for parameter e: Provider[NamesFor[Test]]
println( provider.provide.names )
Any thoughts on how I could make this all work?关于如何使这一切正常工作的任何想法?
You seem to just forget to extend Provider
from LowPriorityProvider
您似乎只是忘记从LowPriorityProvider
扩展Provider
object Provider extends LowPriorityProvider {...}
Then然后
case class Test( int : Int )
val provider = implicitly[ Provider[ NamesFor[ Test ] ] ]
println( provider.provide.names )
compiles编译
https://scastie.scala-lang.org/DmytroMitin/S7LrOQocSSeMZHetCEifDA/1 https://scastie.scala-lang.org/DmytroMitin/S7LrOQocSSeMZHetCEifDA/1
Scala 2.13.8, Shapeless 2.3.9 Scala 2.13.8,无形 2.3.9
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