[英]Implicit resolution of dependent types in Scala
Consider the following piece of code: 考虑以下代码:
trait Foo {
type T
def value: T
}
object Foo {
def apply[A](v: A): Foo = new Foo {
override type T = A
override def value = v
}
}
trait Decode[A] {
def apply(x: A): String
}
object Decode {
def apply[A](f: A => String): Decode[A] = new Decode[A] {
override def apply(x: A) = f(x)
}
implicit val decodeStr: Decode[String] = Decode(identity)
}
class Sandbox {
def decodeFoo(foo: Foo)(implicit decoder: Decode[foo.T]): String =
decoder(foo.value)
val foo = Foo("hello")
println(decodeFoo(foo))
}
The above code should work fine and print hello
but instead it fails to compile: 上面的代码应该工作正常并打印
hello
但它无法编译:
could not find implicit value for parameter decoder: Decode[Sandbox.this.foo.T]
[error] println(decodeFoo(foo))
Even when I explicitly pass in the implicit param: 即使我明确传入隐式参数:
println(decodeFoo(foo = foo)(decoder = Decode.decodeStr))
I still get this error now: 我现在仍然收到此错误:
type mismatch;
[error] found : Decode[String]
[error] required: Decode[Sandbox.this.foo.T]
[error] println(decodeFoo(foo = foo)(decoder = Decode.decodeStr))
[error] ^
Ofcourse, I can make Foo
a Foo[T]
and define decoders for it but that's not the point of this question - I want to understand why the above code fails to compile. 当然,我可以让
Foo
成为Foo[T]
并为它定义解码器,但这不是这个问题的重点 - 我想理解为什么上面的代码无法编译。
The problem exists here: 这里存在的问题是:
object Foo {
def apply[A](v: A): Foo = new Foo {
override type T = A
override def value = v
}
}
There, you've established that you will return a Foo
but not, specifically, which Foo
. 在那里,你已经确定你将返回一个
Foo
但不是,特别是,哪个Foo
。 Hence, that function only knows that it can return a Foo
for any type T
. 因此,该函数只知道它可以返回任何类型
T
的Foo
。 You need an Aux
pattern to recapture the type which is lost when establishing your new Foo
(yes, yes...) 你需要一个
Aux
模式来重新获得在建立新Foo
时丢失的类型(是的,是的......)
object Foo {
type Aux[A] = Foo{ type T = A }
def apply[A](v: A): Aux[A] = new Foo {
type T = A
def value = v
}
}
which then says that for a given A
produce the Foo
which has it's T
dependent type set to A
. 然后说对于给定的
A
产生Foo
,它的T
依赖类型设置为A
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