[英]Scala: Equivalence of path-dependent types
How do I get around with equivalence of two path-dependent types that I know are the same but the compiler does not? 我如何解决两个与路径相关的类型的等价,我知道它们是相同但编译器没有?
Using Scala 2.10.0 M7 I am trying to convert an AST from one universe to another. 使用Scala 2.10.0 M7我试图将AST从一个宇宙转换为另一个宇宙。
case class MacroBridge(context: Context) {
def toMacroTree(tree: treehugger.forest.Tree): context.universe.Tree = ???
def fromMacroTree(tree: context.universe.Tree): treehugger.forest.Tree = ???
}
Within a macro implementation, I can use it as: 在宏实现中,我可以将其用作:
val bridge = treehugger.MacroBridge(c)
def fromMacroTree(tree: c.universe.Tree): Tree = bridge.fromMacroTree(tree)
However, this results to a compiler error: 但是,这会导致编译器错误:
[error] /scalamacros-getting-started/library/Macros.scala:21: type mismatch;
[error] found : c.universe.Tree
[error] required: bridge.context.universe.Tree
[error] possible cause: missing arguments for method or constructor
[error] def fromMacroTree(tree: c.universe.Tree): Tree = bridge.fromMacroTree(tree)
In the above code c
is clearly the same value as bridge.context
, but maybe because it's a value type checker cannot check it. 在上面的代码中,
c
显然与bridge.context
值相同,但也许是因为它是值类型检查器无法检查它。 Putting generalized type constraint did not help: 放置通用类型约束没有帮助:
def fromMacroTree[A](tree: A)(implicit ev: A =:= context.universe.Tree): Tree =
In the macro this still resulted to an error: 在宏中,这仍然导致错误:
[error] /scalamacros-getting-started/library/Macros.scala:21: Cannot prove that c.universe.Tree =:= bridge.context.universe.Tree.
[error] def fromMacroTree(tree: c.universe.Tree): Tree = bridge.fromMacroTree(tree)
I need the access to context.universe
so I can get to other dependent types like TermName
. 我需要访问
context.universe
所以我可以访问其他依赖类型,如TermName
。 Is there a better work around besides casting?: 除了演员之外还有更好的解决方法吗?:
def fromMacroTree(tree: c.universe.Tree): Tree =
bridge.fromMacroTree(tree.asInstanceOf[bridge.context.universe.Tree])
I could the following make to work: 我可以做以下工作:
case class MacroBridge[C <: Context](context: C) {
def fromMacroTree(tree: context.universe.Tree): context.universe.Tree = ???
}
trait MB {
def meth(c: Context) {
val bridge = MacroBridge[c.type](c)
def fromMacroTree(tree: c.universe.Tree): c.universe.Tree =
bridge.fromMacroTree(tree)
}
}
I had nearly the same problem some time ago. 前段时间我遇到了几乎相同的问题 。
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