如何在 Scala 中将元素附加或前置到元组
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原文地址: http://stackoverflow.com/questions/15349439/
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How to append or prepend an element to a tuple in Scala
提问by EECOLOR
I have a tuple and want to add an element without loosing type safety. This is what I want to achieve:
我有一个元组，想在不失去类型安全性的情况下添加一个元素。这就是我想要实现的目标：
val tuple = ("", 1, 1f) // (String, Int, Float)
val newTuple:(String, Int, Float, Double) = tuple :+ 1d
采纳答案by Rex Kerr
It's worth noting that you can also write a code generator for this in a few lines:
值得注意的是，您还可以用几行代码为此编写代码生成器：
val tupadd = for (n <- 2 to 20) yield {
val t = (0 until n).map(i => ('A'+i).toChar).mkString(", ")
val u = ('A'+n).toChar
val i = (0 until n).map(i => "x._"+(i+1)).mkString(", ")
List(
s"implicit class TupOps$n[$t](val x: ($t)) extends AnyVal {",
s" def :+[$u](y: $u) = ($i, y)",
s" def +:[$u](y: $u) = (y, $i)",
"}"
).mkString("\n")
}
Print these out, stick 'em in a file, and you're good to go:
打印出来，把它们放在一个文件中，你就可以开始了：
scala> implicit class TupOps2[A, B](val x: (A, B)) extends AnyVal {
| def :+[C](y: C) = (x._1, x._2, y)
| def +:[C](y: C) = (y, x._1, x._2)
| }
defined class TupOps2
scala> (1,"salmon") :+ true
res15: (Int, String, Boolean) = (1,salmon,true)
回答by Alex Archambault
Shapeless now does it. Adding
无形现在做到了。添加
import shapeless.syntax.std.tuple._
before your code just compiles.
在您的代码编译之前。
回答by EECOLOR
I found a solution with the help of the awesome Shapeless libraryand it's HList
我在很棒的Shapeless 库的帮助下找到了一个解决方案，它是HList
/*
Define an implicit class to add the methods
It accepts any tuple as we have typed it as Product
*/
implicit class TupleOps[A <: Product](t: A) {
/*
Declare the method to append
B - The type of element we want to append
L - The HList representing the tuple A
P - The HList after appending B
R - The final result
*/
def :+[B, L <: HList, P <: HList, R <: Product](b: B)(
/*
We need some tools to help with the conversion
hlister - Converts a tuple into an HList
prepend - Can prepend one HList before another
tupler - Can convert an HList into a tuple
*/
implicit hlister: HListerAux[A, L], prepend: PrependAux[L, B :: HNil, P], tupler: TuplerAux[P, R]):R =
// Let the helpers do their job
tupler(prepend(hlister(t), b :: HNil))
/*
The prepend method is similar to the append method but does not require
the extra effort to append
*/
def +:[B, L <: HList, R <: Product](b: B)(
// Here we use the :: type of shapeless
implicit hlister: HListerAux[A, L], tupler: TuplerAux[B :: L, R]):R =
tupler(b :: hlister(t))
}
// usage is like this
("", 1, 1f) :+ 1d //> res0: (String, Int, Float, Double) = ("",1,1.0,1.0)
1d +: ("", 1, 1f) //> res1: (Double, String, Int, Float) = (1.0,"",1,1.0)
Edit
编辑
In some cases, where you need to deal with implicit conversions this solution will not work in combination with case classes. I now reverted to the following implementation (based on the code of Rex Kerr)
在某些情况下，您需要处理隐式转换，此解决方案无法与案例类结合使用。我现在恢复到以下实现（基于 Rex Kerr 的代码）
def char(n: Int) = ('A' + n).toChar
def prop(n: Int) = "t._" + (n + 1)
val result =
for (n <- 1 to 21) yield {
val range = (0 until n)
val tupleTypeParams = range map char mkString ", "
val tupleProperties = range map prop mkString ", "
val elementType = char(n)
val elementProperty = prop(n)
val first = n == 1
val tupleType = if (first) s"Tuple1[$tupleTypeParams]" else s"($tupleTypeParams)"
val tupleInstance = if (first) s"Tuple1($tupleProperties)" else s"($tupleProperties)"
val resultType = s"($tupleTypeParams, $elementType)"
s"""|implicit def tupleOps$n[$tupleTypeParams, $elementType] =
| new TupleAppendOps[$tupleType, $elementType, $resultType] {
| def append(t: $tupleType, e: $elementType) = ($tupleProperties, e)
| def init(t: $resultType) = $tupleInstance
| def last(t: $resultType) = $elementProperty
| }""".stripMargin
}
println(result mkString "\n")
回答by Leo
You can try my experimental macros library based on macro paradise scala branch at https://github.com/leonardschneider/macrogenwhich does exactly your example.
您可以在https://github.com/leonardschneider/macrogen上尝试基于宏天堂 scala 分支的实验性宏库，它完全符合您的示例。
Right now I still have some bugs though, where I can not chain the calls.
现在我仍然有一些错误，我无法链接调用。
