问题描述
我有一个类型推理问题,并要求帮助。
最初的问题是由于重载。一旦纠正I
仍然有问题。
I have a type inference issue and asked for help here.The initial problem was due to overload. Once corrected Istill had problems.
这里是代码:
class DPipe[ A ]( a: A ) {
def !>[ B ]( f: A => B ) = Try(f( a ))
def #>[ B, C ]( f: B => C )(implicit ev: A =:= Try[B]) : Try[C] = a.map(f)
//def &>[ B, C ]( f: B => C )( implicit ev: A =:= Try[ B ] ) = a.map( f )
}
object DPipe {
def apply[ A ]( v: A ) = new DPipe( v )
}
object DPipeOps {
implicit def toDPipe[ A ]( a: A ): DPipe[ A ] = DPipe( a )
}
这里是测试:
object DPipeDebug {
def main( args: Array[ String ] ) {
import DPipeOps._
val r8 = 100.0 !> {x : Double => x / 0.0}
println(r8)
val r9 = r8 #> {x:Double => x* 3.0}
println(r9)
/*
val r8 = 100.0 !> { x: Double => x / 0.0 }
println( r8.get )
val r9 = r8 &> { x: Double => x * 3.0 }
println( r9 )*/
val r10 = (100.0 !> {x : Double => x / 0.0}) #> {x:Double => x* 3.0}
//val r10 = ( 100.0 !> { x: Double => x / 0.0 } ) &> { x: Double => x * 3.0 }
val r11 = 100.0 !> {x : Double => x / 0.0} #> {x:Double => x* 3.0}
//val r11 = 100.0 !> { x: Double => x / 0.0 } &> { x: Double => x * 3.0 }
}
}
我们在最后一行代码中有以下错误:
As it stands we have the following error in the last code line:
Cannot prove that Double => Double =:= scala.util.Try[Double].
val r11 = 100.0 !> {x : Double => x / 0.0} #> {x:Double => x* 3.0}
^
请注意,在第二个代码行中,添加
圆括号以强制执行左侧左侧相关性
(Scala默认值)。看起来像#>
运算符尝试
使用函数 {x:Double => x / 0.0}
,这的确是一个
Double。
Notice that in the second last code line, I need only addthe parenthesis to enforce left left-hand associativity(Scala default). It seems like the #>
operator tries touse the function {x : Double => x / 0.0}
, which indeed is aDouble.
但是,如果我使用&>运算符,则不会发生错误。在下面的
测试代码中,只需翻转注释。所以我的问题是,为什么
是这种情况发生。这是Scala 2.12.0的新功能吗?
If however I use the "&>" operator, no error occurs. In thetest code below, just flip the comments. So my question is, whyis this happening. Is this something new to Scala 2.12.0?
TIA
推荐答案
问题是与运算符优先级。以下是更多详细信息:
The problem is with the operator precedence. Here are more details: Operator precedence in Scala
这取决于运算符的第一个字符。 #
的优先级高于!
和!
&
。
It depends on the first character of the operator. #
has higher precedence than !
and !
has higher precedence than &
.
因此您会得到
100.0 !> ({x : Double => x / 0.0} #> {x:Double => x* 3.0})
b $ b
而不是
instead of
(100.0 !> {x : Double => x / 0.0}) #> {x:Double => x* 3.0}
这篇关于Scala运算符#>导致编译错误,但无法&> - 为什么?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!