什么是PowerShell ScriptBlock

Question

PowerShell ScriptBlock不是词法闭包,因为它不会关闭其声明环境中引用的变量.相反,似乎利用动态范围和自由变量,这些变量在运行时绑定在lambda表达式中.

function Get-Block {
  $b = "PowerShell"
  $value = {"Hello $b"}
  return $value
}
$block = Get-Block
& $block
# Hello
# PowerShell is not written as it is not defined in the scope
# in which the block was executed.


function foo {
  $value = 5
  function bar {
    return $value
  }
  return bar
}
foo
# 5
# 5 is written $value existed during the evaluation of the bar function
# it is my understanding that a function is a named scriptblock
#  which is also registered to function:

在ScriptBlock上调用GetNewClosure()会返回一个新的ScriptBlock,它会关闭引用的变量.但这在范围和能力上非常有限.

什么是ScriptBlock的分类？

Answer 1

根据文档,scriptblock是"预编译的脚本文本块".因此,默认情况下,您只需要预先解析的脚本块,不多也不少.执行它会创建一个子范围,但除此之外,就像您将代码内联粘贴一样.所以最合适的术语就是"只读源代码".

GetNewClosure在动态生成的模块上调用螺栓,该模块基本上在调用时携带调用者范围内所有变量的快照GetNewClosure.它不是真正的闭包,只是变量的快照副本.scriptblock本身仍然只是源代码,并且在调用之前不会发生变量绑定.您可以根据需要在附加的模块中添加/删除/编辑变量.

function GetSB
{
   $funcVar = 'initial copy'

   {"FuncVar is $funcVar"}.GetNewClosure()

   $funcVar = 'updated value'  # no effect, snapshot is taken when GetNewClosure is called
}

$sb = GetSB

& $sb  # FuncVar is initial copy

$funcVar = 'outside'
& $sb  # FuncVar is initial copy

$sb.Module.SessionState.PSVariable.Remove('funcVar')
& $sb  # FuncVar is outside

Answer 2

一个PowerShell脚本块相当于一个一流的,匿名函数.我见过的大多数混淆不是使用ScriptBlocks,而是使用function关键字.

• PowerShell确实支持函数闭包,但函数关键字却不支持.

例子

功能:

PS> function Hello {
>>      param ([string] $thing)
>>      
>>      return ("Hello " + $thing)
>>  }

PS> Hello "World"
"Hello World"

脚本块:

PS> $HelloSB = {
>>      param ([string] $thing)
>>      
>>      return ("Hello " + $thing)
>>  }

PS> & $HelloSB "World"
"Hello World"

PS> $HelloRef = $HelloSB
PS> & $HelloRef "Universe"
"Hello Universe"

关闭:

PS> $Greeter = {
>>      param ([string] $Greeting)
>>      
>>      return ( {
>>          param ([string] $thing)
>>          
>>          return ($Greeting + " " + $thing)
>>      }.GetNewClosure() )
>>  }

PS> $Ahoy = (& $Greeter "Ahoy")
PS> & $Ahoy "World"
"Ahoy World"

PS> $Hola = (& $Greeter "Hola")
PS> & $Hola "Mundo"
"Hola Mundo"

虽然您可以使用"Set-Item"cmdlet 解决function关键字的限制:

PS> function Greeter = { ... }  # ? Error
PS> function Greeter { ... }.GetNewClosure()  # ? Error

PS> Set-Item -Path "Function:Greeter" -Value $Greeter  # (defined above)  ? OK
PS> $Hola = Greeter "Hola"
PS> & $Hola "Mundo"
"Hola Mundo"

"Set-Item"cmdlet 的Value参数可以是任何ScriptBlock,甚至是另一个函数返回的ScriptBlock.(例如,"Greeter"函数返回一个闭包,如上所示.)

PS> Set-Item -Path "Function:Aloha" -Value (Greeter "Aloha")
PS> Aloha "World"
"Aloha World"

另外两个要点:

• PowerShell使用动态范围,而不是词法范围.

  甲词法闭合锁合在其源代码的环境中,而一个动态闭合是基于当GetNewClosure()被调用时存在的活性/动态环境封闭.(哪种更适合脚本语言.)

• PowerShell可能有"函数"和"返回"语句,但实际上它的输入/输出基于流和管道.将返回使用"写入输出"或"写入"cmdlet从ScriptBlock写出的任何内容.