[英]What exactly is a PowerShell ScriptBlock
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,它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?
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”。
主叫GetNewClosure
上的动态生成的模块,其基本上在携带呼叫的时间在呼叫者的范围内的所有变量的快照螺栓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
PowerShell ScriptBlock相当于一流的 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。
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:
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