What exactly is a PowerShell ScriptBlock

Question

A PowerShell ScriptBlock is not a lexical closure as it does not close over the variables referenced in its declaring environment. Instead it seems to leverage dynamic scope and

Answer 1

A PowerShell ScriptBlock is equivalent to a first-class, anonymous function. Most of the confusion I've seen is not with ScriptBlocks, but with the function keyword.

• PowerShell does support function closures, however the function keyword does not.

Examples

Function:

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

PS> Hello "World"
"Hello World"

ScriptBlock:

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

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

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

Closure:

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"

Although you can get around the limitation of the function keyword with the "Set-Item" cmdlet:

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"

The Value parameter of the "Set-Item" cmdlet can be any ScriptBlock, even one returned by another function. (The "Greeter" function, for example, returns a closure, as shown above.)

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

Two other important points:

• PowerShell uses dynamic scoping, not lexical scoping.

  A lexical closure is closed on its source-code environment, whereas a dynamic closure is closed based on the active/dynamic environment that exists when GetNewClosure() is called. (Which is more appropriate for a scripting language.)

• PowerShell may have "functions" and "return" statements, but actually its input/output is based on streams and piping. Anything written out of a ScriptBlock with the "Write-Output" or "write" cmdlet will be returned.

Answer 2

Per the docs, a scriptblock is a "precompiled block of script text." So by default you just a pre-parsed block of script, no more, no less. Executing it creates a child scope, but beyond that it's as if you pasted the code inline. So the most appropriate term would simply be "readonly source code."

Calling GetNewClosure bolts on a dynamically generated Module which basically carries a snapshot of all the variables in the caller's scope at the time of calling GetNewClosure. It is not a real closure, simply a snapshot copy of variables. The scriptblock itself is still just source code, and variable binding does not occur until it is invoked. You can add/remove/edit variables in the attached Module as you wish.

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