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PowerShell Gotcha! - dynamic scoping

PowerShell uses dynamic scoping. Yet the about_Scopes page doesn't mention the word "dynamic".

Wird (wird - so weird that you need to misspell weird to get your point across).

tl;dr;

In PowerShell variables are copied into the stack frame created for the function you're calling. So the "child" function can use your variables but can only modify its own copies. You can avoid this by setting your variable to private $private:varName=... and using Set-StrictMode -version latest to throw an error if "child" functions try to access a undefined variable.

PowerShell uses dynamic scoping. What we know from most programming languages is lexical scoping.

function Do-InnerFunction  { Write-Host $t }
function Do-OutterFunction {
    $t = "hello"
    Do-InnerFunction
}

Do-OutterFunction

hello

Weird! (this is dynamic scoping)

Set-StrictMode -Version Latest
function Do-InnerFunction  { Write-Host $t }
function Do-OutterFunction {
    $t = "hello"
    Do-InnerFunction
}

Do-OutterFunction
Set-StrictMode -Off # remember to turn strict mode off for further testing

hello

Weird! (but makes sense since in PowerShell's world this is perfectly legal hence "strict" changes nothing here)

function Do-InnerFunction  { Write-Host $t }
function Do-OutterFunction {
    $private:t = "hello"
    Do-InnerFunction
}

Do-OutterFunction

Output is empty. No errors but at least $t behaves more like a variable we know from C#/F#.

Set-StrictMode -Version Latest
function Do-InnerFunction  { Write-Host $t }
function Do-OutterFunction {
    $private:t = "hello"
    Do-InnerFunction
}

Do-OutterFunction

InvalidOperation: C:\Users\...\Temp\44f5ff41-4105-482b-a134-b505049d2c61\test3.ps1:2
Line |
   2 |      Write-Host $t
     |                 ~~
     | The variable '$t' cannot be retrieved because it has not been set.

Finally!

function Do-InnerFunction {
    Write-Host $t
    $t = "world"
    Write-Host $t
}

function Do-OutterFunction {
    $t = "hello"
    Do-InnerFunction
    Write-Host $t
}

Do-OutterFunction

hello
world
hello

Ah! So variables are copied to the next "scope".

function Do-InnerFunction {
    Write-Host $t
    $global:t = "world"
    Write-Host $t
}

function Do-OutterFunction {
    $t = "hello"
    Do-InnerFunction
    Write-Host $t
}

Do-OutterFunction
Write-Host $t

hello
hello
hello
world

Now we have created a global $t variable.

This https://ig2600.blogspot.com/2010/01/powershell-is-dynamically-scoped-and.html explains it nicely.