Array unrolling in PowerShell

Question

I'm trying to understand array unrolling in Posh and the following puzzles me. In this example I return the array wrapped in another array (using a comma), which basically gives me the original array after the outer array gets unrolled on return. Why then do I get a correct result (just the array) when I indirectly wrap the result by using the array operator (@) on a variable with the result, but get an array in an array if I use @ directly on the function? Does @ operator behave differently for different types of its parameter? Is there any PS help article for the @ operator and the array unrolling?

function Unroll
{
    return ,@(1,2,3)
}

Write-Host "No wrapper"
$noWrapper = Unroll #This is array[3]
$noWrapper | % { $_.GetType() }

Write-Host "`nWrapped separately"
$arrayWrapper = @($noWrapper) #No change, still array[3]
$arrayWrapper | % { $_.GetType() }

Write-Host "`nWrapped directly"
$directArrayWrapper = @(Unroll) #Why is this array[1][3] then?
$directArrayWrapper | % { $_.GetType() }
Write-Host "`nThe original array in elem 0"
$directArrayWrapper[0] | % { $_.GetType() }

Thanks

Why are you using the comma operator in your `Unroll` function? — Yannick Blondeau, Jun 22 '12 at 09:31

score 0 · Answer 1 · edited May 23 '17 at 12:02

0

If you remove the comma operator from the Unroll function, it behaves at you should expect.

function Unroll
{
    return @(1,2,3)
}

In this case $arrayWrapper = @($noWrapper) is the same as $directArrayWrapper = @(Unroll).

You may find more information about array unrolling with this SO question and in this article about array literals In PowerShell

edited May 23 '17 at 12:02

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1
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answered Jun 22 '12 at 11:14

Yannick Blondeau

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1

This is of course true, but you don't always have access to the original code, or cannot change it, as it's e.g. a library. The question was more about why this thing happens and how to have a consistent way of handling it regardless of how exactly the array is returned to you. – PScripter Jun 25 '12 at 22:42

score 0 · Answer 2 · edited May 23 '17 at 10:34

0

Looking at this other question, it seems that

Putting the results (an array) within a grouping expression (or subexpression e.g. $()) makes it eligible again for unrolling.

which seems to be what happen here.

edited May 23 '17 at 10:34

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answered Jun 26 '12 at 13:43

Yannick Blondeau

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Array unrolling in PowerShell

2 Answers2