Explicit Return in Powershell

Question

I can write the following code in javascript:

function sum(num1, num2) {
  return num1 + num2;
}

and then get a value

var someNum = sum(2,5);

I would like to do the same thing in Powershell, but I read the following guide:

PowerShell also knows the return keyword; however, it follows a different logic. In general, the purpose of return is to end the execution of a code section and to give the control back to the parent block.

If you add a parameter to the return statement, the value will indeed be returned to the calling subroutine. However, this also applies for all other statements with an output. This means that any output produced in the function will be stored in the variable together with the return parameter.

I want to do this for the sake of having pure functions. However, it seems doing

var someNum = sum(2,5);

is entirely redundant, when I can just call the function above, define someNum inside of it, and it will be available in the global scope.

Am I missing something or is it possible to write pure functions in Powershell that don't return everything inside the function?

A bit tangential, but here is my actual code:

function GetPreviousKeyMD5Hashes() {
    $query = "SELECT Name, MD5, executed FROM [AMagicDb].[dbo].cr_Scripts";

    $command = New-Object System.Data.SQLClient.SQLCommand;
    $command.Connection = $connection;
    $command.CommandText = $query;

    try {
        $reader = $command.ExecuteReader();
        while ($reader.Read()) {
            $key = $reader.GetString(1)
            $previousScripts.Add($key) | Out-Null
        }

        $reader.Close();
        Write-Output "$(Get-Date) Finished querying previous scripts"
    }
    catch {
        $exceptionMessage = $_.Exception.Message;
        Write-Output "$(Get-Date) Error running SQL at with exception $exceptionMessage"
    }
}

and then:

$previousScripts = New-Object Collections.Generic.HashSet[string];
GetPreviousKeyMD5Hashes;

This code isn't clear to me at all - running GetPreviousKeyMD5Hashes does set $previousScripts, but this is entirely unclear to whoever modifies this after me. My only other alternative (afaik) is to have all this in line, which also isn't readable.

mklement0 · Accepted Answer · 2018-08-14T02:58:12.887

is entirely redundant, when I can just call the function above, define someNum inside of it, and it will be available in the global scope.

No: functions execute in a child scope (unless you dot-source them with .), so variables created or assigned to inside a function are local to it.

Am I missing something or is it possible to write pure functions in Powershell that don't return everything inside the function?

Yes: The implicit output behavior only applies to statements whose output is neither captured - $var = ... - nor redirected - ... > foo.txt

If there are statements that happen to produce output that you'd like to discard, use $null = ... or ... > $null

^{Note: ... | Out-Null works in principle too, but will generally perform worse, especially in earlier PowerShell versions - thanks, TheIncorrigible1.}

If there are status messages that you'd like to write without their becoming part of the output, use Write-Host or, preferably Write-Verbose or, in PSv5+, Write-Information, though note that the latter two require opt-in for their output to be visible in the console.
Do NOT use Write-Output to write status messages, as it writes to the success output stream, whose purpose is to output data ("return values").
See this answer of mine for more information about PowerShell's output streams.

The equivalent of your JavaScript code is therefore:

function sum($num1, $num2) {
  Write-Host "Adding $num1 and $num2..."  # print status message to host (console)
  $num1 + $num2 # perform the addition and implicitly output result
}

PS> $someNum = sum 1 2 # NOTE: arguments are whitespace-separated, without (...)
Adding 1 and 2...  # Write-Host output was passed through to console

PS> $someNum  # $someNum captured the success output stream of sum()
3

score 1 · Answer 2 · answered Aug 13 '18 at 19:21

Am I missing something or is it possible to write pure functions in Powershell that don't return everything inside the function?

You can't have your cake and eat it too...

If you have no out put in your function, then it is "pure" like you desire. If you have output, that also becomes part of the return.

You can use [ref] params. See below for example.

function DoStuff([ref]$refObj)
{
    Write-Output "DoStuff: Enter"
    $refObj.Value += $(1 + 2)
    $refObj.Value += "more strings"
    Write-Output "DoStuff: Exit"
}

$refRet = @()
$allRet = DoStuff([ref]$refRet)

"allRet"
$allRet
"refRet"
$refRet

"`n`nagain"
$allRet = DoStuff([ref]$refRet)

"allRet"
$allRet
"refRet"
$refRet

That helps, didn't think of ref params. I will play around with that. — VSO, Aug 13 '18 at 19:40

score 1 · Answer 3 · answered Aug 13 '18 at 19:56

Note: Powershell doesn't need semicolons at the end of each statement; only for separating multiple statements on the same line.

Whenever possible, it's a good idea to avoid changing global state within a function. Pass input as parameters, and return the output, so you aren't tied to using the function in only one way. Your sample could look like this:

function sum
{
    param($num1,$num2)
    return $num1+$num2
}

$somenum=sum 2 5

Now, with Powershell, the return statement isn't needed. The result of every statement that isn't otherwise assigned, captured, redirected, or otherwise used, is just thrown in with the return value. So we could replace the return statement above with simply

$num1+$num2

You're already making use of this in your code with:

$previousScripts.Add($key) | Out-Null

where you are discarding the result of .Add(). Otherwise it would be included in the return value.

Personally, I find using return to explicitly mark the return value makes it easier to read. Powershell's way of putting all if the output in the return caused a lot of trouble for me as I was learning.

So, the only fixes to your code I would make are:

Move $previousScripts = New-Object Collections.Generic.HashSet[string] to inside the function, making it local.

Add return $previousScripts to the end of the function.

Explicit Return in Powershell

3 Answers3