Strange behavior of the function

[eosfor]

Function returns nothing but should return an empty list

[mklement0]

By default, function outputs are enumerated if they are collections, and enumerating an empty list yields "nothing" (more precisely, a special "null collection" value, [System.Management.Automation.Internal.AutomationNull]::Value).

In order to output your list as-is, as a single object, wrap it in an auxiliary array:

function foo { 
    $ret = [System.Collections.Generic.List[long]]::new()
    , $ret  # construct a single-element wrapper array around the list
}

$k = foo
$k.GetType().Name

The above yields List`1, proving that the list was returned.
The aux. wrapper array was enumerated, outputting just the list.

Do note that the conceptually cleaner approach would be to use Write-Output -NoEnumerate, but Write-Output is currently broken - see #5955

function foo { 
    $ret = [System.Collections.Generic.List[long]]::new()
    # This SHOULD work, but is currently BROKEN - see #5955 
    Write-Output -NoEnumerate $ret
}

As of PowerShell Core v6.0.2, the above yields an empty [psobject[]] array in lieu of the list.

[eosfor]

Hm, ok, seems reasonable. But to me using either one is kind of unexpected. I'd prefer it to return a value, even it is is an empty collection.

[mklement0]

@eosfor:

Your being surprised is understandable if you're used to function semantics from other languages, but in PowerShell, functions integrate tightly with the pipeline, a salient feature of which is implicit enumeration of collections.

Thus, unlike in other languages, you have to opt out of this automatic enumeration.

Note that expressions, by contrast, are not automatically enumerated:

$k = [System.Collections.Generic.List[long]]::new()
$k.GetType().Name

The above yields List`1; that is, no enumeration was performed.

[eosfor]

Yes, i agree. I know about this feature. However when you create a collection and expect it to be returned as is, wrapping it into a single-value array is, kind of, counterintuitive, IMHO.

I'd say, the solution you proposed in #5955 is a good idea.

[mklement0]

@eosfor:

Yes, the wrapper-array solution is obscure (though conveniently short).

Note that #5955 describes a regression bug. That is, Write-Output -NoEnumerate has always worked (and has always been the conceptually cleaner approach) - and still does in Windows PowerShell v5.1.