René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Enable simple detection of [AutomationNull]::Value with -is [AutomationNull]","articleBody":"* Follow-up from #9997, which lacked a clear proposal.\r\n* Related to #11921 (`-is [pscustomobject]`) and #10704 (`-is $null`)\r\n\r\n## Summary of the new feature/enhancement\r\n\r\nEnable the following `-is` test to allow for a PowerShell-idiomatic detection of the `[System.Management.Automation.Internal.AutomationNull]::Value` value that commands that output \"nothing\" technically output.\r\n\r\n```powershell\r\n# `\u0026 {}` outputs \"nothing\", which is technically [System.Management.Automation.Internal.AutomationNull]::Value\r\nPS\u003e \u0026 {} -is [AutomationNull]   # WISHFUL THINKING\r\nTrue\r\n```\r\n\r\nIn _expression_ contexts `[System.Management.Automation.Internal.AutomationNull]::Value` is treated like `$null`, but there are **contexts where the distinction matters**:\r\n\r\nIn the **pipeline**:\r\n\r\n```powershell\r\n# A true $null is sent through the pipeline. \r\nPS\u003e $null  | % { 'here' }\r\nhere\r\n\r\n# [System.Management.Automation.Internal.AutomationNull]::Value is NOT sent through the pipeline\r\n#  - it is an empty enumeration.\r\nPS\u003e \u0026 { }  | % { 'here' }\r\n# NO output\r\n```\r\n\r\nAlso, because the **`switch` statement** treats its operand as an _enumeration_, a `[System.Management.Automation.Internal.AutomationNull]::Value` value causes the statement to be skipped altogether; that is, it is effectively _ignored_:\r\n\r\n``` powershell\r\n$val = \u0026 {}\r\n# Because $val contains [System.Management.Automation.Internal.AutomationNull]::Value,\r\n# the switch statement is effectively ignored;\r\nswitch ($val) {\r\n  default { 'hi' }\r\n}\r\n```\r\n\r\n**Operators** behave inconsistently with AutomationNull as the LHS (see #3866):\r\n\r\n```powershell\r\nPS\u003e [System.Management.Automation.Internal.AutomationNull]::Value -match 'foo'\r\n# NO OUTPUT - LHS was interpreted as *collection*\r\n\r\nPS\u003e [System.Management.Automation.Internal.AutomationNull]::Value -eq 'foo'\r\nFalse # LHS was interpreted as a *scalar*\r\n```\r\n\r\nIn short: **Because `[System.Management.Automation.Internal.AutomationNull]::Value` causes observable differences in behavior from `$null`, it must be discoverable.**\r\n\r\n\r\n## Proposed technical implementation details (optional)\r\n\r\nNote: Strictly speaking, `[System.Management.Automation.Internal.AutomationNull]::Value` is of type `[System.Management.Automation.PSObject]`, but treating it as an instance of `AutomationNull` would be a helpful \"white lie\", similar to the one `-is` already tells when it reports a `[psobject]` wrapper's `.BaseObject`  value's type as the type.\r\n\r\nHowever, @SeeminglyScience's  [proposed implementation here](https://github.com/PowerShell/PowerShell/issues/9997#issuecomment-580743572) would actually resolve this issue, via a new, _public_ `System.Management.Automation.AutomationNull` wrapper, for which an `[AutomationNull]` _type accelerator_ would have to be defined.\r\n\r\n\r\n","author":{"url":"https://github.com/mklement0","@type":"Person","name":"mklement0"},"datePublished":"2020-08-17T20:18:26.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":10},"url":"https://github.com/13465/PowerShell/issues/13465"}