Introduce a new conceptual topic that explains to-Boolean coercion logic ("truthiness")

[mklement0]

PowerShell can treat any type implicitly as a Boolean, which is generally very handy, but can be problematic if the rules aren't clear.

The rules are summarized below, and I hope they'll become the basis for a new conceptual topic such as about_Booleans.

The following topics should then be updated to point to the new topic:

• about_If
• about_While
• about_Do
• about_Arrays (the section about the .Where() method)
• Where-Object
• Others?

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To-Boolean conversion summary:

• Among scalars:

  • The following are implicitly $false:

    • ''/"" (empty string)
    • $null
    • 0 (of any numeric type).

  • Pitfall: Any non-empty string evaluates to $true

    • e.g., [bool] 'False' is $true
    • Note that this differs from explicit string parsing: [bool]::Parse('false') does return$false (and $true for 'true', but recognizes nothing else).

  • Instances of any other (non-collection) type are implicitly $true, including of type [pscustomobject].

• Among collections such as arrays (more accurately, collection-like types that implement the IList interface - see the source code):
  • Empty collections are always $false, as is the special "null collection" value indicating the absence of output from a command, [System.Management.Automation.Internal.AutomationNull]::Value.
  • Pitfall: Single-element collections evaluate to the Boolean value of their one and only element.
  • 2+-element collections are always $true.

[sdwheeler]

NOTE: Type conversion is covered in detail in Chapter 6 of the PowerShell Language Specification. It may be best to just convert that chapter to docs.