Invoke-Conditional.ps1

# Syntax:
# ?? -If $true -Then { 'yep' } -Else { 'nah' }
# ?? -If $true { 'yep' } : { 'nah' }
# ?? -If $true { 'yep' } else { 'nah' }
# ?? -If $true { 'yep' } { 'nah' }
#
# $true | ?? -Then { 'yep' } -Else { 'nah' }
# $true | ?? { 'yep' } : { 'nah' }
# $true | ?? { 'yep' } else { 'nah' }
# $true | ?? { 'yep' } { 'nah' }
#
# @() | ?? { 'nothin' }
# @() | ?? -Else { 'nothin' }

function Invoke-Conditional {
    [Alias('??')]
    [CmdletBinding(PositionalBinding = $false, DefaultParameterSetName = 'WithDecoration')]
    param(
        [Parameter(Position = 0)]
        [ValidateNotNull()]
        [scriptblock] $Then,

        [Parameter(DontShow, Position = 1, ParameterSetName = 'WithDecoration')]
        [ValidateSet(':', 'else')]
        [string] $Decoration,

        [Parameter(Position = 2, ParameterSetName = 'WithDecoration')]
        [Parameter(Position = 1, ParameterSetName = 'WithoutDecoration')]
        [ValidateNotNull()]
        [scriptblock] $Else,

        [Parameter(ValueFromPipeline)]
        [AllowNull()]
        [AllowEmptyCollection()]
        [psobject] $If
    )
    begin {
        $firstObject = $null
        $hasSetFirstObject = $false
        $pipelineInputCount = 0


        if ($PSBoundParameters.ContainsKey('Then') -and $PSBoundParameters.ContainsKey('Else')) {
            $onFalse = $Else
            $onTrue = $Then
        } elseif ($PSBoundParameters.ContainsKey('Else')) {
            $onFalse = $Else
            $onTrue = $null
        } elseif ($PSBoundParameters.ContainsKey('Then')) {
            $onFalse = $Then
            $onTrue = $null
        } else {
            $exception = [System.Management.Automation.PSArgumentException]::new(
                'At least one action must be specified. Please specify a value for the ' +
                'parameter "Then", "Else", or both and then try the command again')

            $PSCmdlet.ThrowTerminatingError(
                [System.Management.Automation.ErrorRecord]::new(
                    <# exception:     #> $exception,
                    <# errorId:       #> 'NoActionSpecified',
                    <# errorCategory: #> [System.Management.Automation.ErrorCategory]::InvalidArgument,
                    <# targetObject:  #> $null))

            return
        }
    }
    process {
        if (-not $hasSetFirstObject) {
            $firstObject = $If
            $hasSetFirstObject = $true
        }

        $pipelineInputCount++
    }
    end {
        $isTrue = $pipelineInputCount -gt 1 -or ($pipelineInputCount -eq 1 -and $firstObject)
        
        if ($isTrue) {
            if ($null -ne $onTrue) {
                return . $onTrue
            }

            return
        }

        return . $onFalse
    }
}