嵌套函数调用Powershell

Question

我在PowerShell中探索函数式编程，而且我在执行嵌套函数时遇到问题：

function Get-ThreadsafeMessageAction {
    param(
        [System.Threading.Mutex] $mutex,
        $action)
        $mtx = $mutex
        $act = $action
    return { param([string] $message)
        $mtx.WaitOne()
        &$act $message
        $mtx.ReleaseMutex()}
}

错误：

The expression after '&' in a pipeline element produced an object that was not valid. 
It must result in a command name, a script block, or a CommandInfo object.
At C:\Users\username\Desktop\myscript.ps1:21 char:52
+         Invoke-Command -ScriptBlock {param($act) & $act $message } -A ...
+                                                    ~~~~
    + CategoryInfo          : InvalidOperation: (:) [], RuntimeException
    + FullyQualifiedErrorId : BadExpression