That code is basically equivalent to running this code in the caller's workspace.
W = who global
You can't use command form when you want to get an output argument from the function that you're calling.
Walter has shown you one way to perform this call. But in general, both evalin and global are discouraged. Can you say a little more about why your function is reaching up into its caller's workspace and why you're using global variables? We may be able to offer some alternatives.
