Sorry, Skip, my previous reply was not properly edited...
<post> That said, the error you're getting is because
> USER>w ##class(%ListOfDataTypes).LogicalToDisplay(olist)
this method expects a $LIST, not an instance of %ListOfDataTypes.
The logical representation of a data is simply its $H format. That's how it gets stored in a global. But lists of data types are stored as a nested $LIST structures (nested within the object that references them).
It seems to be you can get the underlying list of the OREF by calling %GetSerial, e.g.
USER>W ##class(%ListOfDataTypes).LogicalToDisplay(olist.%GetSerial()) </post>
That's more like it :)
