How to manipulate data type STRING in CDS views?

Hello,

S4 HANA ABAP DDIC tables could be defined with a field of type STRING - for instance data element RSSTRING could as data type in this case...

The question is how to enable an ABAP CDS view to fetch a sub-string of this field?

None of CDS functions like CAST, LEFT, CONCATENATE,... accepts an argument of type STRING...

How to overcome this situation?

Thanks,

Benoit