Actually we do need expressions that return values other than booleans for
the presentation model (e.g. for constraints), but the fact is this doesn't
appear in the mrl file (e.g. the position of a guide is declared as float
even though it can store an expression that returns a float), so no need for
expression types here.
On the other hand, void procedures, as in:
> > attribute init_procedure : Expression; // Here type is None
> > // stands for void.
can be forced to return a boolean value.
So, we could do without the <...> types for now, but I'm not sure we won't
need it eventually, i.e. need attributes that store procedures that return,
say a guide, or an application data.
Pablo