[plt-scheme] typed scheme vs. PLAI types

> 
> 
> you don't add the tags to turn Relation into RelExpr. This work:
> 
> (eval-relexpr (make-Union (make-Rel suppliers_1-3) (make-Rel suppliers_4-5)))
> 
> Note the use of make-Rel.


Ah, of course, I overlooked that... Thanks a lot!

Still, I don't understand in which cases I have to annotate an expression?
Also, when I try annotating (although in the following case it's not necessary it seems, as suppliers_1-3 is constructed as a Relation) like this: 

(ann suppliers_1_3 Relation)

I get the error

. Type Checker: Unbound type name Relation in: Relation

which I don't understand.

Thanks a lot again,
Sigrid








> 
> HTH,
> N.