David - I've run across them in a somewhat obscure context, I guess.
I've written an XQuery processor that rewrites queries originally parsed
by Saxon, basically serializing them in the process. All I can tell you
is that Saxon generates an internal expression (ItemChecker) that is
documented as corresponding to "treat as" in some cases where there was
no "treat as" in the original supplied expression.
I think this happens, eg, if you declare a variable with a certain
type (so the type will be checked dynamically when the assignment is
done), but later references to the variable's value can be wrapped in
a "treat as" so that the compiler can assume that the variable has the
declared type and perform suitable automatic type inferencing. But
that's just a guess :)
-Mike
"treat as" is basically a dynamic type check. Saxon does optimistic
static typing, which basically means that it automatically inserts
"treat as" expressions into the expression tree whenever it finds a
construct that isn't statically type safe. So if you write
$a/child::node()
and the type of $a isn't statically known, Saxon rewrites it as
($a treat as node()*)/child::node()
which is what you would have to write yourself in a system with
pessimistic type checking (either that, or you would add a type
declaration to the declaration of variable $a).
Michael Kay
Saxonica
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