Re: Constructor functions & `cast as` -- why both?

On 7/5/05, Frans Englich <frans.englich@xxxxxxxxx> wrote:
>
> Hi,
>
> According to the XPath specifications are constructor functions and the
cast
> expression defined to be semantically equivalent. I wonder, why then
provide
> both?
>
> Here my speculation:
>
> * The two uses different default namespaces. Hence, it can be practical on
the
> stylesheet-writing level. But this aspect is not more than convenience,
> right?
>
> * A usability aspect of the language. One might argue that multiple ways of
> expressing the same thing allows users to choose their way which fits them
> the best(a psychological aspect). _For example_, the `cast as` expression
can
> be percepted as changing the type of an existing value, while a constructor
> function can be percepted as creating a new value.
>
> What was the reasoning for creating the two?


A "cast as" expression is a synonym for using a constructor function,
except in two cases, where a constructor function cannot be used:

  - If the "?" must be used after the atomic type name to express that
an empty sequence is allowed.

  - If there is no target namespace for the schema defining the type
(the type is in no namespace) -- in this case a constructor function
with no namespace prefix will bind to the default namespace of the
core functions (F & O).


BTW, this is the second question you ask, the perfect answer to which
can be easily found in Mike Kay's book "XPath 2.0 Programmer's
Reference" -- ISBN: 0-764-56910-4


Cheers,
Dimitre Novatchev