RE: question about subtyping and type annotations

I agree with Michael Rys's reading.

I don't think that constructor functions should be considered as standard
function calls.

Section 3.12.5 Constructor Functions seems to be pretty clear about it:

[Definition: The constructor function for a given type is used to convert 
instances of other atomic types into the given type. The semantics of the 
constructor function T($arg) are defined to be equivalent to the 
expression ($arg cast as T?).]

- Jerome

www-ql-request@w3.org wrote on 02/24/2006 05:23:54 AM:

> 
> 
> Given Michael Rys' intervention, I think the debate hinges on which of 
the
> following statements takes precedence.
> 
> 3.1.5: the value returned by a function retains its most specific type,
> which may be derived from the declared return type of the function. For
> example, a function that has a declared return type of xs:decimal may in
> fact return a value of dynamic type xs:integer.
> 
> (Or more generally 2.2.3.2: The dynamic type of a value may be more 
specific
> than the static type of the expression that computed it.)
> 
> 3.12.3: cast is supported if the input type is a non-primitive atomic 
type
> that is derived by restriction from the target type. In this case, the 
input
> value is mapped into the value space of the target type, unchanged 
except
> for its type. For example, if shoesize is derived by restriction from
> xs:integer, a value of type shoesize can be cast into the schema type
> xs:integer.
> 
> My reading is that 3.12.3 requires that the result is an instance of
> xs:integer, but 3.1.5 allows that it might also be an instance of 
xs:byte.
> The phrase "unchanged except for its type" does not disallow returning 
the
> type unchanged.
> 
> The reason Saxon behaves this way is that the compiler often generates
> unnecessary casts and then optimizes them away in a subsequent phase of
> analysis. If I weren't allowed to optimize the cast away, I would have 
to
> distinguish an cast created by the compiler from one that was written by 
the
> user.
> 
> Michael Kay
> http://www.saxonica.com/
> 
> 
>

Received on Friday, 24 February 2006 16:22:40 UTC