A question about the type system
Wednesday, 15 April 2015 · 4 messages
KK
Kristian Kankainen15 Apr 2015 14:37 #
Hello!
I'm not sure whether this is a question relevant to BaseX or more generally to the XQuery community. Since I don't use other XQuery systems and it might be specific to the BaseX parser, I post it here first.
Why can't I specify xs:positiveIntegers as literals? Or why can't I treat xs:integers as xs:positiveIntegers? Consider a simple function that takes a xs:positiveInteger as input:
<code>
xquery version "3.0" encoding "UTF-8";
declare namespace local = "local";
declare function local:positive(
$input as xs:positiveInteger
) as xs:positiveInteger
{
$input
};
</code>I get an error XPTY0004 Cannot treat xs:integer as xs:positiveInteger if I run it with a literal input, like this:
<code>
local:positive(2)
</code>I get no error if I specify the type explicitly, like this:
<code>
local:positive(xs:positiveInteger(2))
</code>This feel very odd, since the type system is hierarchical and xs:positiveInteger is a subcase of xs:integer, I don't really see a problem of this kind of automatic casting of integer literals if they qualify the cast.
Can you give any reflection on this?
Best regards
Kristian Kankainen
CG
Christian Grün15 Apr 2015 15:19 #
Hi Kristian,
Quoted text · Kristian Kankainen · 2 lines
> Why can't I specify xs:positiveIntegers as literals? Or why can't I treat > xs:integers as xs:positiveIntegers?
The error is correct. Type handling in XQuery is a topic on its own: Values can be cast, promoted, treated as, etc. in this language. If you want to dive into the details, I suggest having a look at the section on type promotion in the specification:
http://www.w3.org/TR/xquery-31/#id-type-promotion-and-operator-mapping
If you are looking for a practical solution, I advise you to simply use with xs:integer, even if a value will always be positive, or if it will have a smaller bit range. Most operations have been optimized for integers, so you won't lose any time for additional type conversion.
Hope this helps,
Christian
Quoted text · Kristian Kankainen · 35 lines
Consider a simple function that takes a > xs:positiveInteger as input: > > <code> > xquery version "3.0" encoding "UTF-8"; > declare namespace local = "local"; > > declare function local:positive( > $input as xs:positiveInteger > ) as xs:positiveInteger > { > $input > }; > </code> > > I get an error XPTY0004 Cannot treat xs:integer as xs:positiveInteger if I > run it with a literal input, like this: > <code> > local:positive(2) > </code> > > I get no error if I specify the type explicitly, like this: > <code> > local:positive(xs:positiveInteger(2)) > </code> > > This feel very odd, since the type system is hierarchical and > xs:positiveInteger is a subcase of xs:integer, I don't really see a problem > of this kind of automatic casting of integer literals if they qualify the > cast. > > Can you give any reflection on this? > > Best regards > Kristian Kankainen
KK
Kristian Kankainen15 Apr 2015 20:36 #
Hello!
Thank you very much, I will read. What I skimmed now, there exists *subtype substitution*, which would allow xs:positiveIntegers be automaticall cast to xs:integers, which is the opposite direction of my case. There exists also *type propagation*, which of one case allows any xs:decimal or subtype of it be cast into either xs:double or xs:float, which can result in a loss of precision.
I guess my intuition put together, that if it's possible to substitute with subtypes && it's possible to propagate from decimals to floats and doubles, then it ought to be possible to move also along the non-destructive parts of sub and supertypes. Nothing is ideal, atleast I can explicitly cast the return value as xs:positiveInteger.
PS for my further investigations, could you point me to the BaseX code for the pseudo-function named derives-from? Thank you!
Best regards
Kristian
15.04.2015 16:19, Christian Grün kirjutas:
Quoted text · Christian Grün · 55 lines
> Hi Kristian, > >> Why can't I specify xs:positiveIntegers as literals? Or why can't I treat >> xs:integers as xs:positiveIntegers? > The error is correct. Type handling in XQuery is a topic on its own: > Values can be cast, promoted, treated as, etc. in this language. If > you want to dive into the details, I suggest having a look at the > section on type promotion in the specification: > > http://www.w3.org/TR/xquery-31/#id-type-promotion-and-operator-mapping > > If you are looking for a practical solution, I advise you to simply > use with xs:integer, even if a value will always be positive, or if it > will have a smaller bit range. Most operations have been optimized for > integers, so you won't lose any time for additional type conversion. > > Hope this helps, > Christian > > > Consider a simple function that takes a >> xs:positiveInteger as input: >> >> <code> >> xquery version "3.0" encoding "UTF-8"; >> declare namespace local = "local"; >> >> declare function local:positive( >> $input as xs:positiveInteger >> ) as xs:positiveInteger >> { >> $input >> }; >> </code> >> >> I get an error XPTY0004 Cannot treat xs:integer as xs:positiveInteger if I >> run it with a literal input, like this: >> <code> >> local:positive(2) >> </code> >> >> I get no error if I specify the type explicitly, like this: >> <code> >> local:positive(xs:positiveInteger(2)) >> </code> >> >> This feel very odd, since the type system is hierarchical and >> xs:positiveInteger is a subcase of xs:integer, I don't really see a problem >> of this kind of automatic casting of integer literals if they qualify the >> cast. >> >> Can you give any reflection on this? >> >> Best regards >> Kristian Kankainen
CG
Christian Grün15 Apr 2015 20:45 #
Quoted text · Kristian Kankainen · 2 lines
> PS for my further investigations, could you point me to the BaseX code for > the pseudo-function named derives-from? Thank you!
There is no direct relationship between functions in the spec and in BaseX, but you could e.g. have a look at the following function and classes:
https://github.com/BaseXdb/basex/blob/master/basex-core/src/main/java/org/basex/query/value/type/SeqType.java#L438 https://github.com/BaseXdb/basex/blob/master/basex-core/src/main/java/org/basex/query/expr/TypeCheck.java
Hope this helps,
Christian
Quoted text · Kristian Kankainen · 63 lines
> 15.04.2015 16:19, Christian Grün kirjutas: >> >> Hi Kristian, >> >>> Why can't I specify xs:positiveIntegers as literals? Or why can't I treat >>> xs:integers as xs:positiveIntegers? >> >> The error is correct. Type handling in XQuery is a topic on its own: >> Values can be cast, promoted, treated as, etc. in this language. If >> you want to dive into the details, I suggest having a look at the >> section on type promotion in the specification: >> >> http://www.w3.org/TR/xquery-31/#id-type-promotion-and-operator-mapping >> >> If you are looking for a practical solution, I advise you to simply >> use with xs:integer, even if a value will always be positive, or if it >> will have a smaller bit range. Most operations have been optimized for >> integers, so you won't lose any time for additional type conversion. >> >> Hope this helps, >> Christian >> >> >> Consider a simple function that takes a >>> >>> xs:positiveInteger as input: >>> >>> <code> >>> xquery version "3.0" encoding "UTF-8"; >>> declare namespace local = "local"; >>> >>> declare function local:positive( >>> $input as xs:positiveInteger >>> ) as xs:positiveInteger >>> { >>> $input >>> }; >>> </code> >>> >>> I get an error XPTY0004 Cannot treat xs:integer as xs:positiveInteger if >>> I >>> run it with a literal input, like this: >>> <code> >>> local:positive(2) >>> </code> >>> >>> I get no error if I specify the type explicitly, like this: >>> <code> >>> local:positive(xs:positiveInteger(2)) >>> </code> >>> >>> This feel very odd, since the type system is hierarchical and >>> xs:positiveInteger is a subcase of xs:integer, I don't really see a >>> problem >>> of this kind of automatic casting of integer literals if they qualify the >>> cast. >>> >>> Can you give any reflection on this? >>> >>> Best regards >>> Kristian Kankainen > >
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