Problems with multiple multiple-level inclusion of DTDs

[junyang@gmail.com]

Hi all,

I have one DTD fragment, base.dtd, that contains a bunch of useful
element definitions (say an element named "base"), and two DTD
fragments, a.dtd and b.dtd, that each build on base.dtd and defines a
few more elements. I use an entity to include base.dtd in a.dtd and
b.dtd. For example, in a.dtd:

<!ENTITY % include.base SYSTEM "base.dtd">
%include.base
<!ELEMENT a (base+)>

In b.dtd:

<!ENTITY % include.base SYSTEM "base.dtd">
%include.base
<!ELEMENT b (#PCDATA|base)* >

In my XML documents, if I just want to use element a (or b), I simply
need to entity-include a.dtd, and that will automatically include
base.dtd, which is nice.

However, the trouble comes in when I entity-include both a.dtd and
b.dtd. The problem is that the element "base" will be included twice
(once via a.dtd->base.dtd and another time via b.dtd->base.dtd). Note
that multiple definitions of the "include.ba se" entity is fine (as per
XML standard), but multiple definitions of any element is disallowed.

I explored a few workarounds but couldn't find any clean solution (for
example, something that resembles C/C++'s #ifndef and #define). Any
suggestions will be appreciated!

--- Jun Y.

[Peter Flynn]

Re: Problems with multiple multiple-level inclusion of DTDs

junyang@gmail.c om wrote:

Hi all,
>
I have one DTD fragment, base.dtd, that contains a bunch of useful
element definitions (say an element named "base"), and two DTD
fragments, a.dtd and b.dtd, that each build on base.dtd and defines a
few more elements. I use an entity to include base.dtd in a.dtd and
b.dtd. For example, in a.dtd:
>
<!ENTITY % include.base SYSTEM "base.dtd">
%include.base
<!ELEMENT a (base+)>
>
In b.dtd:
>
<!ENTITY % include.base SYSTEM "base.dtd">
%include.base
<!ELEMENT b (#PCDATA|base)* >
>
In my XML documents, if I just want to use element a (or b), I simply
need to entity-include a.dtd, and that will automatically include
base.dtd, which is nice.
>
However, the trouble comes in when I entity-include both a.dtd and
b.dtd. The problem is that the element "base" will be included twice
(once via a.dtd->base.dtd and another time via b.dtd->base.dtd). Note
that multiple definitions of the "include.ba se" entity is fine (as per
XML standard), but multiple definitions of any element is disallowed.

The technique is to use parameter-entity switches.

In a.dtd:

<!ENTITY % use.base.from.a "INCLUDE">
<![%use.base.from. a;[
%include.base;
<!ENTITY % use.base.from.b "IGNORE">
]]>

In b.dtd

<!ENTITY % use.base.from.b "INCLUDE">
<![%use.base.from. b;[
%include.base;
<!ENTITY % use.base.from.a "IGNORE">
]]>

This way, whichever one you invoke first will prevent the other one from
being activated.

Better would be to paramaterise a and b alone, and invoke your base in
your document type declaration with a PE switch to include whichever or
a and b (or both) that you require for the instance.

///Peter

[junyang@gmail.com]

Re: Problems with multiple multiple-level inclusion of DTDs

On Jul 26, 4:40 pm, Peter Flynn <peter.n...@m.s ilmaril.iewrote :

>
The technique is to use parameter-entity switches.
>
In a.dtd:
>
<!ENTITY % use.base.from.a "INCLUDE">
<![%use.base.from. a;[
%include.base;
<!ENTITY % use.base.from.b "IGNORE">
]]>
>
In b.dtd
>
<!ENTITY % use.base.from.b "INCLUDE">
<![%use.base.from. b;[
%include.base;
<!ENTITY % use.base.from.a "IGNORE">
]]>
>
This way, whichever one you invoke first will prevent the other one from
being activated.

This would work, except that now a.dtd would need to know about b.dtd,
and vice versa, which is difficult to keep manage if there are many
potential includers of base.dtd...

Better would be to paramaterise a and b alone, and invoke your base in
your document type declaration with a PE switch to include whichever or
a and b (or both) that you require for the instance.

This is what I am doing for now, though users of a.dtd and b.dtd have
to manually manage inclusion.

Another possibility I thought of is to have an exclude-base-
includers.dtd file that sets all PE switches to false. This file would
be included by base.dtd. Each includer of base.dtd would set the its
own PE switch to true, and then include base.dtd. End users are hidden
from all this mess, but the downside of this approach is that the
exclude-base-includers.dtd file needs to be maintained.

--- Jun Y.

[Peter Flynn]

Re: Problems with multiple multiple-level inclusion of DTDs

junyang@gmail.c om wrote:

On Jul 26, 4:40 pm, Peter Flynn <peter.n...@m.s ilmaril.iewrote :

[snip]

>Better would be to paramaterise a and b alone, and invoke your base in
>your document type declaration with a PE switch to include whichever or
>a and b (or both) that you require for the instance.

>
This is what I am doing for now, though users of a.dtd and b.dtd have
to manually manage inclusion.
>
Another possibility I thought of is to have an exclude-base-
includers.dtd file that sets all PE switches to false. This file would
be included by base.dtd. Each includer of base.dtd would set the its
own PE switch to true, and then include base.dtd. End users are hidden
from all this mess, but the downside of this approach is that the
exclude-base-includers.dtd file needs to be maintained.

It's usually far easier to manage if you make base.dtd the external
entity referenced by the DOCTYPE Declaration, and let users add their
own selection of a, b, ... using a PE in the internal subset.

<!DOCTYPE whatever SYSTEM "/some/uri/for/base.dtd" [
<!ENTITY % a "INCLUDE">
]>

and do all your entity management and file inclusion in base.dtd, a.ent,
b.ent, or whatever.

///Peter

[junyang@gmail.com]

Re: Problems with multiple multiple-level inclusion of DTDs

On Jul 27, 4:14 pm, Peter Flynn <peter.n...@m.s ilmaril.iewrote :

It's usually far easier to manage if you make base.dtd the external
entity referenced by the DOCTYPE Declaration, and let users add their
own selection of a, b, ... using a PE in the internal subset.
>
<!DOCTYPE whatever SYSTEM "/some/uri/for/base.dtd" [
<!ENTITY % a "INCLUDE">
]>
>
and do all your entity management and file inclusion in base.dtd, a.ent,
b.ent, or whatever.

Thanks! This is much cleaner than the exclude-base-includers.dtd
trick I thought of earlier.

--- Jun Y.