Re: [xml-dev] Do not use the same tag name for different things ...Do yo

Hi Roger,

> Question: Beside violating the DRY principle, are there other reasons to avoid using the same tag name for different things?

If you mean having the same name() (the XPath function), then yes, this is to be avoided.

However, it is OK to have elements with the same local-name() that are in different namespaces:

restaurant:table, sql:table, word:table, furniture:table, html:table

architecture:column, sql:column, table:column, army:column

css:style, architecture:style, fashion:style

language:greek, student-life:greek

calendar:march, army:march, ..., etc

This is one of the main reasons why we have and use namespaces.

Thanks,

Dimitre

On Wed, Jan 19, 2022 at 10:53 AM Roger L Costello <costello@mitre.org> wrote:

Hi Folks,

Consider this XML document:

<Items>
<Aquarium>
<Tank>40 gallon capacity</Tank>
</Aquarium>
<Abrams>
<Tank>M-1A2C</Tank>
</Abrams>
</Items>

Notice that there are two <Tank> elements. They have completely different meanings: the first is a tank that we put fish in, the second is a tank that is used in military battles.

Same tag name, totally different meanings.

Are you okay with that? That is, are you okay with using the same tag name for two different things?

Below I make this argument: Never use the same tag name for two different things.

Here's why:

Recall the DRY (Don't Repeat Yourself) principle. Consider an XML Schema for the above XML. Assume the type for the aquarium tank is different than the type for the military tank:

<xs:element name="Title" type="AquariumTankType"/>
<xs:element name="Title" type="MilitaryTankType"/>

The DRY principle says: Declare once, ref often. That is, in the XML Schema globally declare the Title element once and ref to it multiple times. But we can't declare Title only once since they have different types. There must be at least one Title element that is locally declared. The DRY principle must be violated. That is bad design.

Question: Beside violating the DRY principle, are there other reasons to avoid using the same tag name for different things?

Here's a better design:

<Items>
<Aquarium>
<Tank>40 gallon capacity</Tank>
</Aquarium>
<Abrams>
<BattleTank>M-1A2C</BattleTank>
</Abrams>
</Items>

Now an XML Schema can declare Tank and BattleTank globally and ref to them. DRY is intact.

Question: do you agree that XML documents should not use the same tag name for different things?

/Roger

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Cheers,

Dimitre Novatchev

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