XLink: behavior must go!
This probably isn't the best week to send this but you can't always choose when inspiration (or religious fervor) will come to you. The message is widely crossposted because it crosses disciplines but I would like to see discussion be restricted to XML-DEV or offline. (I choose XML-DEV over xlxp because more people are subscribed there right now. I believe that the XLink behavioral attributes should be removed. Theoretically they mix presentation and structure. This causes all kinds of practical problems addressed below: Problem: No data model. No processing model. In my experience, the most useless language features are those where the interpretation is completely up to the software. The most useful features are those that either have a well defined underlying data model *or* a well-defined processing model. XML notations are useless not because they are a bad idea but because nobody has defined the data model or processing implications of the associated system identifier. The XLink behavior attributes have this problem. Just what do these behaviors *do*? What do they look like? They take us half-way down the path to inline presentation and arguably that is worse than taking us down that path whole hog. What does it look like to traverse a multi-ended link? How are the anchors represented? Do Netscape and Microsoft get to decide? After working for years to give them pixel-perfect stylesheet languages are designers back to trusting that vendors will give them a reasonable rendition? We could go whole hog and specify exactly what they look like (e.g. popup menus, buttons, hotlinks, etc.) and provide options for styling them. But that would further violate a major tenet of the XML religion: the separation of presentation and structure. Problem: media dependence That this is already a problem can be demonstrated by trying to imagine what it means to do an "auto/replace" in a printed document or "auto/embed" in a command-driven text-based user interface. Problem: stylesheet interoperability if XSL and CSS define to the pixel what the rendition of a document in a graphical browser should look like then what room is there for the browser's automatic anchor display behavior? Can they insert icons or buttons that adjust the flow? Can they override colors specified in the stylesheets? There are dozens of little issues like this where the stylesheet and the built-in linking behavior have the possibility of conflicting. Who wins? What are the rules? Do we have to add a concept like CSS's cascade to XSL? Problem: traversal path the behaviors are attached to *anchors* but the correct behavior probably depends both on the source and target anchors. The XLink specification says: "Link formatting and link behavior are inextricably connected" but then goes on to disconnect them. What will happen when we try to reconnect them? Problem: heresy Even if we could attach behaviors to both the source and target, the document is the *wrong place to do it*. We don't put "FONT" elements in our documents because the right font depends on the context of the viewer. Well the right link behavior also depends on the context of the viewer. Some viewers will have low-bandwidth connections and will want to explicitly initiate embedding. Some viewers will have sophisticated tree browsers an will want to treat what the author considered a "replace" links as an "embed". I have used browsers that treat HTML IMGs as "user/embed" (NS,IE images off), "user/new" (user-invoked helper app), "auto/embed"(NS,IE default) and "auto/new" (auto-invoked helper app). Obviously the right one depends on the situation. Other combinations might also make sense in some situations. I have used user agents that treat HTML "A"s as "user/replace"(typical), "user/embed" (Documentor), "user/new"(www-Emacs), "auto/new" (site downloaders). We've got to move this stuff to stylesheets so that people can make stylesheets that are optimized for various media. Even if we ignore UA-display and bandwidth limitations there are lots of other reasons to have anchors behave differently based on the stylesheet. I think three different site designers and/or end users could all disagree on whether the best way to handle a reference to an item in the glossary is an inline expansion (keeps context), a new window (screen clutter) or a replace. Maybe someone new to the topic would like every glossary entry expanded inline by default. Maybe some people want the glossary entry to pop up when the mouse moves over it. Another example: perhaps one view of some document is for "textual" people (pictures on demand) and another for "visual" people (pictures inline). Problem: incompatible implementations Okay, so to get around the fact that the client will and should do whatever it wants, we could refer to the links behaviors as just "hints". "Hints" are a sure sign that something is wrong in language design. They are usually an excuse for not doing the right thing. In this case, the right thing is to move all behavioral specifications into the stylesheet. Web designers should run screaming from underspecified behavioral attributes of all types. They are still trying to clean up the mess left by the (unavoidable) delay between the creation of HTML: "trust your browser" and CSS: "designers know better than software engineers." The behaviors chosen by browser authors were not always interoperable and then when we tried to "layer" the CSS standard on top it became a total mess because the "built-in" stylesheets of various browsers were different. The only way to get 100% portable rendering is to carefully and completely override the built-in behaviors of the browsers. So what is the use of the hints then? Also, XLink's behavior attribute is a car-wreck waiting to happen. What if Microsoft and Netscape use it differently? Problem: confusion Many, many people want to interpret the auto/embed command as an instruction to embed the nodes at the target at the current location and thus do node reuse. But this interpretation is not supported by the XLink specification. The target of an auto/embed could be a language that has no concept of "nodes" (barring definition of grove property sets) such as a JPEG or MPEG. Furthermore, I've thought about making a transclusion syntax for XML and there are some hard problems that XLink has not solved. For instance what if IDs clash. What happens to pointers and links? etc. The specification also suggests that auto/replace can be used for forwarding. To me this is just further evidence that these attribute are medium-specific presentational tricks. Is AltaVista or Google supposed to recognize an auto/replace as a request to update the entry for a document? The argument that these are "invariant semantics of link types" does not strike me as a good one. There is nothing semantic about "when you click here you go there." That's all about behavior. Problem: non-sensical combinations What if a document has a hundred anchors that all have auto/replace? Should it open a hundred documents in the current window *one at a time*? Problem: complexity Let's radically simplify the XLink specification and *get it out the door*. Taking behaviors out will help. Solution: Take all references to behavior and traversal out of the XLink spec. Move it all into a pair of specification called "Extensions to XSL for Link Rendering" and "Extensions to CSS for Link Rendering." In CSS, the basic model would be that link anchor-handling rules would be processed as part of the cascade. The "ordinary" rule might make some text bold and the "linking" rule would make it blue, underlined and clickable. Some simple rules could also render extended links as popup menus, lists of buttons, lists of icons, etc. In XSL, the model would be that link anchor-handling rules would have a higher priority than ordinary rules (either in general or through manual priority setting). They would specify some details of formatting and behavior (e.g. underline and clickability) and then invoke the rules that would otherwise have applied (e.g. to actually process text and sub-elements). Merging of adjacent clickable links can occur in the back-end. (e.g. <LINK DEST="foo">blah blah blah</LINK><LINK DEST="foo">blah blah blah</LINK> would be rendered as a single link). -- Paul Prescod - ISOGEN Consulting Engineer speaking for only himself http://itrc.uwaterloo.ca/~papresco Diplomatic term: "Emerging Markets" Translation: Poor countries. The great euphemism of the Asian financial meltdown. Investors got much more excited when they thought they could invest in up-and-comers than when they heard they could invest in the Third World.(Brills Content, Apr. 1999) xml-dev: A list for W3C XML Developers. To post, mailto:xml-dev@i... Archived as: http://www.lists.ic.ac.uk/hypermail/xml-dev/ and on CD-ROM/ISBN 981-02-3594-1 To (un)subscribe, mailto:majordomo@i... the following message; (un)subscribe xml-dev To subscribe to the digests, mailto:majordomo@i... the following message; subscribe xml-dev-digest List coordinator, Henry Rzepa (mailto:rzepa@i...)
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