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Idempotent extension functions

Added by Anonymous over 12 years ago

Legacy ID: #10497161 Legacy Poster: Paul Duffin (paulduffin)

I could not find any examples of implementing an ExtensionFunctionDefinition/Call in the provided samples which could have been useful but in the end it was quite easy to get something working. However, I think I may be missing something. I have a function which depends on some externally provided context information (passed in as a parameter), a set of supported features (which are just strings) but if it is called with the same set of strings it will always return the same value, true if they are all supported false otherwise. I have created my extensions of ExtensionFunctionDefinition and ExtensionFunctionCall and in the former I have said that it has no side effects and does not depend on the focus. I then have an example stylesheet that uses that function and calls it in a number of places passing in the same reference to the variable that provides the context as well as the same set of feature strings as literals. I expected the function call to be optimized automatically so that it was only called once (which ever use was processed first) and from then on all other usages would simply return the same value. However, when I tried it and debugged my code I found that the function was being called each time it was processed. is that expected, i.e. a limitation of the existing product, or is it something that I am doing wrong, or a bug. BTW. I tried to add another topic (which was quite long) and was told that it was caught in the SPAM NET. Does that mean it is lost forever or can someone free it somehow? I tried looking for help online but could not find anything about a SPAM NET on the forum topics.


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RE: Idempotent extension functions - Added by Anonymous over 12 years ago

Legacy ID: #10497796 Legacy Poster: Paul Duffin (paulduffin)

Implementation should not be difficult. First, the style sheet compiler would need to identify those function call instances that are idempotent and the same, i.e. have the same definition, which is marked as idempotent (assuming existing focus and side effect options are not enough), and have the same set of literal parameters. Second, for each set of function calls it would need to allocate a slot in an array and replace the function calls with a special call that first checks the slot (in the Transformer) to see if the value has been set, if it has then return it, otherwise call the function and store the returned value in the slot (in the Transformer). Third, each Transformer would need to allocate an array for all the slots needed by its creating Templates.

RE: Idempotent extension functions - Added by Anonymous over 12 years ago

Legacy ID: #10498462 Legacy Poster: Michael Kay (mhkay)

Saxon doesn't implement functions as memo functions by default because it doesn't have the information needed to get the space/time trade-off right. In many cases keeping the results of previous function calls would just be a waste of memory. You can cause your function to behave as a memo function by calling it from within an XSLT wrapper function defined using <xsl:function saxon:memo-function="yes">. (If you do this, all results from the function are saved in memory; there's no LRU cache.) I'm afraid I don't know what happens to messages that SourceForge identifies as spam. On the whole the site seems to do quite a good job. The documentation for integrated extension functions will improve in the next release.

RE: Idempotent extension functions - Added by Anonymous over 12 years ago

Legacy ID: #10501464 Legacy Poster: Paul Duffin (paulduffin)

Ah, ok I did not realise that about the xsl:function. Could you not do the same thing for extension functions, i.e. add a "boolean memoize()" method to ExtensionFunctionDefinition, returns false by default but if you return true it behaves like an xsl:function with saxon:memo-function="yes".

RE: Idempotent extension functions - Added by Anonymous over 12 years ago

Legacy ID: #10501587 Legacy Poster: Michael Kay (mhkay)

Sure, it could be done in theory. But I'm not convinced it's worth the effort.

RE: Idempotent extension functions - Added by Anonymous over 12 years ago

Legacy ID: #10502816 Legacy Poster: Paul Duffin (paulduffin)

Got a bit side tracked, you answered my question, Saxon does not currently support this so I wasn't doing anything wrong. Thanks.

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