Subject: Re: [xsl] Line break algorithm From: "Michael Kay mike@xxxxxxxxxxxx" <xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx> Date: Wed, 6 May 2020 21:37:00 -0000 |
> Saxon does this for templates, but I believe, not for all types of functions. In the past I raised this problem and Dr. Michael Kay said that at the time the XSLT WG didn't mandate recognizing and optimizing tail-recursion, because they didn't have a definition for "tail-recursion". Separate questions here. Firstly, it's true that the spec says nothing about how functions are implemented. This is a tricky area and we hit it a lot with streaming: how do you define language behaviour that is only observable in terms of resource usage, not in terms of functional results? The general rule is that you shouldn't mandate anything unless you know how to write a test for it. Secondly, the question of exactly when templates and functions are tail-recursive in Saxon. The main differences are: * For templates, tail-call optimization applies to mutual recursion, but for functions, it applies only to self-recursion. * For templates, it is permissible to sequence-concatenate the result of the recursive call with other items returned by the template. For functions, any operation performed on the result of the recursive call, including sequence concatenation, means that the call is not regarded as a tail call. * For functions, byte code generation can handle tail call optimization; for templates, it can't. * Type checking and conversion applied to the result of the recursive call can mean it isn't treated as tail recursive. This depends on whether static analysis is able to determine that type checking at run-time isn't needed. Michael Kay Saxonica
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