Re: Bounded-variadic functions and focus-dependence

Sorry for this response, it was on the wrong thread.

I apologize,
Christophe

Le 13/12/2020 à 00:57, Christophe Marchand a écrit :
>
> And the groundhog...
>
> Le 12/12/2020 à 22:55, Michael Kay a écrit :
>> Indeed, it's an open question what the context for evaluation of a 
>> default parameter value expression should be.
>>
>> We've started implementing core functions in XSLT (for portability), 
>> so we would like to eliminate the situation where core functions can 
>> depend on the context in a way that is not allowed for user-defined 
>> functions. A common case is an argument whose default value depends 
>> on the default collation in the static context of the caller.
>>
>> A simple rule that default value expressions are evaluated in the 
>> context of the caller doesn't work, however: we wouldn't want to 
>> allow it to reference local variables in the caller's context, or 
>> (thinking XSLT packages) private functions in the calling package.
>>
>> Michael Kay
>> Saxonica
>>
>>> On 12 Dec 2020, at 11:33, Reece Dunn <msclrhd@googlemail.com 
>>> <mailto:msclrhd@googlemail.com>> wrote:
>>>
>>> A ·focus-dependent· function is a function that accepts the current 
>>> path focus (context item `.`) as a parameter to the function. 
>>> Currently this is only supported for built-in functions, but could 
>>> be useful for user-defined functions.
>>>
>>> Bounded-variadic functions offer a way of doing this succinctly 
>>> without any extension to the existing syntax. The logic would be:
>>>
>>> > If an argument has a default expression equal to the context item, 
>>> or an expression using the context item (e.g. `./test`), the context 
>>> item is evaluated at the point at which that argument is evaluated 
>>> (either from a function call, or a partial function application).
>>>
>>> This would allow the Functions and Operators specification to use 
>>> this to define the ·focus-dependent· functions like:
>>>
>>> |    fn:string|(|$arg|| as ||item()? := .|)| as ||xs:string|
>>>
>>> Note #1: I'm not restricting this to just the context item to 
>>> support things like:
>>>
>>>     declare function to-xsdate($year := ./year, $month := ./month, 
>>> $day := ./day) {};
>>>
>>> so the following is possible:
>>>
>>>     to-xsdate(2020, 12, 10), $doc//date/to-xsdate()
>>>
>>> Note #2: I think it makes sense to have the static context bind at 
>>> the function declaration, and the dynamic context bind at the call 
>>> site (where the argument is passed to the parameter).
>>>
>>> Note #3: I'm not sure whether defaults that depend on parameters 
>>> declared earlier should be valid or not. For example:
>>>
>>>     declare function f($a, $b := $a * 2) {};
>>>
>>> Kind regards,
>>> Reece
>>