Re: Bounded-variadic functions and focus-dependence

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
>