[Xpath-ng] List constructor syntax

[Uche Ogbuji]

> Hi,
> 
> If we should decide to go for lists with arbitrary content, including other
> lists,
> where singleton lists are not treated specially, we have to think a bit
> about the list constructor syntax.
> 
> XPath 2.0 uses a comma separated list of items, often enclosed in
> parenthesis to create a sequence. The problem is how to create a list with
> only one item. Since the comma operator is an infix operator, it requires
> at least two operands. In XPath 2.0 this is no problem, since only flat lists
> are
> allowed, and an item is identicall to it's corresponding singleton sequence,
> so there's little need to create singleton sequences (and you can by '(1,
> ())').
> But if lists are just lists with no magical behavior (the way I want them)
> you'll
> want the ability to create singleton lists. Here are a couple of possibilities:
> 
> Have a list function which takes zero or more arguments:
>   list (1, 17) -- list with two items
>   list (17) -- list with one item
>   list () -- empty list
> 
>   This solution could be used alone or as a complement to the comma operator
> solution.
> 
> Use square brackets or braces
>   [1, 17] or {1, 17}
>   [17] or {17}
>   [] or {}
> 
> Use parenthesis prefixed by a special character e.g. $
>   $(1, 17)
>   $(17)
>   $()
> 
>   A variant of this and the function solution would be to
>   name the function 'L' as in L(1, 17) and thus decrese the verbosity,
>   but on the other hand it would be inconsistent with other core
>   functions, which start with a lowercase character.
> 
> My gut feeling is that the first list function solution is the cleanest and
> simplest.
> I could live with the square bracket version too, but I believe many will
> consider it
> too close to the filter syntax.
>
> Any other ideas?

Python uses an extra comma to disambiguate.  i.e.

(1,2) is a tuple of length 2
(1) is an expression equivalent to "1"
(1,) is a tuple of length 1
(1,2,) is also a tuple of length 2  (i.e. the magic is general case)


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Uche Ogbuji                                    Fourthought, Inc.
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