Re: The map-lookup-forgiveness problem

@Michael Kay <mike@saxonica.com>  I understand this reasoning.

But on the other hand, this is a real use-case submitted by a real user
almost two years ago. They are maybe hoping to get our understanding...

And a chain of map lookups using "?" is really similar to an XPath
path-expression using "/".  One could also object to chaining an XPath
expression with "/" and not reporting errors -- this is equally difficult
to debug as its maps counterpart. But using path expressions the way they
are is really convenient, and using chained map-lookup expressions will
feel the same to XPath users.

We are also thinking about "deep search" inside maps and arrays, and
probably solving the "map forgiveness" problem could be a good step forward,

Thanks,
Dimitre

On Tue, Feb 7, 2023 at 9:29 AM Michael Kay <mike@saxonica.com> wrote:

> "?" is modelled on "/", and "/" requires the LHS to deliver nodes, so this
> would be an inconsistency.
>
> The language is already very inconsistent about whether it's a good idea
> to widen the domain of a function or operator to embrace items that can
> only deliver a trivial result for the operation. For example we widen the
> domain of name() to allow text nodes, but not to allow strings. The reason
> we're inconsistent is that there are benefits in doing it, and there are
> also benefits (type checking) in not doing it. Personally I think that a
> void result, with no error, is one of the hardest things to debug, and as a
> general rule I think it's a good idea to make it a type error to apply any
> operation to inappropriate operands. But there will always be arguments
> both ways.
>
> Michael Kay
> Saxonica
>
> On 7 Feb 2023, at 17:06, Dimitre Novatchev <dnovatchev@gmail.com> wrote:
>
> At today's meeting Michael Kay asked for a detailed use-case for map
> returning a map with no keys as a result of a failed lookup.
>
> Here is this described in https://github.com/qt4cg/qtspecs/issues/105  "[XPath]
> Proposal: Maps with Infinite Number of Keys: Total Maps and Decorated maps
> #105"
>
>
> Using this representation of the empty sequence, we can provide a solution
> for the *"Forgiveness problem"
> <https://xmlcom.slack.com/archives/C011NLXE4DU/p1616167871037100>* raised
> by Jarno Jelovirta in the XML.Com #general channel in *March 2021*:
>
> This expression will raise an error:
>
> [map {"k0": 1}, map{"k0": [1, 2, 3]}]?*?("k0")?*
>
> *[XPTY0004] Input of lookup operator must be map or array: 1.*
>
> To prevent ("forgive", thus "Forgiveness Problem") the raising of such
> errors we could accept the rule that in XPath 4.0 any expression that
> evaluates to something different than a map or an array, could be coerced
> to the following map, which returns the empty sequence as the corresponding
> value for any key requested in a lookup:
>
> map {'\' : ()
> }  (: produces the empty sequence  for any lookup:)
>
>
> Thanks,
> Dimitre
>
>
>