Re: mapping (Was: Re: [xsl] Re: . in for)

[Joerg Pietschmann <joerg.pietschmann@xxxxxx>]

Jeni Tennison <jeni@xxxxxxxxxxxxxxxx> wrote:
> Function names are indistinguishable from element names,
[...]
Well, i realized it somewhat too late as you have seen in my
follow-up...

> You could do:
>   map('xf:lower-case', $departments)
> but of course this will only work with functions (rather than any
> expression), and only with functions whose sole argument was an item.

I was thinking about the equivalent of C function pointers. It may
be inconvenient to wrap simple expressions in functions for the sole
purpose of applying them to a map, but it seems to work for a
reasonable large audience (see below for caveat). However:

> ... rather than any expression ...

This means you want lambda expressions.

>  - define a map expression (rather than a map() function)

Note that in you proposal of a mapping operator
  $coordinates -> (. * 2)
the second operand actually *is* already a lambda expression. Hint:
the . is not bound to the value of the context node as it would
have been in ordinary expressions.
We can argue about inventing an XPath function xf:lambda() for this
purpose, with all the consequences. I'd probably like it to have one,
but i let it to Dmitre to make up the full proposal... :-)

Well, the C++ STL got around not having lambdas using the template
expressions technique. Is there anyone who could to an analysis whether
- C++ template expressions are equivalent to lambdas and
- there is a way to do something similar in XSLT?

> But as Dimitre's explained, if we just had functions as objects in the
> data model we wouldn't be banging our heads against this - we could
> have a map function no problems at all.

I some sense, we already have, just somewhat restricted, in form of
"for" and similar operators. With full lambdas, we could get rid of them.
IMHO this is a laudable goal.

Regards
J.Pietschmann

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