Focus for RHS of thin arrow expressions

[michaelhkay]

We define A -> F(B, C) as being equivalent to A ! F(., B, C) which means that B and C are evaluated with a focus based on the current item in A, not with the outer focus. This is different from the => operator. For example if the $E is an element E, with several children called F, then

namespace-uri(.) -> fn:QName(name())

has a different effect from

namespace-uri(.) => fn:QName(name())

whereas it might reasonably be expected that in the case where the LHS produces a single value, the two operators are equivalent. We can't change the meaning of => because it's defined in 3.1. So should we change the meaning of -> to fall into line?

We could do this easily enough by defining A -> F(B, C) as equivalent to for $a in A return F($a, B, C). I think that as well as being more consistent with =>, the result is probably more intuitive. (We could also define it as equivalent to let $f := F(?, B, C) return A ! $f(.))

For the expression A -> {B}, and for the proposed A => {B}, I don't think we have any choice other than evaluating B with an inner focus based on A. But at least we can do it consistently for both operators.

[ChristianGruen]

A good observation! I fully agree, the for loop is more plausible.

[michaelhkay]

This change has been implemented as a consequence of the revamp of -> to =!>.