Re: [basex-talk] A question about the arrow operator and predicates
> > As always, thank you so much! > As always, you're welcome! As a follow up, for my layman's thinking about XPath/XQuery, is this >> effectively creating a sequence > > It does. The expression is completely equivalent to, and just another writing for: tokenize($s, "/").
Re: [basex-talk] A question about the arrow operator and predicates
Christian -
As always, thank you so much!
On Wed, Mar 31, 2021 at 10:35 AM Christian Grün
wrote:
> Dear Bridger,
>
> > I was wondering if anyone would have an insight for me as to why the
> following expression is wrong:
> >
> > for $s in ("/a/b/c", "/1/2/3")
> > let $t := $s => tokenize("/")[last()]
> > return $t
>
> This is due to the grammar rules of XQuery 3.1, which mandate that
> “=>” is followed by an “ArrowFunctionSpecifier” and an “ArgumentList”.
> Here are some production rules from the spec:
>
> [96] ArrowExpr ::= UnaryExpr ( "=>" ArrowFunctionSpecifier ArgumentList )*
> [127] ArrowFunctionSpecifier ::= EQName | VarRef | ParenthesizedExpr
> [122] ArgumentList ::= "(" (Argument ("," Argument)*)? ")"
> …
>
> The arrow function specifier can be an EQName, a variable reference or
> a parenthesized expression. The last one will do the job:
>
> for $s in ("/a/b/c", "/1/2/3")
> let $t := ($s => tokenize("/"))[last()]
> return $t
>
> It does - I had read some other online notes about parenthesizing part of
the left hand side expression, but hadn't parenthesized properly.
As a follow up, for my layman's thinking about XPath/XQuery, is this
effectively creating a sequence that is then filtered? Or am I mentally
overloading the parentheses?
Thanks for the insight!
Hope this helps,
> Christian
>
> [1] https://www.w3.org/TR/xquery-31/#id-arrow-operator
>
>
Best,
Bridger
>
>
>
> >
> > Thanks for your help!
> > Best,
> > Bridger
> >
> > PS I don't always remember to look at the optimized query in the Info
> window, but when I do I always get a hint about something; "util:last()" in
> this case.
> >
> > correctly
> > ```
> > for $s in ("/a/b/c", "/1/2/3")
> > let $t := tokenize($s, "/")[last()]
> > return $t
> > ```
> > ```
> > for $s in ("/a/b/c", "/1/2/3")
> > let $t := $s => tokenize("/") => util:last()
> > return $t
> > ```
>
Re: [basex-talk] A question about the arrow operator and predicates
Dear Bridger,
> I was wondering if anyone would have an insight for me as to why the
> following expression is wrong:
>
> for $s in ("/a/b/c", "/1/2/3")
> let $t := $s => tokenize("/")[last()]
> return $t
This is due to the grammar rules of XQuery 3.1, which mandate that
“=>” is followed by an “ArrowFunctionSpecifier” and an “ArgumentList”.
Here are some production rules from the spec:
[96] ArrowExpr ::= UnaryExpr ( "=>" ArrowFunctionSpecifier ArgumentList )*
[127] ArrowFunctionSpecifier ::= EQName | VarRef | ParenthesizedExpr
[122] ArgumentList ::= "(" (Argument ("," Argument)*)? ")"
…
The arrow function specifier can be an EQName, a variable reference or
a parenthesized expression. The last one will do the job:
for $s in ("/a/b/c", "/1/2/3")
let $t := ($s => tokenize("/"))[last()]
return $t
Hope this helps,
Christian
[1] https://www.w3.org/TR/xquery-31/#id-arrow-operator
>
> Thanks for your help!
> Best,
> Bridger
>
> PS I don't always remember to look at the optimized query in the Info window,
> but when I do I always get a hint about something; "util:last()" in this case.
>
> correctly
> ```
> for $s in ("/a/b/c", "/1/2/3")
> let $t := tokenize($s, "/")[last()]
> return $t
> ```
> ```
> for $s in ("/a/b/c", "/1/2/3")
> let $t := $s => tokenize("/") => util:last()
> return $t
> ```
[basex-talk] A question about the arrow operator and predicates
Hi all -
I was wondering if anyone would have an insight for me as to why the
following expression is wrong:
```
for $s in ("/a/b/c", "/1/2/3")
let $t := $s => tokenize("/")[last()]
return $t
```
This is because of the predicate filter, but I'm not clear on *why* it's
because of that :)
Thanks for your help!
Best,
Bridger
PS I don't always remember to look at the optimized query in the Info
window, but when I do I always get a hint about something; "util:last()" in
this case.
correctly
```
for $s in ("/a/b/c", "/1/2/3")
let $t := tokenize($s, "/")[last()]
return $t
```
```
for $s in ("/a/b/c", "/1/2/3")
let $t := $s => tokenize("/") => util:last()
return $t
```
