[XML-DEV Mailing List Archive Home] [By Thread] [By Date] [Recent Entries] [Reply To This Message]

Re: [XQuery] FLWOR (for-let-where-order-return) equivalent to

• To: Michael Kay <mike@s...>
• Subject: Re: [XQuery] FLWOR (for-let-where-order-return) equivalent to FOR (for-order-return)?
• From: Daniela Florescu <dflorescu@m...>
• Date: Sun, 5 Dec 2004 22:40:52 -0800
• Cc: xml-dev@l...
• In-reply-to: <E1Cb4P6-0000t8-00@u...>
• References: <E1Cb4P6-0000t8-00@u...>

> The let clause is also redundant because references to the
> variable can be replaced by the expression to which the variable is 
> bound.

  I am not sure I agree Michael.

To be strictly formal,  LET is not syntactic sugar in XQuery.
  It is syntactic sugar in most pure functional programming languages 
(i.e. can be
eliminated by the rule that you cite) but not in XQuery, due to at least
two independent reasons:
(a) side effects as result of node constructors and
(b) context-sensitive semantics (e.g. namespaces and unordered{..})

Here are a couple of counter-examples.

(a) side effects as result of node constructors

let $x := <a/>
return fn:count(($x,$x)/.)

returns 1

while

fn:count((<a/>, <a/>)/.)


returns 2

(b) context sensitive semantics due to unordered {...}

let $x := (fn:doc("foo.xml")//a[@b=3])[1]
return unordered {  $x }

is not equivalent to

unordered {(fn:doc("foo.xml")//a[@b=3])[1]}

The first one is deterministic, the second one not.

But in reality few programmers will realize those small subtle 
differences.
Moreover, a good compiler should be able to detect the cases where
the rule you cite is applicable, and the apply it only under those 
circumstances.

Best regards,
Dana

P.S> It is late in the night and I have no XQuery processor at hand to 
run and
  check those examples  but I hope I got them right. I am sure you know 
what I
mean anyway.

• Follow-Ups:
  • RE: [XQuery] FLWOR (for-let-where-order-return) equivalent to FOR (for-order-return)?
    • From: "Michael Kay" <mike@s...>

• References:
  • RE: [XQuery] FLWOR (for-let-where-order-return) equivalent to FOR (for-order-return)?
    • From: "Michael Kay" <mike@s...>

• Prev by Date: RE: The XML Backlash
• Next by Date: Schema: decimal data type with blank value
• Previous by thread: RE: [XQuery] FLWOR (for-let-where-order-return) equivalent to FOR (for-order-return)?
• Next by thread: RE: [XQuery] FLWOR (for-let-where-order-return) equivalent to FOR (for-order-return)?
• Index(es):
  • Date
  • Thread