[xquery-talk] :=

[Ghislain Fourny]

Hi Benito,

Yes, that's true. The semantics for scripting is different even though the
same symbol is used. This is mostly the reason why I distinguish between
binding and assignment for pedagogical purposes, although I do agree with
Mike's point that in the end understanding the semantics of a language
matters more than the terminology used.

In some scripting extensions, Zorba's for example, assignments are
statements, and mean that there is a change of snapshot following a side
effect (actually changing the value of a variable), and this side effect is
subsequently visible everywhere else in the program (for example through a
global variable).

This is different from a binding, in that binding a value to a variable
merely organizes dynamic contexts in various scopes in which other
expressions are evaluated.

I visualize the difference like so: side effects due to assignments
propagate linearly, left-to-right, in time, whereas variable bindings
propagate top-down in an arborescent manner, in a functional way.

Kind regards,
Ghislain


On Mon, Nov 21, 2016 at 10:12 PM Benito van der Zander <benito at benibela.de>
wrote:

> Hi,
>
>
>   You may find some resources that somewhat misleadingly define ":="
> as the "assignment operator," a phrase that is not found in any XQuery
> or XPath specification. Calling ":=" the "assignment operator" is
> incorrect in two respects. First, ":=" is involved in the *binding*,
> not *assigning*, of values to variables—an important distinction in
> functional and declarative languages like XQuery, in which a variable,
> once bound to a value, cannot be altered, or reassigned a different
> value. Second, the ":=" cannot be described as an *operator* because
> it does not produce an output sequence of items, in contrast to
> operators like "+" (an arithmetic operator), "and" (a boolean
> operator), and "/" (an operator for connecting steps in a path
> expression).
>
>
> although some implementations have a scripting extensions, where there
> actually is an assignment operator, which does both these things.
>
>
>
> Cheers,
> Benito
>
>
>
> On 11/21/2016 06:40 PM, Joe Wicentowski wrote:
>
> Hi Ghislain,
>
> Thanks very much for your reply!  If ":=" is not an "operator," by
> your helpful definition, then is there a category like "symbol,"
> "delimiter," or at worst, "thingy," that it *does* rightly belong to?
> I'm looking to fill in the blank in the following sentence, which
> builds on your summary:
>
>   ":=" has no formal name in the XPath and XQuery specifications but
> is used extensively in clauses and declarations that bind variables.
> Specifically, it is used in let and group-by clauses (which produce
> tuples) as well as variable and context item declarations (which
> contribute to the dynamic context). Rather than leaving this important
> symbol nameless, let's call it the "binding ____."
>
> I'm also thinking of a parenthetical note or footnote like this:
>
>   You may find some resources that somewhat misleadingly define ":="
> as the "assignment operator," a phrase that is not found in any XQuery
> or XPath specification. Calling ":=" the "assignment operator" is
> incorrect in two respects. First, ":=" is involved in the *binding*,
> not *assigning*, of values to variables—an important distinction in
> functional and declarative languages like XQuery, in which a variable,
> once bound to a value, cannot be altered, or reassigned a different
> value. Second, the ":=" cannot be described as an *operator* because
> it does not produce an output sequence of items, in contrast to
> operators like "+" (an arithmetic operator), "and" (a boolean
> operator), and "/" (an operator for connecting steps in a path
> expression).
>
> (For some resources that use the term "assignment operator," see viahttps://www.google.com/search?q=%22assignment+operator%22+xquery.)
>
> It's funny, "/" didn't have a name in XPath 1.0, but got one in XPath
> 3.0 - "path operator."  Perhaps it's time for ":=" to get a name?
>
> Thanks again,
> Joe
>
>
> Syntactically, ":=" is not an operator because it does not produce an output
> sequence of items like "+" or "and" or "/". It is part of let and group-by
> clauses (which produce tuples) as well as variable and context item
> declarations (which contribute to the dynamic context).
>
> From a pedagogical perspective, I would simply call it a variable binding,
> the important part being not to call it "assignment" to make clear that it
> is a functional and declarative language.
>
>
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>
>
>
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