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Re: [xml-dev] RE: Declarative programming requires a different mindset

It seems to me that a declarative language describes where
you want to be (output model) in relation (semantics) to where
you are (input model) - but NOT the steps you might wish to
take to get there (programming steps).

So (X)HTML goes some way to describing output (browser
behaviour) and how output might relate to input (in the 'tag'
names at least - e.g. 'table' by its name invokes our common
understanding of what a table will look like) and it certainly
describes the input model (the (X)HTML markup itself). By the
above tentative definition that makes it declarative.

A programming language is more like a series of steps, many
of which might in themselves, discretely, be declarative (they
invoke definitions of input, output and the mapping between the
two). Their sequencing takes them beyond declarative though.

There are grey areas of course. When you pipe/chain XSLT,
say, it starts to become a programmed sequence of steps,
which in my view is no longer purely declarative.

I think people tend to mix all these concepts together in their
designing of languages. A query language tends to have some
imperative aspects and some declarative aspects - even a purely
declarative one will have, at least implicitly, the specification of
where to put the result (which kinds of makes it like a single
step in an imperative language). A statement like 'Y=X' can be
put as an imperative 'copy X to Y'. When you put that as a query
it might become 'what is X?' but there might be an implicit 'put
the answer into (variable) Y'. Declaratively you might wrap X and
Y in tags which mean that X is the input and Y is the output and
the semantics of the tags means that there is straight mapping
of the input to the output, e.g. <var name="Y"><copy-of>X</...
It ceases to be declarative if you split it into two steps where you
first say declaratively 'copy X' and secondly 'put the copy into Y'
but if your semantics require that the first step be completed
before the second step then it loses the declarative characteristics.
If the semantics require that the second step be started 'as-if' the
first step had been completed (logically) then it becomes more
of a grey area, I think. e.g you could say that the step of
'put the copy into Y' has a prerequisite that 'X has been copied'.
(Still seems to have lost its declarative characteristics.)

Stephen D Green

On 11 April 2010 13:31, Costello, Roger L. <costello@mitre.org> wrote:
> Hi Folks,
> Thank you very much for your comments. I am assimilating them as fast as I can.
> With regard to my statement "XHTML is a declarative programming language" I must confess that I got the idea from the Wikipedia article on Declarative Programming [1]:
> -------------------------------------------------------------------------------
> Domain-specific languages
> Some well-known examples of declarative domain specific languages (DSLs) include regular expressions, CSS, and a subset of SQL (SELECT queries, for example). DSLs have the advantage of being useful while not necessarily needing to be Turing-complete, which makes it easier for a language to be purely declarative.
> Many markup languages such as HTML, MXML, XAML, XSLT, SVG or other user interface markup languages are often declarative. HTML, for example, only describes what should appear on a webpage and doesn't specify the possible interactions with it.
> -------------------------------------------------------------------------------
> Should the Wikipedia page be modified?
> /Roger
> [1] http://en.wikipedia.org/wiki/Declarative_programming
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