RE: Declarative programming requires a different mindset

["Costello, Roger L." <costello@mitre.org>]

Hi Folks,

I've had some additional insights into declarative programming that I would like to share.

The three main constructs used in programming are:

  - Loops 

  - Conditional

  - Sequence

I have always thought of these constructs in terms of "Do this, then that." That is, I thought these are imperative constructs only. But now I understand that these are also declarative constructs. 

Here's how to declaratively think about the constructs.


Ways of Thinking about Loops

Example Loop: 

    for $i in //Member return A, B, C

Imperative: loop through each <Member> element and do actions A, B, C.

Declarative: this A, B, C description applies to each <Member> element.


Ways of Thinking about Conditional Statements

Example Conditional: 

    if (empty(//Member[999])) then A else B

Imperative: if there is no 99th <Member> element then do action A, else do action B.

Declarative:  this description A is for the case where the old document does not contain a 999th <Member> element. And this description B is for the case where the old document _does_ contain a 999th <Member> element.


Ways of Thinking about a Sequence of Statements

Example Sequence: 

    delete node $i/Name,
    replace node $i/@id with 
            attribute {'id'} {data($i/Name)}

Imperative: delete the <Name> element, then replace the value of the id attribute with the value of the <Name> element. (Obviously, this is nonsensical in an imperative programming.)

Declarative: in the new document there will be no <Name> element. In the new document the id attribute's value will be the value of the <Name> element in the old document.


To recap: declarative programming focuses on describing what you want in the new document, relative to what was in the old document. How a machine takes that declarative description and accomplishes it, is transparent.

Question: When you create XSLT transforms or XQueries, how do you maintain a declarative mindset, and avoid slipping back into an imperative mindset?

/Roger