Hi Folks,
A couple of weeks ago Mike Beckerle pointed out that many data formats contain
things like this:
A number, N
N occurrences of something
For example, 3 followed by the names of three students:
3
John Doe
Sally Smith
Judy Jones
How should that be parsed? Using the DFDL occursCount and
occursCountKind="expression" and hiddenGroup you can parse the input to ensure
that exactly three student names are consumed. The output is this XML:
<Students>
<name>John Doe</name>
<name>Sally Smith</name>
<name>Judy Jones</name>
</Students>
But is it really the job of the parser to "ensure that exactly three student
names are consumed"?
I raised this question to the compiler experts on the compilers Usenet list.
Here's what one person wrote:
> I would contend that in your example the /syntax/ of lists is really a number
> followed by zero or more strings (number string*), and that verifying the
> string
> count is semantics, not syntax. I believe that, whenever possible, semantics
> are
> best left until after parsing is finished.
In other words, keep your DFDL schema simple: forget
occursCountKind="expression" and hiddenGroup; just parse the number and the
following strings. The output should be this:
<number>3</number>
<Students>
<name>John Doe</name>
<name>Sally Smith</name>
<name>Judy Jones</name>
</Students>
If you need to "ensure that there are 3 student names" you can do that check
*after* parsing.
This is the Minimalist DFDL philosophy.
/Roger
