Here's a nice real world example of how having an appropriate representation of nil is important:
http://newsfeed.time.com/2012/02/18/mans-vanity-license-plate-attracts-20000-in-erroneous-tickets/ A short summary: DC police use "NO TAGS" as the license plate value when ticketing vehicles that do not have a license plate. But someone had a vanity plate of "NO TAGS" and started getting a bunch of tickets because the system couldn't differentiate the two. In this case, the ticket file format should really be something like HAS_TAGS: YES TAG: NO TAGS or this for when there is no tag: HAS_TAGS: NO TAG: NO TAGS You could maybe consider this an out of band nil, where the tag should be considered nil if HAS_TAGS has a value of NO. On 7/10/19 8:23 AM, Beckerle, Mike wrote: > I did a quick search of the schemas on the github DFDL schemas site. > > > NITF has nillable elements. > > EDIFACT has nillable elements > > > In EDIFACT case, empty strings become nilled elements. That's in-band for > string. > > > NITF has nillable dates, and nillable strings. Nillable strings are empty > strings. Nillable dates are date elements containing all %SP; ie. all spaces. > That's in band, because the dates are textual representation also. > > > Many of the common mil data formats have a concept of "NO STATEMENT" which > can > be represented as either nilled elements, non-existing elements, empty > elements, > or an explicit <NOSTATEMENT/> element that is a marker. We're doing the > latter. > > > Arguably we should change this NO STATEMENT concept to one of the more > conventional forms. I tend not to want to use empty elements because that > creates ambiguity with empty string. Nilled elements are probably the natural > choice here, as these elements are required, not optional. > > > I don't have any examples off-hand of out-of-band nils used in a data format. > But I will point out that XML's > > <foo xsi:nil="true"/> is itself an out-of-band nil representation. The > attribute > is a separate indicator out of band of the value space for the element. > > > > -------------------------------------------------------------------------------- > *From:* Costello, Roger L. <[email protected]> > *Sent:* Wednesday, July 10, 2019 7:33:24 AM > *To:* [email protected] > *Subject:* Examples of file formats that use in-band nil? Examples of file > formats that use out-of-band nil? > > Hello DFDL community, > > There are two ways file formats denote that a region has a nil value: > > * *In-band nil: a symbol inserted into the region indicates nil. A part of > the > region’s value space is reserved for indicating nil.* > * *Out-of-band nil: a symbol, separate from the region, indicates that the > region has a nil value.* > > I am seeking examples of each. Can you tell me a file format that uses > in-band > nil? Can you tell me a file format that uses out-of-band nil? > > /Roger >
