[XML-DEV Mailing List Archive Home] [By Thread] [By Date] [Recent Entries] [Reply To This Message] RE: Strong Typing in SGML and XML
I accidently sent this message before completing it. The full message is here: > Several people have written in recently asking, in effect, "What are > the > purposes of this strong typing? What needs does it solve?" So I > asked > around. Here are the needs that have been cited: > > 1. Storage optimization. Various clients want to be able to > optimize storage by keeping numbers in a binary format, strings in a > preallocated structure, etc. > > 2. Implied semantics. E.g. numbers can be added together, if you > know they are numbers. Also, knowing that a number is meant to have a > fixed versus floating precision affects how operations are performed, > what kind of precision is retained during calculations, what errors > are > reported, etc. Knowing that a string is meant to be a > URL gives hints on its use. Etc. > > 3. Parsing and formatting rules. Dates are expected to be in some > standard representation, such as given by ISO 8601 (e.g.). Floating > point numbers permit scientific notation. Etc. For example, > though the number "0.1234E+20" could have been represented as > "<mantissa>1234</mantissa><exponent>20</exponent>", and the date > "19970508T10:47" could have been similarly broken into year, month, > etc., > and this markup would eliminate the need for special parsers for > numbers and > dates, it has obvious readability and bloat problems. > An explicit data type can signal what the internal elements are and > how > to parse for them without tags. > > 4. Different data types need different supplementary attributes, > such as number of digits precision, total size in characters, whether > time zones are present, etc. (In Tim's proposal, these all overload a > single, generic attribute.) > > 5. Range restrictions. Dates and other kinds of things measured in > numbers can be limited to a range of values. All types can be > potentially limited to a set of descrete values (by enumeration or > rule). For example, an attribute expressing a color > in terms of wavelength could be limited to 400..700 (nanometers). An > attribute listing a US egg size could be limited to be among > "medium," "large," "extra large" and "jumbo." If represented > numerically, > it could be limited to "1," "2," "3" and "4." > > 6. Passthrough. Sometimes XML is a carrier syntax between systems > where some participants need to convey implications not covered in > points > 1 through 5 above. For example, a database may make a distinction > between CHAR and VARCHAR even though other readers of a document > don't. > > > xml-dev: A list for W3C XML Developers Archived as: http://www.lists.ic.ac.uk/hypermail/xml-dev/ To unsubscribe, send to majordomo@i... the following message; unsubscribe xml-dev List coordinator, Henry Rzepa (rzepa@i...)
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