Strong Typing in SGML and XML

Andrew Layman andrewl at
Thu May 8 19:25:21 BST 1997

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 advanced:

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.

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.

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

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