VDM-10 Language Manual

Appendix B
Lexical Specification
B.1 Characters
The characters that comprise a valid VDM specification are defined in terms of Unicode codepoints.
The actual character encoding of a VDM source file (for example UTF-8, ISO-Latin-1 or
Shift-JIS) is not defined, and the tool support is responsible for converting whatever encoding is
used into Unicode during the parse of the file.
All VDM keywords and delimiter tokens are composed of characters from the Basic Latin
block (“ASCII” codepoints less than U+0080). On the other hand, user identifiers (variable names,
function names and so on) can be composed of a rich variety of Unicode codepoints, reflecting the
need for fully internationalized specifications.
All Unicode codepoints have a “category”. Certain categories are entirely excluded from the set
of codepoints that are permitted in identifiers. This prevents, say, puntuation characters from being
used. On the other hand, to provide a degree of compatibility with the original VDM ISO standard,
and for backward compatibility, there are different rules for the formation of user identifiers that
only use ASCII characters. For example, the underscore is permitted in identifiers (U+005F), even
though this is in the connecting punctuation category, which would not normally be allowed.
See http://www.fileformat.info/info/unicode/category/index.htm for
more information about categories.
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