Character Set

NAMEs, TYPEs and the contents of strings in HDS must consist only of printable ASCII characters (i.e. hexadecimal 20 through 7E). Beyond this, HDS itself (see SUN/92) imposes restrictions on NAMEs and TYPES, to which we add the requirement that the first character of a (non-primitive) TYPE must not be underscore.

It is strongly recommended by Starlink that NAMEs and TYPEs be limited to letters, numbers, and the underscore character, and that the first character be a letter. This is to prevent inconvenience to users, who will find themselves having to resort to special syntax (extra quote marks, for example) in order to resolve command-line ambiguity where unconventional NAMEs or TYPEs are present. Worse, the possibility cannot be ruled out that unexpected interactions between command-language features and imaginative NAMEs or TYPEs will, at some stage, cause insurmountable difficulties.

The names of HDS container files should be chosen to be the same as the names of their top-level data structures, or at least to be closely related. Note that filenames must not appear in applications code (see SGP/16).