4.15 Character Sets

Character sets are another composite data type, like strings, built upon the character data type. A character set is a mathematical set of characters with the most important attribute being membership. That is, a character is either a member of a set or it is not a member of a set. The concept of sequence (e.g., whether one character comes before another, as in a string) is completely foreign to a character set. If two characters are members of a set, their order in the set is irrelevant. Also, membership is a binary relation; a character is either in the set or it is not in the set; you cannot have multiple copies of the same character in a character set. Finally, there are various operations that are possible on character sets including the mathematical set operations of union, intersection, difference, and membership test.

HLA implements a restricted form of character sets that allows set members to be any of the 128 standard ASCII characters (i.e., HLA's character set facilities do not support extended character codes in the range #128..#255). Despite this restriction, however, HLA's character set facilities are very powerful and are very handy when writing programs that work with string data. The following sections describe the implementation and use of HLA's character set facilities so you may take advantage of character sets in your own programs.