Character Codes
The following operation is included in order to allow integer arithmetic on
character codes.
[
X]
- Evaluates to X for numeric X. This is
relevant because character strings in Prolog are lists of character
codes, that is, integers. Thus, for those integers that correspond
to character codes, the user can write a string of one character in
place of that integer in an arithmetic expression. For example, the
expression (A) is equivalent to (B), which in turn becomes (C) in
which case X is unified with 2:
X is "c" - "a" (A)
X is [99] - [97] (B)
X is 99 - 97 (C)
A cleaner way to do the same thing is
X is 0'c - 0'a