4.2 Character representation

In traditional (Edinburgh-) Prolog, characters are represented using character-codes. Character codes are integer indices into a specific character set. Traditionally the character set was 7-bits US-ASCII. 8-bit character sets have been allowed for a long time, providing support for national character sets, of which iso-latin-1 (ISO 8859-1) is applicable to many western languages. Text-files are supposed to represent a sequence of character-codes.

ISO Prolog introduces three types, two of which are used for characters and one for accessing binary streams (see open/4). These types are:

• code
  A character-code is an integer representing a single character. As files may use multi-byte encoding for supporting different character sets (utf-8 encoding for example), reading a code from a text-file is in general not the same as reading a byte.
• char
  Alternatively, characters may be represented as one-character-atoms. This is a very natural representation, hiding encoding problems from the programmer as well as providing much easier debugging.
• byte
  Bytes are used for accessing binary-streams.

The current version of SWI-Prolog does not provide support for multi-byte character encoding. This implies for example that it is not capable of breaking a multi-byte encoded atom into characters. For SWI-Prolog, bytes and codes are the same and one-character-atoms are simple atoms containing one byte.

To ease the pain of these multiple representations, SWI-Prolog's built-in predicates dealing with character-data work as flexible as possible: they accept data in any of these formats as long as the interpretation is unambiguous. In addition, for output arguments that are instantiated, the character is extracted before unification. This implies that the following two calls are identical, both testing whether the next input characters is an a.

peek_code(Stream, a).
peek_code(Stream, 97).

These multiple-representations are handled by a large number of built-in predicates, all of which are ISO-compatible. For converting between code and character there is char_code/2. For breaking atoms and numbers into characters are are atom_chars/2, atom_codes/2, number_codes/2 and number_chars/2. For character I/O on streams there is get_char/[1,2], get_code/[1,2], get_byte/[1,2], peek_char/[1,2], peek_code/[1,2], peek_byte/[1,2], put_code/[1,2], put_char/[1,2] and put_byte/[1,2]. The Prolog flag double_quotes controls how text between double-quotes is interpreted.