As discussed in section
2.18, SWI-Prolog supports international character handling. Its
internal encoding is UNICODE. I/O streams convert to/from this internal
format. This section discusses the options for source files not in
US-ASCII.
SWI-Prolog can read files in any of the encodings described in
section 2.18. Two
encodings are of particular interest. The
text encoding deals with the current locale, the
default used by this computer for representing text files. The encodings
utf8, unicode_le and unicode_be
are
UNICODE encodings: they can represent---in the same
file---characters of virtually any known language. In addition, they do
so unambiguously.
If one wants to represent non US-ASCII text as Prolog terms in a
source file, there are several options:
- Use escape sequences
This approach describes NON-ASCII as sequences of the form
\octal\. The numerical argument is
interpreted as a UNICODE character.44To
my knowledge, the ISO escape sequence is limited to 3 octal digits,
which means most characters cannot be represented. The
resulting Prolog file is strict 7-bit US-ASCII, but if there are many
NON-ASCII characters it becomes very unreadable.
- Use local conventions
Alternatively the file may be specified using local conventions, such as
the EUC encoding for Japanese text. The disadvantage is portability. If
the file is moved to another machine, this machine must use the same locale
or the file is unreadable. There is no elegant way if files from
multiple locales must be united in one application using this technique.
In other words, it is fine for local projects in countries with uniform
locale conventions.
- Using UTF-8 files
The best way to specify source files with many NON-ASCII characters is
definitely the use of UTF-8 encoding. Prolog can be notified of this
encoding in two ways, using a UTF-8 BOM (see section
2.18.1.1) or using the directive :- encoding(utf8).
Many of today's text editors, including PceEmacs, are capable of editing
UTF-8 files. Projects that were started using local conventions can be
re-coded using the Unix
iconv tool or often using commands offered by the editor.
