[m-rev.] for review: document char__to_int caveats

[Fergus Henderson]

Branches: main
Estimated hours taken: 0.25

library/char.m:
	Add some documentation warning about issues with char__to_int
	and Unicode.

Workspace: /home/ceres/fjh/mercury
Index: library/char.m
===================================================================
RCS file: /home/mercury1/repository/mercury/library/char.m,v
retrieving revision 1.38
diff -u -d -r1.38 char.m
--- library/char.m	14 Jun 2002 10:18:46 -0000	1.38
+++ library/char.m	21 Oct 2002 09:31:26 -0000
@@ -11,6 +11,9 @@
 
 % This module defines some predicates that manipulate characters.
 
+% The set of characters which are supported and the mapping from
+% characters to integer values are both implementation dependent.
+
 % Originally we used `character' rather than `char' for the type name
 % because `char' was used by NU-Prolog to mean something different.
 % But now we use `char' and the use of `character' is discouraged.
@@ -33,7 +36,18 @@
 :- mode char__to_int(in, out) is det.
 :- mode char__to_int(in, in) is semidet.	% implied
 :- mode char__to_int(out, in) is semidet.
-	% Convert a character to it's corresponding numerical code.
+	% Convert a character to its corresponding numerical code
+	% (integer value).
+	% Beware that the mapping from characters to numerical codes
+	% is implementation dependent; there is no guarantee that
+	% the integer values for characters will fit in 8 bits.
+	% Furthermore, the value returned from char__to_int might be
+	% different than the byte(s) used to store the character in a file.
+	% There is also no guarantee that characters created using 
+	% `char__to_int(out, in)' can be written to files or
+	% to the standard output or standard error streams.
+	% For example, an implementation might use represent characters
+	% using Unicode, but store files in an 8-bit national character set.
 
 :- func char__max_char_value = int.
 :- pred char__max_char_value(int).

-- 
Fergus Henderson <fjh at cs.mu.oz.au>  |  "I have always known that the pursuit
The University of Melbourne         |  of excellence is a lethal habit"
WWW: <http://www.cs.mu.oz.au/~fjh>  |     -- the last words of T. S. Garp.
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