I am building some test cases for integer/1 using SWI-Prolog.
ISO/IEC 13211-1 gives a BNF definition for integer and one of the alternatives for integer is for a character code constant.
I am able to create and test examples of all the other alternatives using integer/1 but for character code constant I cannot create a valid example. (see below)
How is an integer as a character code constant created that will return true using integer/1?
Answer
Thanks to @false.
integer(0'0).
true.
integer(0'9).
true.
integer(0'a).
true.
integer(0'\n).
true.
Usefulness
X is 0'\n.
X = 10.
X is 0b010101.
X = 21.
X is 0xFFF1.
X = 65521.
X is 0o7423.
X = 3859.
and thanks to j4n bur53 via link from @false
with SWI-Prolog other radix can be used besides 2,8, or 16.
X is 5'1234012340.
X = 3032220.
X is 32'123456789ABCDEFGHIJKLMNOPQRSTU.
X = 47525417447024678661670292427038339608998846.
What I tried
integer("0").
false.
integer('0').
false.
integer(`0`).
false.
integer("1").
false.
integer('1').
false.
integer(`1`).
false.
ISO
INTERNATIONAL STANDARD ISO/IEC 13211-1 First edition 1995-06-01
Information technology - Programming languages - Prolog
Part 1: General Core
INTERNATIONAL STANDARD ISO/IEC 13211-1:1995
TECHNICAL CORRIGENDUM 1
Published 2007-11-15
INTERNATIONAL STANDARD ISO/IEC 13211-1:1995
TECHNICAL CORRIGENDUM 2
Published 2012-02-15
BNF for integer
integer token (* 6.4.4 *) =
integer constant (* 6.4.4 *)
| character code constant (* 6.4.4 *)
| binary constant (* 6.4.4 *)
| octal constant (* 6.4.4 *)
| hexadecimal constant (* 6.4.4 *) ;
BNF for character code constant
character code constant (* 6.4.4 *) =
"0" , single quote char (* 6.5.5 *), single quoted character (* 6.4.2.1 *)
I suspect the BNF is wrong in ISO/IEC 13211-1 but checking the CORRIGENDUM shows no corrections.
Integer test cases
% <integer constant> examples
number(1).
% true.
number(0).
% true.
number(01).
% true.
number(12345678901234567890123456789012345678901234567890).
% true.
% <character code constant> examples
% ???
% <binary constant> examples
number(0b0).
% true.
number(0b10101010101010101010101010101010101010101010101010).
% true.
integer(0b2).
% ERROR: Syntax error: Illegal number
% ERROR: integer
% ERROR: ** here **
% ERROR: (0b2) .
% <octal constant> examples
integer(0o7).
% true.
integer(0o1234567012345670123456701234567012345670123456701234567).
% true.
integer(0o8).
% ERROR: Syntax error: Illegal number
% ERROR: integer
% ERROR: ** here **
% ERROR: (0o8) .
% <hexadecimal constant>
integer(0x0).
% true.
integer(0xF).
% true.
integer(0xf).
% true.
integer(0x123456789ABCDEF012345670123456789ABCDEF012345670123456789ABCDEF).
% true.
integer(0xG).
% ERROR: Syntax error: Illegal number
% ERROR: integer
% ERROR: ** here **
% ERROR: (0xG) .
This was answered by false in a comment.
Reposting here so that others can see an answer exist.
integer(0'0).
true.
integer(0'9).
true.
integer(0'a).
true.
integer(0'\n).
true.
来源：https://stackoverflow.com/questions/41637402/how-is-a-integer-created-as-a-character-code-constant