问题
I have a problem, I've got an escaped string for example "\\u0026" and I need this to transform to unicode char '\u0026'.
Tricks like string_concat('\\', S, "\\u0026"), write(S). didn't help, because it will remove \ not only the escape . So basically my problem is, how to remove escape chars from the string.
EDIT: Oh, I've just noticed, that stackoverflow also plays with escape \.
write_canonical/1 gives me "\\u0026", how to transform that into a single '&' char?
回答1:
In ISO Prolog a char is usually considered an atom of length 1. Atoms and chars are enclosed in single quotes, or written without quotes if possible. Here are some examples:
?- X = abc. /* an atom, but not a char */
X = abc
?- X = a. /* an atom and also a char */
X = a
?- X = '\u0061'.
X = a
The \u notation is SWI-Prolog specific, and not found in the ISO Prolog. In SWI-Prolog there is a data type string again not found in the ISO Prolog, and always enclosed in double quotes. Here are some examples:
?- X = "abc". /* a string */
X = "abc"
?- X = "a". /* again a string */
X = "a"
?- X = "\u0061".
X = "a"
If you have a string at hand of length 1, you can convert it to a char via the predicate atom_string/2. This is a SWI-Prolog specific predicate, not in ISO Prolog:
?- atom_string(X, "\u0061").
X = a
?- atom_string(X, "\u0026").
X = &
Some recommendation. Start learning the ISO Prolog atom predicates first, there are quite a number. Then learn the SWI-Prolog atom and string predicates.
You dont have to learn so many new SWI-Prolog predicates, since in SWI-Prolog most of the ISO Prolog predicates also accept strings. Here is an example of the ISO Prolog predicate atom_codes/2 used with a string in the first argument:
?- atom_codes("\u0061\u0026", L).
L = [97, 38].
?- L = [0'\u0061, 0'\u0026].
L = [97, 38].
?- L = [0x61, 0x26].
L = [97, 38].
P.S: The 0' notation is defined in the ISO Prolog, its neither a char, atom or string, but it represents an integer data type. The value is the code of the given char after the 0'. I have combined it with the SWI-Prolog \u notation.
P.P.S: The 0' notation in connection of the \u notation is of course redundant, in ISO Prolog one can directly use the hex notation prefix 0x for integer values.
回答2:
The thing is that "\\u0026" is already what you are searching for because it represents \u0026.
来源：https://stackoverflow.com/questions/39910950/swi-prolog-how-to-get-unicode-char-from-escaped-string