What is the difference between ' and " in Prolog?

Question

I am new to Prolog and noticed that ' and " give different behavior, but am curious as to why. Specifically, when loading a file, ?- ['test1.pl']. works, while ?- ["test1.pl"]. doesn't.

Answer 1

Single quoted items are always atoms.

The meaning of double quotes depends on the Prolog flag double_quotes:

---

atom — with this value "a" = a. Nowadays, this is rarely used. But you will find Prolog books where ["abc.pl"] is written.

---

codes — a list of character codes. This is frequently the default, but it leads to very unreadable answers like

?- set_prolog_flag(double_quotes,codes).
true.

?- phrase(("Ja tvoi ",("sluga"|"rabotnik"),"!"), Satz).
Satz = [74,97,32,116,118,111,105,32,115,108,117,103,97,33] ;
Satz = [74,97,32,116,118,111,105,32,114,97,98,111,116,110,105,107,33].

Even worse, if you use characters beyond ASCII:

?- phrase(("Я твой ",("слуга"|"работник"),"!"), Satz).
Satz = [1071,32,1090,1074,1086,1081,32,1089,1083,1091,1075,1072,33] ;
Satz = [1071,32,1090,1074,1086,1081,32,1088,1072,1073,1086,1090,1085,1080,1082,33].

---

chars — a list of one-char atoms. See this for more about it.

?- set_prolog_flag(double_quotes,chars).
true.

?- phrase(("Ja tvoi ",("sluga"|"rabotnik"),"!"), Satz).
Satz = ['J',a,' ',t,v,o,i,' ',s,l,u,g,a,!] ;
Satz = ['J',a,' ',t,v,o,i,' ',r,a,b,o,t,n,i,k,!].

?- phrase(("Я твой ",("слуга"|"работник"),"!"), Satz).
Satz = ['Я',' ',т,в,о,й,' ',с,л,у,г,а,!] ;
Satz = ['Я',' ',т,в,о,й,' ',р,а,б,о,т,н,и,к,!].

This notation gives more readable answers and is the default in Scryer which displays them even more compactly with the double quote notation for printing any list of one-char atoms. For SICStus and SWI this can be emulated with the following library.

?- use_module(library(double_quotes)).
true.

?- phrase(("Ja tvoi ",("sluga"|"rabotnik"),"!"), Satz).
Satz = "Ja tvoi sluga!" ;
Satz = "Ja tvoi rabotnik!".

?- phrase(("Я твой ",("слуга"|"работник"),"!"), Satz).
Satz = "Я твой слуга!" ;
Satz = "Я твой работник!".

If you have difficulties installing double_quotes.pl as a library, simply put it into the directory of your other Prolog files and say: use_module(double_quotes).

Answer 2

Strings in Prolog are written in single quotes. Terms written in double quotes are immediately converted to a list of character codes.

?- write('sdf').
sdf
true.

?- write("sdf").
[115, 100, 102]
true.