SWI-Prolog has the special string data type. However, in Prolog, text may be represented more traditionally as a list of character-codes, i.e. (small) integers (in SWI-Prolog specifically, those are Unicode code points). This results in output like the following (here using the backquote notation which maps text to a list of codes):
?- writeln(`hello`).
[104, 101, 108, 108, 111]
?- atom_codes("hello",X).
X = [104,101,108,108,111].
Unless you know the Unicode tables by heart, this is pretty
unpleasant for debugging. Loading library(portray_text)
makes the toplevel and debugger consider certain lists of integers as
text and print them as text. This is called "portraying". Of course,
interpretation is imperfect as there is no way to tell in general
whether [65,66] should written as `AB` or as [65,66].
Therefore it is important that the user be aware of the fact that this
conversion is enabled. This is why this library must be loaded
explicitly.
To be able to copy the printed representation and paste it back,
printed text is enclosed in back quotes if current_prolog_flag/2
for the flag
back_quotes is codes (the default), and
enclosed in double quotes otherwise. Certain control characters
are printed out in backslash-escaped form.
The default heuristic only considers list of codes as text if the codes are all from the set of 7-bit ASCII without most of the control characters. A code is classified as text by text_code/1, which in turn calls is_text_code/1. Define portray_text:is_text_code/1 to succeed on additional codes for more flexibility (by default, that predicate succeeds nowhere). For example:
?- maplist([C,R]>>(portray_text:text_code(C)->R=y;R=n),
`G\u00e9n\u00e9rateur`,Results).
Results = [y,n,y,n,y,y,y,y,y,y].
Now make is_text_code/1 accept anything:
?- [user]. |: portray_text:is_text_code(_). |: ^D % user://3 compiled 0.00 sec, 1 clauses true.
Then:
?- maplist([C,R]>>(portray_text:text_code(C)->R=y;R=n),
`G\u00e9n\u00e9rateur`,Results).
Results = [y,y,y,y,y,y,y,y,y,y].
true, consider lists of
integers as list of Unicode code points and print them as corresponding
text inside quotes: `text` or "text". Quoting
depends on the value of current_prolog_flag/2 back_quotes.
Same as
?- set_portray_text(enabled, true).
start...end.