问题
Need example using SWI-Prolog ./2.
The signature is
.(+Int,[])
Also if there is a name for this operator it would be nice to know. Searching for '.' is senseless.
The closest to a name is from SWI documentation section F.3 Arithmetic Functions
List of one character: character code
What I tried
?- X is .(97,[]).
results in
ERROR: Type error: `dict' expected, found `97' (an integer)
ERROR: In:
ERROR: [11] throw(error(type_error(dict,97),_5812))
ERROR: [8] '<meta-call>'(user:(...,...)) <foreign>
ERROR: [7] <user>
ERROR:
ERROR: Note: some frames are missing due to last-call optimization.
ERROR: Re-run your program in debug mode (:- debug.) to get more detail.
Also tried
?- X is .(97,['a']).
with same result.
回答1:
In standard Prolog, the '.' functor is the actual list term functor. Specifically, [H|T] is equivalent to '.'(H, T). If you run GNU Prolog, for example, you will get this:
| ?- write_canonical([H|T]).
'.'(_23,_24)
yes
| ?- X = .(H,T).
X = [H|T]
yes
| ?-
The is/2 operator will allow you to read the character code directly from a list consisting of a single character code. So the following works in Prolog:
| ?- X is [97].
X = 97
yes
| ?- X is .(97,[]).
X = 97
yes
Where it starts to get interesting is that, in SWI Prolog, they've introduced dictionaries that rely on the '.' as a means of designating dictionary keys. This creates a conflict with the '.' functor being used as a list functor. This is explained in the link on dictionaries. Thus, when you attempt to use the '.' functor for a list, you get something like this:
1 ?- X is [97].
X = 97.
2 ?- X is .(97, []).
ERROR: Type error: `dict' expected, found `97' (an integer)
ERROR: In:
ERROR: [11] throw(error(type_error(dict,97),_5184))
ERROR: [9] '$dicts':'.'(97,[],_5224) at /usr/local/lib/swipl-7.4.2/boot/dicts.pl:46
ERROR: [8] '<meta-call>'(user:(...,...)) <foreign>
ERROR: [7] <user>
ERROR:
ERROR: Note: some frames are missing due to last-call optimization.
ERROR: Re-run your program in debug mode (:- debug.) to get more detail.
3 ?- X = .(H,T).
ERROR: Arguments are not sufficiently instantiated
ERROR: In:
ERROR: [11] throw(error(instantiation_error,_6914))
ERROR: [9] '$dicts':'.'(_6944,_6946,_6948) at /usr/local/lib/swipl-7.4.2/boot/dicts.pl:46
ERROR: [8] '<meta-call>'(user:(...,...)) <foreign>
ERROR: [7] <user>
ERROR:
ERROR: Note: some frames are missing due to last-call optimization.
ERROR: Re-run your program in debug mode (:- debug.) to get more detail.
SWI has, therefore, defined another functor, [|], to serve the purpose that '.' traditionally served, which is the list functor.
4 ?- X is '[|]'(97, []).
X = 97.
5 ?- X = '[|]'(H, T).
X = [H|T].
6 ?-
Oddly, you might think then that this would happen:
7 ?- write_canonical([H|T]).
'[|]'(_, _)
However, what happens is this:
7 ?- write_canonical([H|T]).
[_|_]
true.
So to SWI Prolog, the list representation [H|T] is already the canonical representation of the list.
回答2:
The arithmetic function '.'/2 maps
lists holding a single character or character code (terms like [C] or '.'(C,[])) to the corresponding character code.
The particular irratic behavior that you were observing is SWI-Prolog specific.
You can disable the non-standard SWI-Prolog "extensions" (like dicts) by adding the command-line argument --traditional, like so:
$ swipl --traditional Welcome to SWI-Prolog (threaded, 64 bits, version 8.0.0) [...] ?- C = 97, V is [C]. C = 97, V = 97. ?- C = 0'a, V is [C]. C = 97, V = 97. ?- C = a, V is [C]. C = a, V = 97.
This also works with string literals:
:- set_prolog_flag(double_quotes, codes). ?- Str = "a", Str = [C], V is Str. Str = [97], C = 97, V = 97. :- set_prolog_flag(double_quotes, chars). ?- Str = "a", Str = [C], V is Str. Str = [a], C = a, V = 97.
来源：https://stackoverflow.com/questions/49243643/how-to-use-swi-prolog-2-function