# Using Xs\\Ys for difference list results in compile error **URL:** \ **Category:** Help!\ **Tags:** difference-list\ **Created:** [November 2, 2024, 2:23pm UTC](https://swi-prolog.discourse.group/t/using-xs-ys-for-difference-list-results-in-compile-error/8576 "2024-11-02T14:23:30Z")\ **Posts on this page:** 4\ **Page:** 1
**Author:** ![JS\_Nielsen](https://avatars.discourse-cdn.com/v4/letter/j/dc4da7/32.png) [@JS\_Nielsen](https://swi-prolog.discourse.group/u/JS_Nielsen)\ **Post date:** [November 2, 2024, 2:23pm UTC](https://swi-prolog.discourse.group/t/using-xs-ys-for-difference-list-results-in-compile-error/8576/1 "2024-11-02T14:23:30Z")
I am reading “the art of Prolog” by Sterling and Shapiro and is stuck on chapter 15.1 ‘Difference-lists’. In short the use of Xs\Ys for a difference-list does not compile as expected. I tried compiling both from swipl and emacs, and both fails. I noticed that `help(flatten).` specifically mentions difference lists, so what am I doing wrong? I’m using: SWI-Prolog version 9.2.8 for x86\_64-darwin, and sweepl (threaded, 64 bits, version 9.2.6) for emacs I want the code to: implement difference-lists But what I’m getting is: compile error. ```prolog ?- [flatten_dl]. ERROR: /Users/neo/Library/Mobile Documents/com~apple~CloudDocs/prolog/the_art/flatten_dl.pl:20:12: Syntax error: Operator expected ERROR: /Users/neo/Library/Mobile Documents/com~apple~CloudDocs/prolog/the_art/flatten_dl.pl:27:19: Syntax error: Operator expected ERROR: /Users/neo/Library/Mobile Documents/com~apple~CloudDocs/prolog/the_art/flatten_dl.pl:29:20: Syntax error: Operator expected ERROR: /Users/neo/Library/Mobile Documents/com~apple~CloudDocs/prolog/the_art/flatten_dl.pl:32:19: Syntax error: Operator expected ERROR: /Users/neo/Library/Mobile Documents/com~apple~CloudDocs/prolog/the_art/flatten_dl.pl:35:13: Syntax error: Operator expected true. ``` My code looks like this: ```prolog %program 15.2 %append_dl(As,Bs,Cs) :- The difference-list Cs is the result of appending Bs to As, where As and Bs are compatible difference-lists. append_dl(Xs\Ys,Ys\Zs,Xs\Zs). % program 15.2 %flatten(Xs,Ys) :- %Ys is the flattened list containing the elements in Xs. %@ flatten(Xs,Ys) :- flatten_dl(Xs,Ys\[]). flatten_dl([X|Xs],Ys\Zs) :- flatten_dl(X,Ys\Ys1), flatten_dl(Xs,Ys1\Zs). flatten_dl(X,[X|Xs]\Xs) :- constant(X), X\=[]. flatten([],Xs\Xs). constant(X) :- \+ var(X). ``` ---
**Author:** ![brebs](https://avatars.discourse-cdn.com/v4/letter/b/e9c0ed/32.png) [@brebs](https://swi-prolog.discourse.group/u/brebs)\ **Post date:** [November 2, 2024, 3:14pm UTC](https://swi-prolog.discourse.group/t/using-xs-ys-for-difference-list-results-in-compile-error/8576/2 "2024-11-02T15:14:01Z")
Can’t use that backslash character, it seems: ```prolog ?- X = A\B. ERROR: Syntax error: Operator expected ERROR: X = ERROR: **here** ERROR: A\B . ?- X = A/B. X = A/B. ``` ---
**Author:** ![Boris](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/boris/32/7486_2.png) [@Boris](https://swi-prolog.discourse.group/u/Boris)\ **Post date:** [November 2, 2024, 3:14pm UTC](https://swi-prolog.discourse.group/t/using-xs-ys-for-difference-list-results-in-compile-error/8576/3 "2024-11-02T15:14:52Z")
If you want to just move on, replace every “X\Y” with “X, Y”. For example, instead of writing: ```prolog flatten_dl(Xs, Ys\[]). ``` write: ```prolog flatten_dl(Xs, Ys, []). ``` Do this consistently throughout your program. ~~Your original code should have worked, however. I wonder what is the issue, exactly. The `\` is defined as an operator…~~ This notation, `X\Y`, is not some special Prolog syntax, this is a term `\(X, Y)`. In order for `X\Y` to be correctly parsed you need to define `\` as a binary operator. Currently it is only defined as a unary operator. See [the docs on operators](https://www.swi-prolog.org/pldoc/man?section=operators). If I add the following on the top of the program you show: ```prolog :- op(400, yfx, (\)). ``` then the syntax errors are gone and there are other warnings instead. However, modern Prolog code seems to use separate arguments to represent the two terms that comprise a difference list. One example in the standard library is findall/4. The third and fourth argument are the “list” and the “tail” that in “The Art of Prolog” would have been written as `Bag\Tail` I assume. “The Art of Prolog” is a good book but sadly a bit outdated to enjoy without help from others. ---
**Author:** ![JS\_Nielsen](https://avatars.discourse-cdn.com/v4/letter/j/dc4da7/32.png) [@JS\_Nielsen](https://swi-prolog.discourse.group/u/JS_Nielsen)\ **Post date:** [November 3, 2024, 7:52am UTC](https://swi-prolog.discourse.group/t/using-xs-ys-for-difference-list-results-in-compile-error/8576/4 "2024-11-03T07:52:50Z")
Thank you, Boris. Adding the `:- op(400, yfx, (\)).` did the trick. ```prolog %test flatten_dl([1,2,[2,3,4]],Ys\[]). % Ys = [1,2,2,3,4] ```