# \`w\` write mode not working? **URL:** \ **Category:** General\ **Created:** [April 13, 2026, 1:45am UTC](https://swi-prolog.discourse.group/t/w-write-mode-not-working/9569 "2026-04-13T01:45:34Z")\ **Posts on this page:** 19\ **Page:** 1
**Author:** ![emummel20](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/emummel20/32/4952_2.png) [@emummel20](https://swi-prolog.discourse.group/u/emummel20)\ **Post date:** [April 13, 2026, 1:45am UTC](https://swi-prolog.discourse.group/t/w-write-mode-not-working/9569/1 "2026-04-13T01:45:34Z")
Sorry if I’m missing something obvious but write mode doesn’t seem to be working for me here: I have a very small txt file which I’m reading with ```prolog file_lines([]) --> []. file_lines([L|Ls]) --> [L], file_lines(Ls). test(File,Content) :- phrase_from_file(file_lines(Content), File). ``` I’m getting an abbreviated list of codes in the repl which I used to be able to expand with `w` but now I’m getting ```prolog 46 ?- test('file.txt',Content). Content = [102, 111, 111, 10, 98, 97, 114, 10, 104|…] [write] Content = [102, 111, 111, 10, 98, 97, 114, 10, 104|…] [write] Content = [102, 111, 111, 10, 98, 97, 114, 10, 104|…] . ``` Is there a different command for this now or a flag or anything? I recently upgraded from 9.2.X (I think) to 10.0.2. I tried checking the changelog but don’t believe I’m seeing anything on this. ---
**Author:** ![jan](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/jan/32/4_2.png) [@jan](https://swi-prolog.discourse.group/u/jan)\ **Post date:** [April 13, 2026, 6:53am UTC](https://swi-prolog.discourse.group/t/w-write-mode-not-working/9569/2 "2026-04-13T06:53:25Z")
See [Effefct of "write" option at the top-level - #2 by jan](https://swi-prolog.discourse.group/t/effefct-of-write-option-at-the-top-level/9530/2) ---
**Author:** ![emummel20](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/emummel20/32/4952_2.png) [@emummel20](https://swi-prolog.discourse.group/u/emummel20)\ **Post date:** [April 13, 2026, 12:07pm UTC](https://swi-prolog.discourse.group/t/w-write-mode-not-working/9569/3 "2026-04-13T12:07:24Z")
> [@jan](#): > > Effefct of “write” option at the top-level - #2 by jan `+` works, thanks @jan . Do you want me to delete this post? ---
**Author:** ![emummel20](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/emummel20/32/4952_2.png) [@emummel20](https://swi-prolog.discourse.group/u/emummel20)\ **Post date:** [April 13, 2026, 2:07pm UTC](https://swi-prolog.discourse.group/t/w-write-mode-not-working/9569/4 "2026-04-13T14:07:14Z")
@jan can I ask one more quick question? I keep getting args not instantiated for this and can’t tell what I’m doing wrong ```prolog file_lines([]) --> []. file_lines([L|Ls]) --> { string_codes(Char,[L]) }, [Char], file_lines(Ls). test(File,Content) :- phrase_from_file(file_lines(Content), File). ``` I’m trying to get a list of characters rather than the character codes above. ---
**Author:** ![CapelliC](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/capellic/32/17_2.png) [@CapelliC](https://swi-prolog.discourse.group/u/CapelliC)\ **Post date:** [April 13, 2026, 2:40pm UTC](https://swi-prolog.discourse.group/t/w-write-mode-not-working/9569/5 "2026-04-13T14:40:18Z")
You should swap two lines: `file_lines([L|Ls]) →` ` [Char],` ` { string_codes(Char,[L]) },` ` file_lines(Ls).` ---
**Author:** ![emummel20](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/emummel20/32/4952_2.png) [@emummel20](https://swi-prolog.discourse.group/u/emummel20)\ **Post date:** [April 13, 2026, 2:48pm UTC](https://swi-prolog.discourse.group/t/w-write-mode-not-working/9569/6 "2026-04-13T14:48:30Z")
Like this? ```prolog file_lines([]) --> []. file_lines([L|Ls]) --> [Char], { string_codes(Char,[L]) }, file_lines(Ls). test(File,Content) :- phrase_from_file(file_lines(Content), File). ``` I’m getting false ```prolog 131 ?- trace, test('file.txt',Content). Call: (13) test('file.txt', _7680) ? creep ^ Call: (14) pure_input:phrase_from_file(file_lines(_7680), 'file.txt') ? creep Call: (19) open('file.txt', read, _10326, []) ? creep Exit: (19) open('file.txt', read, (0x600001124300), []) ? creep Call: (21) file_lines(_7680, _12174{pure_input = ...}, []) ? creep Call: (22) _12174{pure_input = ...}=[] ? creep Fail: (22) _12174{pure_input = ...}=[] ? creep Redo: (21) file_lines(_7680, _12174{pure_input = ...}, []) ? creep Call: (22) string_codes(102, [_15942]) ? creep Fail: (22) string_codes(102, [_15942]) ? creep Fail: (21) file_lines(_7680, _12174{pure_input = ...}, []) ? creep Call: (18) close((0x600001124300)) ? creep Exit: (18) close((0x600001124300)) ? creep ^ Fail: (14) pure_input:phrase_from_file(user:file_lines(_7680), 'file.txt') ? creep Fail: (13) test('file.txt', _7680) ? creep false. ``` By the way, what does the caret ^ indicate in the trace output? I couldn’t find that in the docs. ---
**Author:** ![jamesnvc](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/jamesnvc/32/14_2.png) [@jamesnvc](https://swi-prolog.discourse.group/u/jamesnvc)\ **Post date:** [April 13, 2026, 4:03pm UTC](https://swi-prolog.discourse.group/t/w-write-mode-not-working/9569/7 "2026-04-13T16:03:26Z")
I think that should be ```prolog file_lines([]) --> []. file_lines([Char|Ls]) --> [Code], { string_codes(Char, [Code]) }, file_lines(Ls). ``` because in a DCG `[Code]` will unify `Code` with a single code and so you want to turn that into a string (i.e. in CapeliC’s code, `Char` and `L` should be swapped). The reason you got an instantiantion error and CapelliC suggested you change the order of the lines is that, in your original code, you’re calling string\_codes/2 with both arguments uninstantiated, which raises an error; therefore you want to match the code _first_, then convert to a string. ---
**Author:** ![emummel20](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/emummel20/32/4952_2.png) [@emummel20](https://swi-prolog.discourse.group/u/emummel20)\ **Post date:** [April 13, 2026, 4:28pm UTC](https://swi-prolog.discourse.group/t/w-write-mode-not-working/9569/8 "2026-04-13T16:28:01Z")
Thanks, yep that works, but it’s definitely counter intuitive. ```prolog file_lines([]) --> []. file_lines([L|Ls]) --> [L], file_lines(Ls). ``` L seems to unify with a code ```prolog 109 ?- test('file.txt',Content). Content = [102, 111, 111, 10, 98, 97, 114, 10, 104|…] . ``` So I thought my goal is to take the head, where `Content = [L|_], L = 102` and pass it to the second argument as [L], since ```prolog 110 ?- string_codes(Char,[102]). Char = "f". ``` I’ll go with your solution but I’m a little confused about why this works. ---
**Author:** ![jamesnvc](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/jamesnvc/32/14_2.png) [@jamesnvc](https://swi-prolog.discourse.group/u/jamesnvc)\ **Post date:** [April 13, 2026, 5:47pm UTC](https://swi-prolog.discourse.group/t/w-write-mode-not-working/9569/9 "2026-04-13T17:47:32Z")
Ah, sorry, I may have confused things by changing variable names. Your original code: ```prolog file_lines([]) --> []. file_lines([L|Ls]) --> [L], file_lines(Ls). ``` But we want `L` to be a single-character string instead: ```prolog file_lines([]) --> []. file_lines([L|Ls]) --> [Char], % changed from L to Char { string_codes(L, [Char]) }, % added this line file_lines(Ls). ``` There are two differences here from what you had above: 1. The DCG matches `[Char]` _before_ calling string\_codes/2, so we don’t get the instantiation error (because at least one of the arguments must be ground). 2. The order of the arguments to string\_codes/2 - the first argument is the string, the second is the list of codes. > [@emummel20](#): > > So I thought my goal is to take the head, where `Content = [L|_], L = 102` and pass it to the second argument as [L], since To clarify, the `L` in head is bound by `[L]` matching the next code in the stream; at the beginning of the DCG body, `L` is still unbound; does that make sense? ---
**Author:** ![jan](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/jan/32/4_2.png) [@jan](https://swi-prolog.discourse.group/u/jan)\ **Post date:** [April 13, 2026, 6:42pm UTC](https://swi-prolog.discourse.group/t/w-write-mode-not-working/9569/10 "2026-04-13T18:42:31Z")
Even if we get this right though, the file is translated into a list of strings, each holding one character. I doubt that is the intend of the OP. What is the intend though? The name suggests a list of strings, one for each line? ---
**Author:** ![emummel20](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/emummel20/32/4952_2.png) [@emummel20](https://swi-prolog.discourse.group/u/emummel20)\ **Post date:** [April 13, 2026, 7:49pm UTC](https://swi-prolog.discourse.group/t/w-write-mode-not-working/9569/11 "2026-04-13T19:49:40Z")
Sorry @jamesnvc not completely sure I’m following. With ```prolog file_lines([]) --> []. file_lines([L|Ls]) --> [L], file_lines(Ls). ``` I’m seeing ```prolog 105 ?- test('file.txt',Content), Content = [L|_]. Content = [102, 111, 111, 10, 98, 97, 114, 10, 104|…], L = 102 . ``` Again, what I was attempting to do was wrap L in a list, as [102], then convert it to a char by ```prolog 106 ?- string_codes(Char,[102]). Char = "f". ``` That was the thought behind ```prolog file_lines([]) --> []. file_lines([L|Ls]) --> [Char], { string_codes(Char,[L]) }, file_lines(Ls). ``` because `string_codes(?String, ?Codes)` file\_lines is unifying with the codes, the head unifies with a code literal, so I wanted to put in a list and pass it in to the second argument of string\_codes/2 to get a string. In your solution we’re passing 102 into the first argument which is expecting a string but it still works? @jan yes, sorry about the deceptively named predicates. I’m working my way to a different behavior than what we have here. Ultimately I’m trying to get to a list of lines from a file akin to python’s `readlines()`. ---
**Author:** ![jamesnvc](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/jamesnvc/32/14_2.png) [@jamesnvc](https://swi-prolog.discourse.group/u/jamesnvc)\ **Post date:** [April 13, 2026, 8:19pm UTC](https://swi-prolog.discourse.group/t/w-write-mode-not-working/9569/12 "2026-04-13T20:19:23Z")
> [@emummel20](#): > > because `string_codes(?String, ?Codes)` file\_lines is unifying with the codes, the head unifies with a code literal, so I wanted to put in a list and pass it in to the second argument of [string\_codes/2](https://www.swi-prolog.org/pldoc/doc_for?object=string_codes/2) to get a string. I think this is where the misunderstanding lies: In the code above, `[Char]` is what’s unifying the next code in the file with the variable `Char`; at that point, `L` has not been unified with anything. In your previous code block, where you have `file_lines([L|Ls]) --> [L], …` it’s the second occurrence of `L`, in the body of the DCG that actually does the parsing, which then unifies with the same `L` in head. ---
**Author:** ![emummel20](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/emummel20/32/4952_2.png) [@emummel20](https://swi-prolog.discourse.group/u/emummel20)\ **Post date:** [April 13, 2026, 10:15pm UTC](https://swi-prolog.discourse.group/t/w-write-mode-not-working/9569/13 "2026-04-13T22:15:48Z")
Oh I see what you’re saying. I thought variables in file\_lines([L|Ls]) were unified first. If everything after → gets unified first then this makes sense. Thanks. ---
**Author:** ![emummel20](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/emummel20/32/4952_2.png) [@emummel20](https://swi-prolog.discourse.group/u/emummel20)\ **Post date:** [April 14, 2026, 5:46am UTC](https://swi-prolog.discourse.group/t/w-write-mode-not-working/9569/14 "2026-04-14T05:46:39Z")
And one last thing: This basically gets me what I want ```prolog lines([]) --> []. lines([Str|Lines]) --> line(Line), { atomic_list_concat(Line,Str) }, lines(Lines). line(["\n"]) --> [Code], { string_codes("\n", [Code]) }. line([Char|Rest]) --> [Code], { string_codes(Char, [Code]) }, line(Rest). readlines(File,Content) :- phrase_from_file(lines(Content),File), !. ``` For a file like file.txt with ```prolog foo bar hello world ``` I get ```prolog ?- readlines('file.txt',Content). Content = ['foo\n', 'bar\n', 'hello\n', 'world\n']. ``` but then I run this on a 26.7MB (\>303k rows csv) data set file and get “Stack limit (1.0Gb) exceeded” after about 7 seconds. The analogous readlines() python function handles the same file with no issue ```prolog import time def readlines_time(): start = time.perf_counter() file = open("health.csv", "r") lines = file.readlines() file.close() print(lines) end = time.perf_counter() elapsed = end - start print(f'Time taken: {elapsed:.6f} seconds') ``` runs in 1.365822 seconds cold. How do I optimize this? ---
**Author:** ![CapelliC](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/capellic/32/17_2.png) [@CapelliC](https://swi-prolog.discourse.group/u/CapelliC)\ **Post date:** [April 14, 2026, 6:50am UTC](https://swi-prolog.discourse.group/t/w-write-mode-not-working/9569/15 "2026-04-14T06:50:44Z")
> [@emummel20](#): > > How do I optimize this? E.g. ```prolog lines([]) --> []. lines([String|Strings]) --> line(Codes), {string_codes(String,Codes)}, !, lines(Strings). line([]) --> "\n". line([Code|Rest]) --> [Code], line(Rest). readlines(File,Strings) :- phrase_from_file(lines(Strings),File). ``` that yields ```prolog 112 ?- time(readlines(large,Ls)). % 16,691,295 inferences, 1.750 CPU in 1.791 seconds (98% CPU, 9537883 Lips) ``` on a 16mb file. But… are you aware of [library(csv)](https://www.swi-prolog.org/pldoc/man?section=csv) ? ---
**Author:** ![jan](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/jan/32/4_2.png) [@jan](https://swi-prolog.discourse.group/u/jan)\ **Post date:** [April 14, 2026, 7:11am UTC](https://swi-prolog.discourse.group/t/w-write-mode-not-working/9569/16 "2026-04-14T07:11:23Z")
> [@CapelliC](#): > > ```prolog > {string_codes(String,Codes)}, > !, lines(Strings) > > ``` At least that gets rid of the choicepoint per line, which avoids the stack overflow. Better yet is to make `line//1` deterministic though by using ``` line([]) --> "\n", !. ``` P.s. The quickest (be it a bit ugly) is probably ``` read_lines(File, Lines) :- read_file_to_string(File, String, []), split_string(String, "\n", "", Lines). ``` See also read\_line\_to\_string/2. But if CSV is the target, library(csv) is probably the best answer. It is fairly efficient and deals with various subtle details in handling various CSV dialects and edge cases. ---
**Author:** ![peter.ludemann](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/peter.ludemann/32/48_2.png) [@peter.ludemann](https://swi-prolog.discourse.group/u/peter.ludemann)\ **Post date:** [April 14, 2026, 9:02am UTC](https://swi-prolog.discourse.group/t/w-write-mode-not-working/9569/17 "2026-04-14T09:02:10Z")
> [@emummel20](#): > > I thought variables in file\_lines([L|Ls]) were unified first. They are unified first but they don’t necessarily become _ground_ first. For example, in `X=Y`, the `X` and `Y` are unified, but they don’t have any value. In `X=Y,Y=1`, because `X` and `Y` are unified, `Y=1` results in `X` having the value 1 also. ---
**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:** [April 14, 2026, 10:07am UTC](https://swi-prolog.discourse.group/t/w-write-mode-not-working/9569/18 "2026-04-14T10:07:45Z")
Yes, please use library(csv) as suggested by Jan if you parse a CSV file. It also has backtracking predicate, csv\_read\_file\_row/3, and also csv\_read\_row/3. You can use those if you don’t need the full content of the file in memory. You can see the example under repeat/0 to see how to process a file with csv\_read\_row/3. If you have a well-defined task it would be easier to provide a good solution. ---
**Author:** ![emummel20](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/emummel20/32/4952_2.png) [@emummel20](https://swi-prolog.discourse.group/u/emummel20)\ **Post date:** [April 14, 2026, 12:10pm UTC](https://swi-prolog.discourse.group/t/w-write-mode-not-working/9569/19 "2026-04-14T12:10:58Z")
Thanks all, really appreciate it. For sure, I’ll check out the csv library but the goal here is to put together a generic utilities library for myself for my work to parse my clients’ data, some of which tends to be structured but not of a well known specification like csv or json. I chose csv in this case at random for testing on a large data set.