SWI-Prolog -- current_prolog

Convert a CSV with column names to JSON

With the help of library(csv) and library(http/json), we read a CSV file and use the column names in the first row to emit JSON.

For this example, we use the file weather.csv:

city,temp_lo,temp_hi,prcp,date San Francisco,46,50,0.25,1994-11-27 San Francisco,43,57,0,1994-11-29 Hayward,37,54,,1994-11-29

The script csv_to_json.pl reads the CSV from standard input or from the file provided as the first argument of the script. It converts the contents to a list of dicts, then writes this as JSON to standard output:

:- initialization(main, main). :- use_module(library(csv)). :- use_module(library(http/json)). main :- ( current_prolog_flag(argv, [CSV_file|_]) -> csv_read_file(CSV_file, CSV, []) ; csv_read_stream(current_input, CSV, []) ), CSV = [Colnames|Rows], Colnames =.. [row|Names], maplist(row_dict(Names), Rows, Dicts), json_write_dict(current_output, Dicts, [null('')]). row_dict(Names, Row, Dict) :- Row =.. [row|Fields], pairs_keys_values(Data, Names, Fields), dict_create(Dict, _, Data).

The null('') option to json_write_dict/3 is necessary to convert the empty "prcp" field in the last row to a missing value represented as null in the JSON output.

This is how we can use it to convert weather.csv to JSON:

$ swipl csv_to_json.pl weather.csv [ { "city":"San Francisco", "date":"1994-11-27", "prcp":0.25, "temp_hi":50, "temp_lo":46 }, { "city":"San Francisco", "date":"1994-11-29", "prcp":0, "temp_hi":57, "temp_lo":43 }, { "city":"Hayward", "date":"1994-11-29", "prcp":null, "temp_hi":54, "temp_lo":37 } ]

LogicalCaptain said (2020-04-14T04:25:43):

Doc needs help

From the DCG page:

The flags

influence interpretation of "string" and `string` in "standard text".

In DCGs, both "string" and `string` are interpreted as "codes" by default.

Without any settings changed, consider this DCG:

representation(double_quotes)    --> "bar".            % SWI-Prolog decomposes this into CODES
representation(back_quotes)      --> `bar`.            % SWI-Prolog decomposes this into CODES
representation(explicit_codes_1) --> [98,97,114].      % explicit CODES (as obtained via atom_codes(bar,Codes))
representation(explicit_codes_2) --> [0'b,0'a,0'r].    % explicit CODES
representation(explicit_chars)   --> ['b','a','r'].    % explicit CHARS

Which of the above matches codes?

?-
findall(X,
   (atom_codes(bar,Codes),
    phrase(representation(X),Codes,[])),
   Reps).

Reps = [double_quotes,back_quotes,explicit_codes_1,explicit_codes_2].

Which of the above matches chars?

?- findall(X,
   (atom_chars(bar,Chars),phrase(representation(X),Chars,[])),
   Reps).
Reps = [explicit_chars].

This page is difficult to search

"answer_write_options" is somehow lost in between "toplevel_print_factorized" and "toplevel_prompt".
also, the option names are not found when searching and/or have no links that can be used to refer to them: "answer_write_options" -> no matches.
for the next iteration of the website, the flags listed really need their individually addressable URLs.

Make toplevel printer print lists fully (instead of redacted using `...` )

Instead of:

OutColl = [0xE3,0x82,0xA2,0xE3,0x82,0xA4,0xE3,0x82,0xA6|...].

I want

OutColl = [0xE3,0x82,0xA2,0xE3,0x82,0xA4,0xE3,0x82,0xA6,0xE3,0x82,0xA8,0xE3,0x82,0xAA].

And the command is:

(note that you have to set all the options, if you just set max_depth for example you will scratch portray(true) for example, leading to output like

'$VAR'('Suffix')

..this took me some time to find out.

Anyway:

?-
Depth=100,
Options=[quoted(true), portray(true), max_depth(Depth), attributes(portray)],
set_prolog_flag(answer_write_options,Options),
set_prolog_flag(debugger_write_options,Options).

or even unset any depth limit completely by setting Depth to 0 above.

As code:

my_print :-
   Depth=100,
   Options=[quoted(true), portray(true), max_depth(Depth), attributes(portray)],
   set_prolog_flag(answer_write_options,Options),
   set_prolog_flag(debugger_write_options,Options).
   
:- my_print.

TODO: Write a predicate to create a new options list with max_depth(Depth) only changed.

Actually, if this follows "options list semantics", just prepending max_depth(100) to the existing list should upgrade the depth. Try it:

?- current_prolog_flag(answer_write_options,Options),NewOptions=[max_depth(200)|Options],set_prolog_flag(answer_write_options,NewOptions).
Options = [quoted(true),portray(true),max_depth(100),attributes(portray)],
NewOptions = [max_depth(200),quoted(true),portray(true),max_depth(100),attributes(portray)].

?- current_prolog_flag(answer_write_options,Options).
Options = [max_depth(200),quoted(true),portray(true),max_depth(100),attributes(portray)].

The multiple values are retained... Not sure whether this is a bug.

Consider adding the instructions to your init.pl ($HOME/.config/swi-prolog/init.pl) file so that it is run at startup.

`home` is Prolog's "home", not the user's "home"

This is not the user's home directory:

?- current_prolog_flag(home, X).
X = '/usr/local/logic/swipl/lib/swipl'.

Use this instead:

?- expand_file_name("~", Home).
Home = ['/home/paquette'].

Use file_search_path/2 for others:

bagof(X,file_search_path(user_app_data,X),UAD),
bagof(X,file_search_path(common_app_data,X),CAD),
bagof(X,file_search_path(user_app_config,X),UAC),
(file_search_path(common_app_config,_) -> bagof(X,file_search_path(common_app_config,X),CAC); CAC = []),
bagof(X,file_search_path(app_data,X),AD),
bagof(X,file_search_path(app_config,X),AC).

UAD = ['/home/paquette/.local/share/swi-prolog'],
CAD = ['/usr/local/share/swi-prolog', '/usr/share/swi-prolog', '/etc/xdg/swi-prolog'],
UAC = ['/home/paquette/.config/swi-prolog'],
CAC = [],
AD = [user_app_data('.'), common_app_data('.')],
AC = [user_app_config('.'), common_app_config('.')].

Anniepoo said (2020-10-14T04:37:24):

the toplevel_extra_white_line flag is missing

abathologist said (2019-05-12T06:20:42):

Since only some flags are module local, we should be careful to document when a flag is so. E.g., under the double_quotes flag, it says

This flag determines how double quoted strings are read by Prolog and is ---like character_escapes and back_quotes--- maintained for each module

but this fact isn't mentioned in the documentation for back_quotes itself.

It might be good to just include a note on every flag, saying whether it is global or local to a module.

`~`m	Type in module if not `user` (see module/1)
`~`l	Break level if not 0 (see break/0)
`~`d	Debugging state if not normal execution (see debug/0, trace/0)
`~`!	History event if history is enabled (see flag history)

Examples

Convert a CSV with column names to JSON

Doc needs help

This page is difficult to search

Make toplevel printer print lists fully (instead of redacted using ... )

home is Prolog's "home", not the user's "home"

Make toplevel printer print lists fully (instead of redacted using `...` )

`home` is Prolog's "home", not the user's "home"