Fill a list

?- length(L, 5), maplist(=(foo), L).
L = [foo, foo, foo, foo, foo].

Convert a CSV with column names to JSON

With the help of library(csv) and library(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(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).

Note: versions before 9.3.33 must use library(http/json) instead of library(json).

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
  }
]

Backtrack over lines in a file

file_line(File, Line) :-
    setup_call_cleanup(open(File, read, In),
        stream_line(In, Line),
        close(In)).

stream_line(In, Line) :-
    repeat,
    (   read_line_to_string(In, Line0),
        Line0 \== end_of_file
    ->  Line0 = Line
    ;   !,
        fail
    ).

This will backtrack over consecutive lines in the input. If we use the Unix dictionary words:

?- file_line("/usr/share/dict/words", Word).
Word = "A" ;
Word = "a" ;
Word = "aa" ;
Word = "aal" ; % and so on

This doesn't load all lines to memory, so it can be used with very large files.

See also

- read_line_to_codes/2 to read to a code list instead of a string

- read_string/5 for more control over the separator and stripping