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Predicate ->/2
Availability:built-in
Source[ISO]:Condition -> :Action
If-then and If-Then-Else. The ->/2 construct commits to the choices made at its left-hand side, destroying choice points created inside the clause (by ;/2), or by goals called by this clause. Unlike !/0, the choice point of the predicate as a whole (due to multiple clauses) is not destroyed. Disregarding the interaction with !/0, the combination ;/2 and ->/2 acts as if defined as: 
If -> Then; _Else :- If, !, Then.
If -> _Then; Else :- !, Else.
If -> Then :- If, !, Then.

Please note that (If -> Then) acts as (If -> Then ; fail), making the construct fail if the condition fails. This unusual semantics is part of the ISO and all de-facto Prolog standards.

Please note that (if->then;else) is read as ((if->then);else) and that the combined semantics of this syntactic construct as defined above is different from the simple nesting of the two individual constructs, i.e., the semantics of ->/2 changes when embedded in ;/2. See also once/1.

As with ;/2, this construct is always nested in parenthesis. Here is an example of the preferred layout for SWI-Prolog. 

p :-
    a,
    (   b,
        c
    ->  d,
        e
    ;   f
    ->  g
    ;   h
    ).

Examples

Convert a CSV with column names to JSON

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
  }
]
Backtrack over lines in a file

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
Convert a string of 0s and 1s to an integer

Convert a string of 0s and 1s to an integer

This shows how to convert almost any "string" of 0s and 1s to an integer.

It takes advantage of the built-in term reading, thus avoiding an explicit loop and arithmetic.

binary_string_to_int(Str, Int) :-
    (   ( string(Str) ; atom(Str) )
    ->  atomics_to_string(["0b", Str], Literal)
    ;   is_list(Str)
    ->  atomics_to_string(["0b"|Str], Literal)
    ;   type_error(string_type, Str)
    ),
    catch(term_string(Int, Literal, []),
          error(syntax_error(_), _),
          type_error(string_of_0s_and_1s, Str)).
?- binary_string_to_int('101', X).
X = 5.

?- binary_string_to_int([0, 0, 1, 0, 1], X).
X = 5.

?- binary_string_to_int(["1", "1", "0"], X).
X = 6.
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