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Predicate read_string/5
Availability:built-in
read_string(+Stream, +SepChars, +PadChars, -Sep, -String)
Read a string from Stream, providing functionality similar to split_string/4. The predicate performs the following steps:

  1. Skip all characters that match PadChars
  2. Read up to a character that matches SepChars or end of file
  3. Discard trailing characters that match PadChars from the collected input
  4. Unify String with a string created from the input and Sep with the code of the separator character read. If input was terminated by the end of the input, Sep is unified with -1.

The predicate read_string/5 called repeatedly on an input until Sep is -1 (end of file) is equivalent to reading the entire file into a string and calling split_string/4, provided that SepChars and PadChars are not partially overlapping.171Behaviour that is fully compatible would require unlimited look-ahead. Below are some examples:

Read a line: 

read_string(Input, "\n", "\r", Sep, String)

Read a line, stripping leading and trailing white space: 

read_string(Input, "\n", "\r\t ", Sep, String)

Read up to‘,’or‘)’, unifying Sep with 0', i.e. Unicode 44, or 0'), i.e. Unicode 41: 

read_string(Input, ",)", "\t ", Sep, String)

Examples

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
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LogicalCaptain said (2020-12-10T23:21:20):0 upvotes 0 0 downvotes
Picture of user LogicalCaptain. 
?-
open_string("Hello@",Input),
read_string(Input, ",@", "\t ", End, String)

Input = <stream>(0x1ca11d0),
End = 41,
String = "Hello".

Reading user input:

?- read_string(user_input, "\n", "\t ", End, String).
|: Hello
End = 10,
String = "Hello".