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Predicate read/1
Availability:built-in
[ISO]read(-Term)
Read the next Prolog term from the current input stream and unify it with Term. On reaching end-of-file Term is unified with the atom end_of_file. This is the same as read_term/2 using an empty option list.

[NOTE] You might have found this while looking for a predicate to read input from a file or the user. Quite likely this is not what you need in this case. This predicate is for reading a Prolog term which may span multiple lines and must end in a full stop (dot character followed by a layout character). The predicates for reading and writing Prolog terms are particularly useful for storing Prolog data in a file or transferring them over a network communication channel (socket) to another Prolog process. The libraries provide a wealth of predicates to read data in other formats. See e.g., library(readutil), library(pure_input) or libraries from the extension packages to read XML, JSON, YAML, etc.

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LogicalCaptain said (2021-02-10T09:00:13):0 upvotes 0 0 downvotes
Picture of user LogicalCaptain.

Note

This corresponds to "serialization/deserialization" in Java and other languages.

You CAN but shouldn't use this for reading input typed in by the user.

Example:

?-
TermOut = a(b,c,[1,2,3],var(ZZ)),
with_output_to(
   string(Buf),
   write_term(TermOut,[])),             % serialize "TermOut" out to "Buf"
read_term_from_atom(Buf, TermIn, []).   % unserialize "TermIn" from "Buf"

Result:

TermOut = a(b,c,[1,2,3],var(ZZ)),
Buf = "a(b,c,[1,2,3],var(_8700))",
TermIn = a(b,c,[1,2,3],var(_9646)).

When asking the user

Use library(readutil) for more freewheeling I/O like user querying, using read_line_to_string/2.

Example:

?-
format("Answer me, Domon!!!!\n"),
read_line_to_string(user_input,S1),
string_lower(S1,S2),
(member(S2,["yes","1","ok","y","ja","oui"])
   -> format("OK!")
   ; (format("NOK"), fail)).

Then:

Answer me, Domon!!!!
|: YES
OK!
S1 = "YES",
S2 = "yes".

Here is example code for reading terms from a file

:- use_module(library(filesex)).

% I'm using strings wherever there is supposed to be "text whose only meaning is itself"

perform_catch(Terms,Encoding) :-
   catch(perform(Terms,Encoding),ExceptionTerm,(format(user_error,"~q",[ExceptionTerm]),fail)).
   
perform(Terms,Encoding) :-
   WhereDir = "/home/user/",
   directory_file_path(WhereDir,"prepacked",PrepackedDir),
   directory_file_path(PrepackedDir,"onepointfour",D1), % directory_file_path has bugs so go step by step here
   directory_file_path(D1,"basics",D2), % directory_file_path has bugs so go step by step here
   directory_file_path(D2,"pack.pl",File),
   exists_file_or_throw(File),
   open_or_throw(File,Stream,Encoding),
   assertion(is_stream(Stream)),
   % Exceptions due to syntax problems are left to bubble "up".
   % If this happens in an interactive sessions, the tracer is triggered
   % (unless you did "set_prolog_flag(debug_on_error,false)") and stops
   % right before the cleanup goal
   setup_call_cleanup(
      true,
      slurp_terms(Stream,Terms),
      (format(user_error,"Closing ~q~n",[File]),close(Stream))),
   assertion(\+is_stream(Stream)).
   
% ---

slurp_terms(Stream,Terms) :-
   read_term(Stream,Term,[]),  % will throw on syntax error; unifies Term with "end_of_file" at EOF
   slurp_terms_again(Term,Stream,Terms).

% ---

slurp_terms_again(end_of_file,_Stream,[]) :- !.
slurp_terms_again(Term,Stream,[Term|Terms]) :-
   slurp_terms(Stream,Terms).

% ---

open_or_throw(File,Stream,Encoding) :-
   open(File,read,Stream,[encoding(Encoding)]),
   !.
   
open_or_throw(File,_Stream,_Encoding) :-
   domain_error("Opening file for reading failed",File). % but how do I know WHY it failed?

% ---

exists_file_or_throw(File) :-
   exists_file(File),!.
   
exists_file_or_throw(File) :-
   domain_error("No such file",File). % domain error is as good as any
Wouter Beek said (2017-10-28T13:40:36):0 upvotes 0 0 downvotes
Picture of user Wouter Beek.
This requires the last two characters to be dot and newline (in that order). When leaving out the dot the input prompt will not go away. The dot is not part of the read term (nor is the newline).