1/*  Part of SWI-Prolog
    2
    3    Author:        Jan Wielemaker
    4    E-mail:        J.Wielemaker@vu.nl
    5    WWW:           http://www.swi-prolog.org
    6    Copyright (c)  1985-2013, University of Amsterdam
    7                              VU University Amsterdam
    8    All rights reserved.
    9
   10    Redistribution and use in source and binary forms, with or without
   11    modification, are permitted provided that the following conditions
   12    are met:
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   14    1. Redistributions of source code must retain the above copyright
   15       notice, this list of conditions and the following disclaimer.
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   17    2. Redistributions in binary form must reproduce the above copyright
   18       notice, this list of conditions and the following disclaimer in
   19       the documentation and/or other materials provided with the
   20       distribution.
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   22    THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
   23    "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
   24    LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
   25    FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
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   34*/
   35
   36:- module(gensym,
   37        [ reset_gensym/0,
   38          reset_gensym/1,               % +Base
   39          gensym/2                      % +Base, -Symbol
   40        ]).   41:- set_prolog_flag(generate_debug_info, false).

 gensym(+Base, -Unique)

Generate <Base>1, <Base>2, etc atoms on each subsequent call. Note that there is nothing that prevents other parts of the application to `invent' the same identifier. The predicate gensym/2 is thread-safe in the sense that two threads generating identifiers from the same Base will never generate the same identifier.

See also

- uuid/1, term_hash/2, variant_sha1/2 may be used to generate various unique or content-based identifiers safely.

   62gensym(Base, Atom) :-
   63    atom_concat('$gs_', Base, Key),
   64    flag(Key, Old, Old+1),
   65    record_gensym(Key, Old),
   66    New is Old+1,
   67    atom_concat(Base, New, Atom).
   68
   69record_gensym(Key, 0) :-
   70    !,
   71    recordz('$gensym', Key).
   72record_gensym(_, _).

 reset_gensym

Reset all gensym counters. Please beware this is dangerous: gensym may be in use by other modules that do not expect their counter to be reset!

   80reset_gensym :-
   81    with_mutex('$gensym', do_reset_gensym).
   82
   83do_reset_gensym :-
   84    (   recorded('$gensym', Key, Ref),
   85        erase(Ref),
   86        set_flag(Key, 0),
   87        fail
   88    ;   true
   89    ).

 reset_gensym(+Base)

Reset a specific gensym counter. Please beware this still is dangerous as other code may use gensym with the same atom!

   96reset_gensym(Base) :-
   97    atom_concat('$gs_', Base, Key),
   98    set_flag(Key, 0).
   99
  100:- multifile sandbox:safe_primitive/1.  101
  102sandbox:safe_primitive(gensym:gensym(_,_))