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\section{library(gensym): Generate unique symbols}

\label{sec:gensym}

Gensym (\textit{Generate Symbols}) is an old library for generating unique
symbols (atoms). Such symbols are generated from a base atom which gets
a sequence number appended. Of course there is no guarantee that
\const{catch22} is not an already defined atom and therefore one must be aware
these atoms are only unique in an isolated context.

The SWI-Prolog gensym library is thread-safe. The sequence numbers are
global over all threads and therefore generated atoms are unique over
all threads.\vspace{0.7cm}

\begin{description}
    \predicate{gensym}{2}{+Base, -Unique}
Generate $<$\arg{Base}$>$1, $<$\arg{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
\predref{gensym}{2} is thread-safe in the sense that two threads generating
identifiers from the same \arg{Base} will never generate the same
identifier.

\begin{tags}
    \tag{See also}
\predref{uuid}{1}, \predref{term_hash}{2}, \predref{variant_sha1}{2} may be used to
generate various unique or content-based identifiers
safely.
\end{tags}

    \predicate{reset_gensym}{0}{}
Reset gensym for all registered keys. This predicate is available
for compatibility only. New code is strongly advised to avoid the
use of reset_gensym or at least to reset only the keys used by your
program to avoid unexpected side effects on other components.

    \predicate{reset_gensym}{1}{+Base}
Restart generation of identifiers from \arg{Base} at $<$\arg{Base}$>$1. Used to make
sure a program produces the same results on subsequent runs. Use
with care.
\end{description}