A.18 library(gensym): Generate unique symbols

Gensym (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 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.

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.

reset_gensym

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.

reset_gensym(+Base)

Restart generation of identifiers from Base at <Base>1. Used to make sure a program produces the same results on subsequent runs. Use with care.