I'm currently trying to learn some Prolog (using ECLiPSe). From time to time I come across the clause/2 predicate, but I fail to understand what it is used for. I read some references (e.g. this one), but I still don't get in what case it could be useful. Can someone provide me with an easy example or explanation of this?
This predicate allows metaprogramming, i.e. reasoning about your Prolog program.
SWI-Prolog uses clause/2 in, a.o., the explain predicate:
?- explain(member).
"member" is an atom
Referenced from 12-th clause of pce_meta:pce_to_pl_type/3
lists:member/2 is a predicate defined in
c:/program files/swi-prolog/library/lists.pl:81
Referenced from 1-th clause of prolog_dialect:source_exports/2
Referenced from 1-th clause of pce_config:term_description/2
Referenced from 1-th clause of pce_config:config_attribute/2
Referenced from 1-th clause of pce_config:load_config_key/2
Referenced from 1-th clause of pce_config:term_description/3
Referenced from 1-th clause of pce_config:current_config_path/1
Referenced from 4-th clause of persistent_frame:has_specifier/1
true.
and in the implementation of Constraint Handling Rules. I suspect it's also useful for doing inductive logic programming and various other Prolog extensions.
For a thorough introduction to metaprogramming in Prolog, see The Art of Prolog by Sterling and Shapiro.