SWI-Prolog
supports rational trees, also known as cyclic terms.‘Supports’is
so defined that most relevant built-in predicates terminate when faced
with rational trees. Almost all SWI-Prolog's built-in term manipulation
predicates process terms in a time that is linear to the amount of
memory used to represent the term on the stack. The following set of
predicates safely handles rational trees:
=../2,
==/2,
=@=/2,
=/2,
@</2,
@=</2,
@>=/2,
@>/2,
\==/2,
\=@=/2,
\=/2,
acyclic_term/1,
bagof/3,
compare/3,
copy_term/2,
cyclic_term/1,
dif/2,
duplicate_term/2,
findall/3,
ground/1,
term_hash/2,
numbervars/3,
numbervars/4,
recorda/3,
recordz/3,
setof/3,
subsumes_term/2,
term_variables/2,
throw/1,
unify_with_occurs_check/2,
unifiable/3,
when/2,
write/1
(and related predicates) .
In addition, some built-ins recognise rational trees and raise an
appropriate exception. Arithmetic evaluation belongs to this group. The
compiler (asserta/1,
etc.) also raises an exception. Future versions may support rational
trees. Predicates that could provide meaningful processing of rational
trees raise a representation_error. Predicates for which
rational trees have no meaningful interpretation raise a type_error.
For example:
1 ?- A = f(A), asserta(a(A)).
ERROR: asserta/1: Cannot represent due to `cyclic_term'
2 ?- A = 1+A, B is A.
ERROR: is/2: Type error: `expression' expected, found
`@(S_1,[S_1=1+S_1])' (cyclic term)
