SWI-Prolog offers three different database mechanisms. The first one
is the common assert/retract mechanism for manipulating the clause
database. As facts and clauses asserted using assert/1
or one of its derivatives become part of the program these predicates
compile the term given to them. retract/1
and retractall/1
have to unify a term and therefore have to decompile the program. For
these reasons the assert/retract mechanism is expensive. On the other
hand, once compiled, queries to the database are faster than querying
the recorded database discussed below. See also dynamic/1.
The second way of storing arbitrary terms in the database is using
the ``recorded database''. In this database terms are associated with a
key. A key can be an atom, small integer or term. In the last
case only the functor and arity determine the key. Each key has a chain
of terms associated with it. New terms can be added either at the head
or at the tail of this chain.
The third mechanism is a special purpose one. It associates an
integer or atom with a key, which is an atom, integer or term. Each key
can only have one atom or integer associated with it.
- [ISO]abolish(:PredicateIndicator)
-
Removes all clauses of a predicate with functor Functor and
arity
Arity from the database. All predicate attributes (dynamic,
multifile, index, etc.) are reset to their defaults. Abolishing an
imported predicate only removes the import link; the predicate will keep
its old definition in its definition module.
According to the ISO standard, abolish/1
can only be applied to dynamic procedures. This is odd, as for dealing
with dynamic procedures there is already retract/1
and retractall/1.
The abolish/1
predicate has been introduced in DEC-10 Prolog precisely for dealing
with static procedures. In SWI-Prolog, abolish/1
works on static procedures, unless the prolog flag iso
is set to true.
It is advised to use retractall/1
for erasing all clauses of a dynamic predicate.
- abolish(+Name,
+Arity)
-
Same as
abolish(Name/Arity). The predicate abolish/2
conforms to the Edinburgh standard, while abolish/1
is ISO compliant.
- redefine_system_predicate(+Head)
-
This directive may be used both in module
user and in
normal modules to redefine any system predicate. If the system
definition is redefined in module user, the new definition
is the default definition for all sub-modules. Otherwise the
redefinition is local to the module. The system definition remains in
the module system.
Redefining system predicate facilitates the definition of
compatibility packages. Use in other context is discouraged.
- [ISO]retract(+Term)
-
When Term is an atom or a term it is unified with the first
unifying fact or clause in the database. The fact or clause is removed
from the database.
- retractall(+Head)
-
All facts or clauses in the database for which the head
unifies with Head are removed.
- assert(+Term)
-
Assert a fact or clause in the database. Term is asserted as
the last fact or clause of the corresponding predicate.
- [ISO]asserta(+Term)
-
Equivalent to assert/1,
but Term is asserted as first clause or fact of the
predicate.
- [ISO]assertz(+Term)
-
Equivalent to assert/1.
- assert(+Term,
-Reference)
-
Equivalent to assert/1,
but Reference is unified with a unique reference to the
asserted clause. This key can later be used with
clause/3
or erase/1.
- asserta(+Term,
-Reference)
-
Equivalent to assert/2,
but Term is asserted as first clause or fact of the
predicate.
- assertz(+Term,
-Reference)
-
Equivalent to assert/2.
- recorda(+Key,
+Term, -Reference)
-
Assert Term in the recorded database under key Key.
Key is a small integer (range min_tagged_integer
...max_tagged_integer,
atom or compound term. If the key is a compound term, only the name and
arity define the key.
Reference is unified with a unique reference to the record
(see
erase/1).
- recorda(+Key,
+Term)
-
Equivalent to
recorda(Key, Value, _).
- recordz(+Key,
+Term, -Reference)
-
Equivalent to recorda/3,
but puts the Term at the tail of the terms recorded under Key.
- recordz(+Key,
+Term)
-
Equivalent to
recordz(Key, Value, _).
- recorded(+Key,
-Value, -Reference)
-
Unify Value with the first term recorded under Key
which does unify. Reference is unified with the memory
location of the record.
- recorded(+Key,
-Value)
-
Equivalent to
recorded(Key, Value, _).
- erase(+Reference)
-
Erase a record or clause from the database. Reference is an
integer returned by recorda/3
or recorded/3, clause/3, assert/2,
asserta/2
or assertz/2.
Other integers might conflict with the internal consistency of the
system. Erase can only be called once on a record or clause. A second
call also might conflict with the internal consistency of the system.bugThe
system should have a special type for pointers, thus avoiding the Prolog
user having to worry about consistency matters. Currently some simple
heuristics are used to determine whether a reference is valid.
- flag(+Key,
-Old, +New)
-
Key is an atom, integer or term. As with the recorded
database, if
Key is a term, only the name and arity are used to locate the
flag. Unify Old with the old value associated with Key.
If the key is used for the first time Old is unified with the
integer 0. Then store the value of New, which should be an
integer, float, atom or arithmetic expression, under Key. flag/3
is a fast mechanism for storing simple facts in the database. The flag
database is shared between threads and updates are atomic, making it
suitable for generating unique integer counters.32The flag/3
predicate is not portable. Non-backtrackable global variables (nb_setval/2)
and non-backtrackable assignment (nb_setarg/3)
are more widely recognised special-purpose alternatives for
non-backtrackable and/or global state.
Traditionally,
Prolog systems used the immediate update view: new clauses
became visible to predicates backtracking over dynamic predicates
immediately and retracted clauses became invisible immediately.
Starting with SWI-Prolog 3.3.0 we adhere the logical update view,
where backtrackable predicates that enter the definition of a predicate
will not see any changes (either caused by assert/1
or
retract/1)
to the predicate. This view is the ISO standard, the most commonly used
and the most `safe'.33For example,
using the immediate update view, no call to a dynamic predicate is
deterministic. Logical updates are realised by keeping
reference-counts on predicates and generation information on
clauses. Each change to the database causes an increment of the
generation of the database. Each goal is tagged with the generation in
which it was started. Each clause is flagged with the generation it was
created as well as the generation it was erased. Only clauses with
`created' ... `erased' interval that encloses the generation of the
current goal are considered visible.
By default, SWI-Prolog, as most other implementations, indexes
predicates on their first argument. SWI-Prolog allows indexing on other
and multiple arguments using the declaration index/1.
Dedicated index schemas can be build using term_hash/2
or term_hash/4.
- [det]term_hash(+Term,
-HashKey)
-
If Term is a ground term (see ground/1), HashKey
is unified with a positive integer value that may be used as a hash-key
to the value. If Term is not ground, the predicate leaves HashKey
an unbound variable. Hash keys are in the range 0 ... 16,777,215,
the maximal integer that can be stored efficiently on both 32 and 64 bit
platforms.
This predicate may be used to build hash-tables as well as to exploit
argument-indexing to find complex terms more quickly.
The hash-key does not rely on temporary information like addresses of
atoms and may be assumed constant over different invocations and
versions of SWI-Prolog.34Last
change: version 5.6.53 The
term_hash/2
predicate is cycle-safe. Hashes for numbers differ between bit and
little endian machines.
- [det]term_hash(+Term,
+Depth, +Range, -HashKey)
-
As term_hash/2,
but only considers Term to the specified
Depth. The toplevel term has depth 1, its arguments have
depth 2, etc. I.e. Depth = 0 hashes nothing; Depth
= 1 hashes atomic values or the functor and arity of a compound
term, not its arguments; Depth = 2 also indexes
the immediate arguments, etc. Using Depth = -1
makes term_hash/4
behave as term_hash/2,
hashing ground terms to the full depth.
HashKey is in the range [0 ...Range-1]. Range
must be in the range [1 ... 2147483647]
