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<H2><A NAME="sec:4.14"><SPAN class="sec-nr">4.14</SPAN> <SPAN class="sec-title">Declaring 
predicates properties</SPAN></A></H2>

<A NAME="ch:dynamic"></A>
<A NAME="sec:declare"></A>

<P>This section describes directives which manipulate attributes of 
predicate definitions. The functors <A NAME="idx:dynamic1:630"></A><A class="pred" href="dynamic.html#dynamic/1">dynamic/1</A>, <A NAME="idx:multifile1:631"></A><A class="pred" href="dynamic.html#multifile/1">multifile/1</A> 
and
<A NAME="idx:discontiguous1:632"></A><A class="pred" href="dynamic.html#discontiguous/1">discontiguous/1</A> 
are operators of priority 1150 (see <A NAME="idx:op3:633"></A><A class="pred" href="operators.html#op/3">op/3</A>), 
which implies the list of predicates they involve can just be a comma 
separated list:

<PRE class="code">
:- dynamic
        foo/0, 
        baz/2.
</PRE>

<P>On SWI-Prolog all these directives are just predicates. This implies 
they can also be called by a program. Do not rely on this feature if you 
want to maintain portability to other Prolog implementations.

<DL>
<DT class="pubdef"><A NAME="dynamic/1"><STRONG>dynamic</STRONG> <VAR>+PredicateIndicator, \ldots</VAR></A></DT>
<DD class="defbody">
Informs the interpreter that the definition of the predicate(s) may 
change during execution (using <A NAME="idx:assert1:634"></A><A class="pred" href="db.html#assert/1">assert/1</A> 
and/or <A NAME="idx:retract1:635"></A><A class="pred" href="db.html#retract/1">retract/1</A>). 
In the multi-threaded version, the clauses of dynamic predicates are 
shared between the threads. The directive <A NAME="idx:threadlocal1:636"></A><A class="pred" href="threadcom.html#thread_local/1">thread_local/1</A> 
provides an alternative where each threads has its own clause-list for 
the predicate. Dynamic predicates can be turned into static ones using
<A NAME="idx:compilepredicates1:637"></A><A class="pred" href="dynamic.html#compile_predicates/1">compile_predicates/1</A>.</DD>
<DT class="pubdef"><A NAME="compile_predicates/1"><STRONG>compile_predicates</STRONG>(<VAR>:ListOfNameArity</VAR>)</A></DT>
<DD class="defbody">
Compile a list of specified dynamic predicates (see <A NAME="idx:dynamic1:638"></A><A class="pred" href="dynamic.html#dynamic/1">dynamic/1</A> 
and
<A NAME="idx:assert1:639"></A><A class="pred" href="db.html#assert/1">assert/1</A>) 
into normal static predicates. This call tells the Prolog environment 
the definition will not change anymore and further calls to <A NAME="idx:assert1:640"></A><A class="pred" href="db.html#assert/1">assert/1</A> 
or <A NAME="idx:retract1:641"></A><A class="pred" href="db.html#retract/1">retract/1</A> 
on the named predicates raise a permission error. This predicate is 
designed to deal with parts of the program that is generated at runtime 
but does not change during the remainder of the program execution.<SUP class="fn">35<SPAN class="fn-text">The 
specification of this predicate is from Richard O'Keefe. The 
implementation is allowed to optimise the predicate. This is not yet 
implemented. In multi-threaded Prolog however, static code runs faster 
as it does not require synchronisation. This is particularly true on SMP 
hardware.</SPAN></SUP></DD>
<DT class="pubdef"><A NAME="multifile/1"><STRONG>multifile</STRONG> <VAR>+PredicateIndicator, \ldots</VAR></A></DT>
<DD class="defbody">
Informs the system that the specified predicate(s) may be defined over 
more than one file. This stops <A NAME="idx:consult1:642"></A><A class="pred" href="consulting.html#consult/1">consult/1</A> 
from redefining a predicate when a new definition is found.</DD>
<DT class="pubdef"><A NAME="discontiguous/1"><STRONG>discontiguous</STRONG> <VAR>+PredicateIndicator, \ldots</VAR></A></DT>
<DD class="defbody">
Informs the system that the clauses of the specified predicate(s) might 
not be together in the source file. See also <A NAME="idx:stylecheck1:643"></A><A class="pred" href="debugger.html#style_check/1">style_check/1</A>.</DD>
<DT class="pubdef"><A NAME="index/1"><STRONG>index</STRONG>(<VAR>+Head</VAR>)</A></DT>
<DD class="defbody">
Index the clauses of the predicate with the same name and arity as
<VAR>Head</VAR> on the specified arguments. <VAR>Head</VAR> is a term of 
which all arguments are either `1' (denoting `index this argument') or 
`0' (denoting `do not index this argument'). Indexing has no 
implications for the semantics of a predicate, only on its performance. 
If indexing is enabled on a predicate a special purpose algorithm is 
used to select candidate clauses based on the actual arguments of the 
goal. This algorithm checks whether indexed arguments might unify in the 
clause head. Only atoms, integers and compound terms are considered. 
Compound terms are indexed on the combination of their name and arity. 
Indexing is very useful for predicates with many clauses representing 
facts.

<P>Due to the representation technique used at most 4 arguments can be 
indexed. All indexed arguments should be in the first 32 arguments of 
the predicate. If more than 4 arguments are specified for indexing only 
the first 4 will be accepted. Arguments above 32 are ignored for 
indexing.

<P>Indexing as specified by this predicate uses a quick but linear scan. 
Without explicit specification the system uses an algorithm depending on 
the structure of the first argument and number of clauses, In 
particular, for predicates that can be indexed on the first argument and 
have many clauses, the system will use an automatically resizing 
hash-table to provide access time independent from the number of 
clauses.<SUP class="fn">36<SPAN class="fn-text">SWI-Prolog indexing is 
no longer state-of-the-art. Better schemas for multi-argument as well as 
indexing <EM>inside</EM> compound terms are known. We hope to integrate 
this in future versions.</SPAN></SUP> If---for example---one wants to 
represents sub-types using a fact list `sub_type(Sub, Super)' that 
should be used both to determine sub- and super types one should declare sub_type/2 
as follows:

<PRE class="code">
:- index(sub_type(1, 1)).

sub_type(horse, animal).
...
...
</PRE>

<P>Note that this type of indexing makes selecting clauses much faster 
but remains <EM>linear</EM> with respect to the number of clauses, while 
hashing as described with <A NAME="idx:hash1:644"></A><A class="pred" href="dynamic.html#hash/1">hash/1</A> 
provides constant access time. See also <A NAME="idx:hash1:645"></A><A class="pred" href="dynamic.html#hash/1">hash/1</A> 
and <A NAME="idx:termhash2:646"></A><A class="pred" href="db.html#term_hash/2">term_hash/2</A>.</DD>
<DT class="pubdef"><A NAME="hash/1"><STRONG>hash</STRONG>(<VAR>+Head</VAR>)</A></DT>
<DD class="defbody">
Index the given predicate by hashing on the first argument. This is done 
by default on any predicate with more than 5 clauses having a first 
argument that can be indexed and at most two that can not be indexed. On 
dynamic predicates the hash-table is resized as the number of clauses 
grows, providing roughly constant-time access regardless of the number 
of clauses predicates that can be indexed on the first argument. See 
also <A NAME="idx:index1:647"></A><A class="pred" href="dynamic.html#index/1">index/1</A>, <A NAME="idx:termhash2:648"></A><A class="pred" href="db.html#term_hash/2">term_hash/2</A> 
and <A NAME="idx:predicateproperty2:649"></A><A class="pred" href="examineprog.html#predicate_property/2">predicate_property/2</A>.

<P></DD>
</DL>

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