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Dynamic clause management
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<H3 CLASS="subsection"><A NAME="htoc91">7.7</A> Dynamic clause management</H3><UL>
<LI><A HREF="gprolog030.html#toc63">Introduction</A>
<LI><A HREF="gprolog030.html#toc64"><TT>asserta/1</TT>,
<TT>assertz/1</TT></A>
<LI><A HREF="gprolog030.html#toc65"><TT>retract/1</TT></A>
<LI><A HREF="gprolog030.html#toc66"><TT>retractall/1</TT></A>
<LI><A HREF="gprolog030.html#toc67"><TT>clause/2</TT></A>
<LI><A HREF="gprolog030.html#toc68"><TT>abolish/1</TT></A>
</UL>
<A NAME="toc63"></A>
<H4 CLASS="subsubsection"><A NAME="htoc92">7.7.1</A> Introduction</H4>
<A NAME="Introduction:(Dynamic-clause-management)"></A>
<B>Static and dynamic procedures</B>: a procedure is either dynamic or
static. All built-in predicates are static. A user-defined procedure is
static by default unless a <TT>dynamic/1</TT> directive precedes its
definition (section <A HREF="gprolog021.html#dynamic/1">6.1.2</A>). Adding a clause to a non-existent procedure
creates a dynamic procedure. The clauses of a dynamic procedure can be
altered (e.g. using <TT>asserta/1</TT>), the clauses of a static procedure
cannot be altered. <BR>
<BR>
<B>Private and public procedures</B>: each procedure is either public or
private. A dynamic procedure is always public. Each built-in predicate is
private, and a static user-defined procedure is private by default unless a
<TT>public/1</TT> directive precedes its definition (section <A HREF="gprolog021.html#public/1">6.1.3</A>). If a
dynamic declaration exists it is unnecessary to add a public declaration
since a dynamic procedure is also public. A clause of a public procedure can
be inspected (e.g. using <TT>clause/2</TT>), a clause of a private procedure
cannot be inspected.<BR>
<BR>
<B>A logical database update view</B>: any change in the database that
occurs as the result of executing a goal (e.g. when a sub-goal is a call of
<TT>assertz/1</TT> or <TT>retract/1</TT>) only affects subsequent
activations. The change does not affect any activation that is currently
being executed. Thus the database is frozen during the execution of a goal,
and the list of clauses defining a predication is fixed at the moment of its
execution.<BR>
<BR>
<A NAME="toc64"></A>
<H4 CLASS="subsubsection"><A NAME="htoc93">7.7.2</A> <TT>asserta/1</TT>,
<TT>assertz/1</TT></H4>
<B>Templates</B>
<DL CLASS="list" COMPACT="compact"><DT CLASS="dt-list"><DD CLASS="dd-list"><TT>
asserta(+clause)<BR>
assertz(+clause)</TT></DL>
<B>Description</B><BR>
<BR>
<TT>asserta(Clause)</TT> first converts the term <TT>Clause</TT> to a clause
and then adds it to the current internal database. The predicate concerned
must be dynamic (section <A HREF="#Introduction:(Dynamic-clause-management)">7.7.1</A>) or
undefined and the clause is inserted before the first clause of the
predicate. If the predicated is undefined it is created as a dynamic
procedure.<BR>
<BR>
<TT>assertz(Clause)</TT> acts like <TT>asserta/1</TT> except that the clause
is added at the end of all existing clauses of the concerned predicate.<BR>
<BR>
<B>Converting a term <TT>Clause</TT> to a clause <TT>Clause1</TT>:</B>
<UL CLASS="itemize"><LI CLASS="li-itemize">extract the head and the body of <TT>Clause</TT>: either
<TT>Clause</TT> = <TT>(Head :- Body)</TT> or <TT>Clause</TT> = <TT>Head</TT>
and <TT>Body</TT> = <TT>true</TT>.<BR>
<BR>
<LI CLASS="li-itemize"><TT>Head</TT> must be a callable term (or else the conversion fails).<BR>
<BR>
<LI CLASS="li-itemize">convert <TT>Body</TT> to a body clause (i.e. a goal) <TT>Body1</TT>.<BR>
<BR>
<LI CLASS="li-itemize">the converted clause <TT>Clause1</TT> = <TT>(Head :- Body1)</TT>.</UL>
<B>Converting a term <TT>T</TT> to a goal:</B>
<UL CLASS="itemize"><LI CLASS="li-itemize">if <TT>T</TT> is a variable it is replaced by the term
<TT>call(T)</TT>.<BR>
<BR>
<LI CLASS="li-itemize">if <TT>T</TT> is a control construct <TT>(',')/2</TT>, <TT>(;)/2</TT>
or <TT>(->)/2</TT> each argument of the control construct is recursively
converted to a goal.<BR>
<BR>
<LI CLASS="li-itemize">if <TT>T</TT> is a callable term it remains unchanged.<BR>
<BR>
<LI CLASS="li-itemize">otherwise the conversion fails (<TT>T</TT> is neither a variable nor a
callable term).</UL>
<B>Errors</B><BR>
<TABLE CELLSPACING=2 CELLPADDING=0>
<TR><TD BGCOLOR=black COLSPAN=3><TABLE BORDER=0 WIDTH="100%" CELLSPACING=0 CELLPADDING=1><TR><TD></TD></TR></TABLE></TD>
</TR>
<TR><TD VALIGN=top ALIGN=left><TT>Head</TT> is a variable</TD>
<TD VALIGN=top ALIGN=center NOWRAP> </TD>
<TD VALIGN=top ALIGN=left><TT>instantiation_error</TT></TD>
</TR>
<TR><TD BGCOLOR=black COLSPAN=3><TABLE BORDER=0 WIDTH="100%" CELLSPACING=0 CELLPADDING=1><TR><TD></TD></TR></TABLE></TD>
</TR>
<TR><TD VALIGN=top ALIGN=left><TT>Head</TT> is neither a variable nor a callable term</TD>
<TD VALIGN=top ALIGN=center NOWRAP> </TD>
<TD VALIGN=top ALIGN=left><TT>type_error(callable, Head)</TT></TD>
</TR>
<TR><TD BGCOLOR=black COLSPAN=3><TABLE BORDER=0 WIDTH="100%" CELLSPACING=0 CELLPADDING=1><TR><TD></TD></TR></TABLE></TD>
</TR>
<TR><TD VALIGN=top ALIGN=left><TT>Body</TT> cannot be converted to a goal</TD>
<TD VALIGN=top ALIGN=center NOWRAP> </TD>
<TD VALIGN=top ALIGN=left><TT>type_error(callable, Body)</TT></TD>
</TR>
<TR><TD BGCOLOR=black COLSPAN=3><TABLE BORDER=0 WIDTH="100%" CELLSPACING=0 CELLPADDING=1><TR><TD></TD></TR></TABLE></TD>
</TR>
<TR><TD VALIGN=top ALIGN=left>The predicate indicator <TT>Pred</TT> of <TT>Head</TT> is that of a
static procedure</TD>
<TD VALIGN=top ALIGN=center NOWRAP> </TD>
<TD VALIGN=top ALIGN=left><TT>permission_error(modify, static_procedure, Pred)</TT></TD>
</TR>
<TR><TD BGCOLOR=black COLSPAN=3><TABLE BORDER=0 WIDTH="100%" CELLSPACING=0 CELLPADDING=1><TR><TD></TD></TR></TABLE></TD>
</TR></TABLE><BR>
<B>Portability</B><BR>
<BR>
ISO predicates.<BR>
<BR>
<A NAME="toc65"></A>
<H4 CLASS="subsubsection"><A NAME="htoc94">7.7.3</A> <TT>retract/1</TT></H4>
<B>Templates</B>
<DL CLASS="list" COMPACT="compact"><DT CLASS="dt-list"><DD CLASS="dd-list"><TT>
retract(+clause)</TT></DL>
<B>Description</B><BR>
<BR>
<TT>retract(Clause)</TT> erases the first clause of the database
that unifies with <TT>Clause</TT>. The concerned predicate must be a
dynamic procedure
(section <A HREF="#Introduction:(Dynamic-clause-management)">7.7.1</A>). Removing all clauses
of a procedure does not erase the procedure definition. To achieve
this use <TT>abolish/1</TT> (section <A HREF="#abolish/1">7.7.6</A>). <TT>retract/1</TT> is
re-executable on backtracking.<BR>
<BR>
<B>Errors</B><BR>
<TABLE CELLSPACING=2 CELLPADDING=0>
<TR><TD BGCOLOR=black COLSPAN=3><TABLE BORDER=0 WIDTH="100%" CELLSPACING=0 CELLPADDING=1><TR><TD></TD></TR></TABLE></TD>
</TR>
<TR><TD VALIGN=top ALIGN=left><TT>Head</TT> is a variable</TD>
<TD VALIGN=top ALIGN=center NOWRAP> </TD>
<TD VALIGN=top ALIGN=left><TT>instantiation_error</TT></TD>
</TR>
<TR><TD BGCOLOR=black COLSPAN=3><TABLE BORDER=0 WIDTH="100%" CELLSPACING=0 CELLPADDING=1><TR><TD></TD></TR></TABLE></TD>
</TR>
<TR><TD VALIGN=top ALIGN=left><TT>Head</TT> is neither a variable nor a callable term</TD>
<TD VALIGN=top ALIGN=center NOWRAP> </TD>
<TD VALIGN=top ALIGN=left><TT>type_error(callable, Head)</TT></TD>
</TR>
<TR><TD BGCOLOR=black COLSPAN=3><TABLE BORDER=0 WIDTH="100%" CELLSPACING=0 CELLPADDING=1><TR><TD></TD></TR></TABLE></TD>
</TR>
<TR><TD VALIGN=top ALIGN=left>The predicate indicator <TT>Pred</TT> of <TT>Head</TT> is that of a
static procedure</TD>
<TD VALIGN=top ALIGN=center NOWRAP> </TD>
<TD VALIGN=top ALIGN=left><TT>permission_error(modify, static_procedure, Pred)</TT></TD>
</TR>
<TR><TD BGCOLOR=black COLSPAN=3><TABLE BORDER=0 WIDTH="100%" CELLSPACING=0 CELLPADDING=1><TR><TD></TD></TR></TABLE></TD>
</TR></TABLE><BR>
<B>Portability</B><BR>
<BR>
ISO predicate. In the ISO reference, the operation associated with the
<TT>permission_error</TT> is <TT>access</TT> while it is <TT>modify</TT> in
GNU Prolog. This seems to be an error of the ISO reference since for
<TT>asserta/1</TT> (which is similar in spirit to <TT>retract/1</TT>) the
operation is also <TT>modify</TT>.<BR>
<BR>
<A NAME="toc66"></A>
<H4 CLASS="subsubsection"><A NAME="htoc95">7.7.4</A> <TT>retractall/1</TT></H4>
<B>Templates</B>
<DL CLASS="list" COMPACT="compact"><DT CLASS="dt-list"><DD CLASS="dd-list"><TT>
retractall(+head)</TT></DL>
<B>Description</B><BR>
<BR>
<TT>retractall(Head)</TT> erases all clauses whose head unifies with
<TT>Head</TT>. The concerned predicate must be a dynamic procedure
(section <A HREF="#Introduction:(Dynamic-clause-management)">7.7.1</A>). The procedure definition
is not removed so that it is found by <TT>current_predicate/1</TT>
(section <A HREF="gprolog031.html#current-predicate/1">7.8.1</A>). <TT>abolish/1</TT> should be used to remove the
procedure (section <A HREF="#abolish/1">7.7.6</A>).<BR>
<BR>
<B>Errors</B><BR>
<TABLE CELLSPACING=2 CELLPADDING=0>
<TR><TD BGCOLOR=black COLSPAN=3><TABLE BORDER=0 WIDTH="100%" CELLSPACING=0 CELLPADDING=1><TR><TD></TD></TR></TABLE></TD>
</TR>
<TR><TD VALIGN=top ALIGN=left><TT>Head</TT> is a variable</TD>
<TD VALIGN=top ALIGN=center NOWRAP> </TD>
<TD VALIGN=top ALIGN=left><TT>instantiation_error</TT></TD>
</TR>
<TR><TD BGCOLOR=black COLSPAN=3><TABLE BORDER=0 WIDTH="100%" CELLSPACING=0 CELLPADDING=1><TR><TD></TD></TR></TABLE></TD>
</TR>
<TR><TD VALIGN=top ALIGN=left><TT>Head</TT> is not a callable term</TD>
<TD VALIGN=top ALIGN=center NOWRAP> </TD>
<TD VALIGN=top ALIGN=left><TT>type_error(callable, Head)</TT></TD>
</TR>
<TR><TD BGCOLOR=black COLSPAN=3><TABLE BORDER=0 WIDTH="100%" CELLSPACING=0 CELLPADDING=1><TR><TD></TD></TR></TABLE></TD>
</TR>
<TR><TD VALIGN=top ALIGN=left>The predicate indicator <TT>Pred</TT> of <TT>Head</TT> is that of a
static procedure</TD>
<TD VALIGN=top ALIGN=center NOWRAP> </TD>
<TD VALIGN=top ALIGN=left><TT>permission_error(modify, static_procedure, Pred)</TT></TD>
</TR>
<TR><TD BGCOLOR=black COLSPAN=3><TABLE BORDER=0 WIDTH="100%" CELLSPACING=0 CELLPADDING=1><TR><TD></TD></TR></TABLE></TD>
</TR></TABLE><BR>
<B>Portability</B><BR>
<BR>
GNU Prolog predicate.<BR>
<BR>
<A NAME="toc67"></A>
<H4 CLASS="subsubsection"><A NAME="htoc96">7.7.5</A> <TT>clause/2</TT></H4>
<B>Templates</B>
<DL CLASS="list" COMPACT="compact"><DT CLASS="dt-list"><DD CLASS="dd-list"><TT>
clause(+head, ?callable_term)</TT></DL>
<B>Description</B><BR>
<BR>
<TT>clause(Head, Body)</TT> succeeds if there exists a clause in the
database that unifies with <TT>Head :- Body</TT>. The predicate in question
must be a public procedure (section <A HREF="#Introduction:(Dynamic-clause-management)">7.7.1</A>). Clauses are delivered from the first to the last. This
predicate is re-executable on backtracking. <BR>
<BR>
<B>Errors</B><BR>
<TABLE CELLSPACING=2 CELLPADDING=0>
<TR><TD BGCOLOR=black COLSPAN=3><TABLE BORDER=0 WIDTH="100%" CELLSPACING=0 CELLPADDING=1><TR><TD></TD></TR></TABLE></TD>
</TR>
<TR><TD VALIGN=top ALIGN=left><TT>Head</TT> is a variable</TD>
<TD VALIGN=top ALIGN=center NOWRAP> </TD>
<TD VALIGN=top ALIGN=left><TT>instantiation_error</TT></TD>
</TR>
<TR><TD BGCOLOR=black COLSPAN=3><TABLE BORDER=0 WIDTH="100%" CELLSPACING=0 CELLPADDING=1><TR><TD></TD></TR></TABLE></TD>
</TR>
<TR><TD VALIGN=top ALIGN=left><TT>Head</TT> is neither a variable nor a callable term</TD>
<TD VALIGN=top ALIGN=center NOWRAP> </TD>
<TD VALIGN=top ALIGN=left><TT>type_error(callable, Head)</TT></TD>
</TR>
<TR><TD BGCOLOR=black COLSPAN=3><TABLE BORDER=0 WIDTH="100%" CELLSPACING=0 CELLPADDING=1><TR><TD></TD></TR></TABLE></TD>
</TR>
<TR><TD VALIGN=top ALIGN=left>The predicate indicator <TT>Pred</TT> of <TT>Head</TT> is that of a
private procedure</TD>
<TD VALIGN=top ALIGN=center NOWRAP> </TD>
<TD VALIGN=top ALIGN=left><TT>permission_error(access, private_procedure, Pred)</TT></TD>
</TR>
<TR><TD BGCOLOR=black COLSPAN=3><TABLE BORDER=0 WIDTH="100%" CELLSPACING=0 CELLPADDING=1><TR><TD></TD></TR></TABLE></TD>
</TR>
<TR><TD VALIGN=top ALIGN=left><TT>Body</TT> is neither a variable nor a callable term</TD>
<TD VALIGN=top ALIGN=center NOWRAP> </TD>
<TD VALIGN=top ALIGN=left><TT>type_error(callable, Body)</TT></TD>
</TR>
<TR><TD BGCOLOR=black COLSPAN=3><TABLE BORDER=0 WIDTH="100%" CELLSPACING=0 CELLPADDING=1><TR><TD></TD></TR></TABLE></TD>
</TR></TABLE><BR>
<B>Portability</B><BR>
<BR>
ISO predicate.<BR>
<BR>
<A NAME="toc68"></A>
<H4 CLASS="subsubsection"><A NAME="htoc97">7.7.6</A> <TT>abolish/1</TT></H4>
<A NAME="abolish/1"></A>
<B>Templates</B>
<DL CLASS="list" COMPACT="compact"><DT CLASS="dt-list"><DD CLASS="dd-list"><TT>
abolish(+predicate_indicator)</TT></DL>
<B>Description</B><BR>
<BR>
<TT>abolish(Pred)</TT> removes from the database the procedure whose
predicate indicator is <TT>Pred</TT>. The concerned predicate must be a
dynamic procedure (section <A HREF="#Introduction:(Dynamic-clause-management)">7.7.1</A>).<BR>
<BR>
<B>Errors</B><BR>
<TABLE CELLSPACING=2 CELLPADDING=0>
<TR><TD BGCOLOR=black COLSPAN=3><TABLE BORDER=0 WIDTH="100%" CELLSPACING=0 CELLPADDING=1><TR><TD></TD></TR></TABLE></TD>
</TR>
<TR><TD VALIGN=top ALIGN=left><TT>Pred</TT> is a variable</TD>
<TD VALIGN=top ALIGN=center NOWRAP> </TD>
<TD VALIGN=top ALIGN=left><TT>instantiation_error</TT></TD>
</TR>
<TR><TD BGCOLOR=black COLSPAN=3><TABLE BORDER=0 WIDTH="100%" CELLSPACING=0 CELLPADDING=1><TR><TD></TD></TR></TABLE></TD>
</TR>
<TR><TD VALIGN=top ALIGN=left><TT>Pred</TT> is a term <TT>Name/Arity</TT> and either
<TT>Name</TT> or <TT>Arity</TT> is a variable</TD>
<TD VALIGN=top ALIGN=center NOWRAP> </TD>
<TD VALIGN=top ALIGN=left><TT>instantiation_error</TT></TD>
</TR>
<TR><TD BGCOLOR=black COLSPAN=3><TABLE BORDER=0 WIDTH="100%" CELLSPACING=0 CELLPADDING=1><TR><TD></TD></TR></TABLE></TD>
</TR>
<TR><TD VALIGN=top ALIGN=left><TT>Pred</TT> is neither a variable nor a predicate indicator</TD>
<TD VALIGN=top ALIGN=center NOWRAP> </TD>
<TD VALIGN=top ALIGN=left><TT>type_error(predicate_indicator, Pred)</TT></TD>
</TR>
<TR><TD BGCOLOR=black COLSPAN=3><TABLE BORDER=0 WIDTH="100%" CELLSPACING=0 CELLPADDING=1><TR><TD></TD></TR></TABLE></TD>
</TR>
<TR><TD VALIGN=top ALIGN=left><TT>Pred</TT> is a term <TT>Name/Arity</TT> and <TT>Arity</TT> is
neither a variable nor an integer</TD>
<TD VALIGN=top ALIGN=center NOWRAP> </TD>
<TD VALIGN=top ALIGN=left><TT>type_error(integer, Arity)</TT></TD>
</TR>
<TR><TD BGCOLOR=black COLSPAN=3><TABLE BORDER=0 WIDTH="100%" CELLSPACING=0 CELLPADDING=1><TR><TD></TD></TR></TABLE></TD>
</TR>
<TR><TD VALIGN=top ALIGN=left><TT>Pred</TT> is a term <TT>Name/Arity</TT> and <TT>Name</TT> is
neither a variable nor an atom</TD>
<TD VALIGN=top ALIGN=center NOWRAP> </TD>
<TD VALIGN=top ALIGN=left><TT>type_error(atom, Name)</TT></TD>
</TR>
<TR><TD BGCOLOR=black COLSPAN=3><TABLE BORDER=0 WIDTH="100%" CELLSPACING=0 CELLPADDING=1><TR><TD></TD></TR></TABLE></TD>
</TR>
<TR><TD VALIGN=top ALIGN=left><TT>Pred</TT> is a term <TT>Name/Arity</TT> and <TT>Arity</TT> is
an integer < 0</TD>
<TD VALIGN=top ALIGN=center NOWRAP> </TD>
<TD VALIGN=top ALIGN=left><TT>domain_error(not_less_than_zero, Arity)</TT></TD>
</TR>
<TR><TD BGCOLOR=black COLSPAN=3><TABLE BORDER=0 WIDTH="100%" CELLSPACING=0 CELLPADDING=1><TR><TD></TD></TR></TABLE></TD>
</TR>
<TR><TD VALIGN=top ALIGN=left><TT>Pred</TT> is a term <TT>Name/Arity</TT> and <TT>Arity</TT> is
an integer > <TT>max_arity</TT> flag (section <A HREF="gprolog045.html#set-prolog-flag/2">7.22.1</A>)</TD>
<TD VALIGN=top ALIGN=center NOWRAP> </TD>
<TD VALIGN=top ALIGN=left><TT>representation_error(max_arity)</TT></TD>
</TR>
<TR><TD BGCOLOR=black COLSPAN=3><TABLE BORDER=0 WIDTH="100%" CELLSPACING=0 CELLPADDING=1><TR><TD></TD></TR></TABLE></TD>
</TR>
<TR><TD VALIGN=top ALIGN=left>The predicate indicator <TT>Pred</TT> is that of a static
procedure</TD>
<TD VALIGN=top ALIGN=center NOWRAP> </TD>
<TD VALIGN=top ALIGN=left><TT>permission_error(modify, static_procedure, Pred)</TT></TD>
</TR>
<TR><TD BGCOLOR=black COLSPAN=3><TABLE BORDER=0 WIDTH="100%" CELLSPACING=0 CELLPADDING=1><TR><TD></TD></TR></TABLE></TD>
</TR></TABLE><BR>
<B>Portability</B><BR>
<BR>
ISO predicate.<BR>
<BR>
<HR SIZE=2>
Copyright (C) 1999-2007 Daniel Diaz
<BR>
<BR>
Verbatim copying and distribution of this entire article is permitted in any
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