/usr/lib/swi-prolog/doc/Manual/extendarith.html

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<H2><A NAME="sec:4.26"><SPAN class="sec-nr">4.26</SPAN> <SPAN class="sec-title">Adding 
Arithmetic Functions</SPAN></A></H2>

<A NAME="sec:extendarith"></A>

<P>Prolog predicates can be given the role of arithmetic function. The 
last argument is used to return the result, the arguments before the 
last are the inputs. Arithmetic functions are added using the predicate <A NAME="idx:arithmeticfunction1:1040"></A><A class="pred" href="extendarith.html#arithmetic_function/1">arithmetic_function/1</A>, 
which takes the head as its argument. Arithmetic functions are module 
sensitive, that is they are only visible from the module in which the 
function is defined and declared. Global arithmetic functions should be 
defined and registered from module <CODE>user</CODE>. Global definitions 
can be overruled locally in modules. The built-in functions described 
above can be redefined as well.

<DL class="latex">
<DT class="pubdef"><A NAME="arithmetic_function/1"><STRONG>arithmetic_function</STRONG>(<VAR>:Head</VAR>)</A></DT>
<DD class="defbody">
Register a Prolog predicate as an arithmetic function (see <A NAME="idx:is2:1041"></A><A class="pred" href="arith.html#is/2">is/2</A>,
<A class="pred" href="arith.html#>/2">&gt;/2</A>, etc.). The Prolog 
predicate should have one more argument than specified by <VAR>Head</VAR>, 
which it either a term
<VAR>Name/Arity</VAR>, an atom or a complex term. This last argument is 
an unbound variable at call time and should be instantiated to an 
integer or floating point number. The other arguments are the 
parameters. This predicate is module sensitive and will declare the 
arithmetic function only for the context module, unless declared from 
module <CODE>user</CODE>. Example:

<PRE class="code">
1 ?- [user].
:- arithmetic_function(mean/2).

mean(A, B, C) :-
        C is (A+B)/2.
user compiled, 0.07 sec, 440 bytes.

Yes
2 ?- A is mean(4, 5).

A = 4.500000
</PRE>

</DD>
<DT class="pubdef"><A NAME="current_arithmetic_function/1"><STRONG>current_arithmetic_function</STRONG>(<VAR>:Head</VAR>)</A></DT>
<DD class="defbody">
True if <VAR>Head</VAR> is a function that is visible from the current 
module. Built-in functions are visible from all modules, while 
user-defined functions (see <A NAME="idx:arithmeticfunction1:1042"></A><A class="pred" href="extendarith.html#arithmetic_function/1">arithmetic_function/1</A>) 
obey the module visibility rules. E.g.,

<PRE class="code">
?- current_arithmetic_function(sin(_)).
true.
</PRE>

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