The built-in predicate `=../2`

is a clear way of building terms
and taking them apart. However, it is almost never the most
efficient way.
`functor/3`

and `arg/3`

are generally much more
efficient, though less direct. The best blend of efficiency and
clarity is to write a clearly-named predicate that
implements the desired operation and to use `functor/3`

and `arg/3`

in that predicate.

Here is an actual example. The task is to reimplement the
built-in predicate `==/2`

. The first variant uses
`=../2`

(this symbol is pronounced "univ" for historical
reasons). Some Prolog textbooks recommend code similar to this.

ident_univ(X, Y) :- var(X), % If X is a variable, !, var(Y), % so must Y be, and samevar(X, Y). % they must be the same. ident_univ(X, Y) :- % If X is not a variable, nonvar(Y), % neither may Y be; X =.. [F|L], % they must have the Y =.. [F|M], % same function symbol F ident_list(L, M). % and identical arguments ident_list([], []). ident_list([H1|T1], [H2|T2]) :- ident_univ(H1, H2), ident_list(T1, T2). samevar(29, Y) :- % If binding X to 29 var(Y), % leaves Y unbound, !, % they were not the same fail. % variable. samevar(_, _). % Otherwise they were.

This code performs the function intended; however,
every time it touches a non-variable term of arity `N`, it
constructs a list with `N`+1 elements, and if the two terms are
identical, these lists are reclaimed only when backtracked over or
garbage-collected.

Better code uses `functor/3`

and `arg/3`

.

ident_farg(X, Y) :- ( var(X) -> % If X is a variable, var(Y), % so must Y be, and samevar(X, Y) % they must be the same; ; nonvar(Y), % otherwise Y must be nonvar functor(X, F, N), % The principal functors of X functor(Y, F, N), % and Y must be identical, ident_farg(N, X, Y) % including the last N args. ). ident_farg(0, _, _) :- !. ident_farg(N, X, Y) :- % The last N arguments are arg(N, X, Xn), % identical arg(N, Y, Yn), % if the Nth arguments ident_farg(Xn, Yn), % are identical, M is N-1, % and the last N-1 arguments ident_farg(M, X, Y). % are also identical.

This approach to walking through terms using
`functor/3`

and `arg/3`

avoids the construction of useless
lists.

The pattern shown in the example, in which a predicate of arity `K` calls
an auxiliary
predicate of the same name of arity `K`+1 (the additional argument denoting
the number of items remaining to process), is very common.
It is not necessary to use the same name for this auxiliary predicate,
but this convention is generally less prone to confusion.

In order to simply find out the principal function symbol of a term, use

| ?-the_term_is(Term),|functor(Term, FunctionSymbol, _).

The use of `=../2`

, as in

| ?-the_term_is(Term),|Term =.. [FunctionSymbol|_].

is wasteful, and should generally be avoided. The same remark applies if the arity of a term is desired.

`=../2`

should not be used to locate a particular argument of
some term. For example, instead of

Term =.. [_F,_,ArgTwo|_]

you should write

arg(2, Term, ArgTwo)

It is generally easier to get the explicit number "2" right
than to write the correct number of "don't care" variables in
the call to `=../2`

. Other people reading the program
will find the call to `arg/3`

a much clearer expression of
the program's intent. The program will also be more efficient.
Even if several arguments of a term must be located, it
is clearer and more efficient to write

arg(1, Term, First), arg(3, Term, Third), arg(4, Term, Fourth)

than to write

Term =.. [_,First,_,Third,Fourth|_]

Finally, `=../2`

should not be used when the functor of
the term to be operated on is known (that is,
when both the function symbol and the arity are known).
For example, to make a new term with the same function symbol and first
arguments as another term, but one additional argument,
the obvious solution might seem to be to write something like the following:

add_date(OldItem, Date, NewItem) :- OldItem =.. [item,Type,Ship,Serial], NewItem =.. [item,Type,Ship,Serial,Date].

However, this could be expressed more clearly and more efficiently as

add_date(OldItem, Date, NewItem) :- OldItem = item(Type,Ship,Serial), NewItem = item(Type,Ship,Serial,Date).

or even

add_date(item(Type,Ship,Serial), Date, item(Type,Ship,Serial,Date) ).