PLOGHELP UNIV                                  Robert Duncan, March 1988


    :- op(40, xfx, '=..').

    ?- T =.. L .


The infix operator '=..' (pronounced "univ") can be used either to
decompose a term into a list containing its functor and arguments or
else to construct a term from such a list.

In a goal of the form

    ?- T =.. L .

if T is a term, then L is unified with a list consisting of the functor
of T and its arguments. So for example:

    ?- p(0, a) =.. L .
    L = [p, 0, a] ?
    yes

If T is atomic, then it counts as its own functor and L is unified with
a singleton list:

    ?- 3 =.. L .
    L = [3] ?
    yes

If T is a variable, then L must be instantiated to a proper list (i.e
one terminated by nil) whose head is atomic; in this case T is unified
with a term constructed from the elements of L, the first element of L
being the functor and the remaining elements its arguments. Thus:

    ?- T =.. [p, 0, a].
    T = p(0, a) ?
    yes

    ?- T =.. [3].
    T = 3 ?
    yes

It is an error for both T and L to be uninstantiated, or for T to be
uninstantiated and L not to be a proper list. Also, if the head of L is
not an atom, then its tail must be nil in order to construct a term,
since objects other than atoms cannot be used as functors of arity
greater than zero.

A "univ" goal cannot be re-satisfied.

An alternative approach to constructing and destructing terms is to use
functor/3 and arg/3 (see PLOGHELP * FUNCTOR, * ARG). These can be
written in terms of "univ" and vice-versa.


-- RELATED DOCUMENTATION ----------------------------------------------

PLOGHELP * ARG
    Determines the nth argument of a term

PLOGHELP * DOLLAR
    Library package which allows variables to stand for predicate names

PLOGHELP * FUNCTOR
    Determines the functor and arity of a term

PLOGHELP * OP
    Discussion of operators and how to declare them

PLOGHELP * OPERATORS
    Summary of the standard operator declarations

PLOGHELP * SYMBOLS
    Summary and index of HELP files for symbolic atoms used in Prolog

--- C.all/plog/help/univ -----------------------------------------------
--- Copyright University of Sussex 1988. All rights reserved. ----------